1 Mendelelian Genetics Dr. R. Siva - PowerPoint Presentation

1. 1Mendelelian GeneticsDr. R. SivaVIT University, INDIArsiva77in@rediffmail.com

2. 2Gregor Mendel(1822-1884)Responsible for the Laws governing Inheritance of Traits

3. 31822 - Brunn in Austria, now, Brno (Czecholovakia)Financial and Health ProblemUnable to continue studiesJoined St.Augustinian Monastery1847 – Priest1851 – Higher studies at Vienna University1854- Science teacher

4. 4Gregor Johann MendelBetween 1856 and 1863, Mendel cultivated and tested some 28,000 pea plantsHe found that the plants' offspring retained traits of the parents

5. 5Gregor Johann MendelAustrian monkDeveloped the laws of inheritancePresented his findings in the Natural History Society of Brunn – 1865 Paper entitled, “Experiments in Plant Hybridization” – 1866German language.

6. 6Mendel's work was not recognized until the turn of the 20th century1900 - Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirmCalled the “Father of Genetics“

7. 7Site of Gregor Mendel’s experimental garden in the Czech Republic

8. 8Mendel’s workplaceFig. 2.5

9. 9Mendel stated that physical traits are inherited as “particles”Mendel did not know that the “particles” were actually Chromosomes & DNAParticulate Inheritance

10. 10Genetic TerminologyTrait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity

11. 11Types of Genetic CrossesMonohybrid cross - cross involving a single traite.g. flower color Dihybrid cross - cross involving two traits e.g. flower color & plant height

12. 12Designer “Genes”Alleles - two forms of a gene (dominant & recessive) Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)

13. 13More TerminologyGenotype - gene combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white)

14. 14GenotypesHomozygous genotype – When the two alleles are same (dominant or 2 recessive genes) e.g. TT or tt; also called pure  Heterozygous genotype – When the 2 alleles are different- one dominant & one recessive allele    (e.g. Tt); also called hybrid

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16. 16Punnett SquareUsed to help solve genetics problems

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18. 18 EquationThe formula 2n can be used, where “n” = the number of heterozygous traits.Ex: TtRr, n=2 22 or 4 different kinds of gametes are possible. TR, tR, Tr, tr

19. 19Dihybrid CrossTtRr X TtRrEach parent can produce 4 types of gametes. TR, Tr, tR, trCross is a 4 X 4 with 16 possible offspring.

20. 20RESULTS9 Tall, Red flowered3 Tall, white flowered3 short, Red flowered1 short, white floweredOr: 9:3:3:1

21. 21Genes and Environment Determine Characteristics

22. 22Mendel’s Pea Plant Experiments

23. 23Why peas, Pisum sativum?Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate several generations Can be artificially cross-pollinatedBisexual.Many traits known.Above all, easy to grow

24. 24Reproduction in Flowering PlantsPollen contains spermProduced by the stamenOvary contains eggsFound inside the flowerPollen carries sperm to the eggs for fertilizationSelf-fertilization can occur in the same flowerCross-fertilization can occur between flowers

25. 25Mendel’s Experimental MethodsMendel hand-pollinated flowers using a paintbrushHe could snip (cut) the stamens to prevent self-pollinationHe traced traits through the several generations

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27. 27How Mendel Began?Mendel produced pure strains by allowing the plants to self-pollinate for several generations

28. 28Eight Pea Plant TraitsSeed shape --- Round (R) or Wrinkled (r)Seed Color ---- Yellow (Y) or  Green (y)Pod Shape --- Smooth (S) or wrinkled (s)Pod Color ---  Green (G) or Yellow (g)Seed Coat Color ---Gray (G) or White (g)Flower position---Axial (A) or Terminal (a)Plant Height --- Tall (T) or Short (t)Flower color --- Purple (P) or white (p)

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31. 31Mendel’s Experimental Results

32. 32Did the observed ratio match the theoretical ratio?The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkledMendel’s observed ratio was 2.96:1The discrepancy is due to statistical errorThe larger the sample the more nearly the results approximate to the theoretical ratio

33. 33Generation “Gap”Parental P1 Generation = the parental generation in a breeding experiment.F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation)From breeding individuals from the P1 generationF2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F1 generation

34. 34Following the GenerationsCross 2 Pure PlantsTT x ttResults in all HybridsTtCross 2 Hybridsget3 Tall & 1 ShortTT, Tt, tt

35. 35Monohybrid Crosses

36. 36Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Wrinkled seeds RR x rrP1 Monohybrid CrossRRrrRrRrRrRrGenotype: RrPhenotype: RoundGenotypicRatio: All alikePhenotypicRatio: All alike

37. 37P1 Monohybrid Cross ReviewHomozygous dominant x Homozygous recessiveOffspring all Heterozygous (hybrids)Offspring called F1 generationGenotypic & Phenotypic ratio is ALL ALIKE

38. 38Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Round seeds Rr x RrF1 Monohybrid CrossRrrRRRrrRrRrGenotype: RR, Rr, rrPhenotype: Round & wrinkledG.Ratio: 1:2:1P.Ratio: 3:1

39. 39F1 Monohybrid Cross ReviewHeterozygous x heterozygousOffspring:25% Homozygous dominant RR50% Heterozygous Rr25% Homozygous Recessive rrOffspring called F2 generationGenotypic ratio is 1:2:1Phenotypic Ratio is 3:1

40. 40What Do the Peas Look Like?

41. 41…And Now the Test CrossMendel then crossed a pure & a hybrid from his F2 generationThis is known as an F2 or test crossThere are two possible testcrosses:Homozygous dominant x HybridHomozygous recessive x Hybrid

42. 42Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Round seeds RR x RrF2 Monohybrid Cross (1st)RRrRRRRrRRRrGenotype: RR, RrPhenotype: RoundGenotypicRatio: 1:1PhenotypicRatio: All alike

43. 43Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Wrinkled seeds x Round seeds rr x RrF2 Monohybrid Cross (2nd)rrrRRrrrRrrrGenotype: Rr, rrPhenotype: Round & WrinkledG. Ratio: 1:1P.Ratio: 1:1

44. 44F2 Monohybrid Cross ReviewHomozygous x heterozygous(hybrid)Offspring:50% Homozygous RR or rr50% Heterozygous RrPhenotypic Ratio is 1:1Called Test Cross because the offspring have SAME genotype as parents

45. 45Practice Your CrossesWork the P1, F1, and both F2 Crosses for each of the other Seven Pea Plant Traits

46. 46Mendel’s Laws1. Law of Dominance2. Law of Segregation3. Law of Independent assortment

47. 47Law of DominanceStates that on crossing homozygous organisms for single pair of contrasting characters, only one characters make its appearance in F1 generation and is name as Dominant character.

48. 48Results of Monohybrid CrossesInheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the same

49. 49Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.All the offspring will be heterozygous and express only the dominant trait.RR x rr yields all Rr (round seeds)

50. 50Law of Dominance

51. 51DrosophilaManCharacterBody ColourEye colourColour of hairForm of hairColour of eyeLipsBlood groupDominantGrayRedDarkCurlyBrownBroad and thickenA,B,ABRecessiveBlackWhiteLightStraightBlueThinO

52. 52.Law of SegregationTwo allele of a gene remain separate and do not contaminate each other in F1 or hybrid. ORThe two alleles for each trait separate during gamete formation.

53. 53Law of SegregationDuring the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

54. 54Law of segregation

55. 55Applying the Law of Segregation

56. 56Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses.

57. 57Dihybrid CrossTraits: Seed shape & Seed colorAlleles: R round r wrinkled Y yellow y green RrYy x RrYyRY Ry rY ryRY Ry rY ryAll possible gamete combinations

58. 58Dihybrid CrossRYRyrYryRYRyrYry

59. 59Dihybrid CrossRRYYRRYyRrYYRrYyRRYyRRyyRrYyRryyRrYYRrYyrrYYrrYyRrYyRryyrrYyrryyRound/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 19:3:3:1 phenotypic ratioRYRyrYryRYRyrYry

60. 60Dihybrid CrossRound/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 19:3:3:1

61. 61Question:How many gametes will be produced for the following allele arrangements?Remember: 2n (n = # of heterozygotes) 1. RrYy 2. AaBbCCDd 3. MmNnOoPPQQRrssTtQq

62. 62Answer:1. RrYy: 2n = 22 = 4 gametes RY Ry rY ry2. AaBbCCDd: 2n = 23 = 8 gametes ABCD ABCd AbCD AbCd aBCD aBCd abCD abCD 3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes

63. 63Summary of Mendel’s lawsLAWPARENT CROSSOFFSPRINGDOMINANCETT x tt tall x short100% Tt tallSEGREGATIONTt x Tt tall x tall75% tall 25% shortINDEPENDENT ASSORTMENTRrGg x RrGg round & green x round & green9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods

64. 64Summary of Mendel’s Hypothesis1. Genes can have alternate versions called alleles.Each offspring inherits two alleles, one from each parent.If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation

65. 65The elegance of mendel’s experiments was partly due to the complete consistency between his observation and Hypotheses he developed. However, after mendel’s work was rediscovered, it became clear that simple medelian model did not adequately predict experimental observations in all situations.

66. 66Variations on Mendel1. Incomplete Dominance2. Codominance3. Multiple Alleles4. Epistasis5. Polygenic Inheritance

67. 67Incomplete DominanceandCodominance

68. 68Incomplete DominanceF1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.Carl Correns Crossed 4’O Clock Plant (flower)red (RR) x white (rr) RR = red flower rr = white flowerrrRR

69. 69Incomplete DominanceRrRrRrRrrrRRAll Rr = pink(heterozygous pink)produces theF1 generation

70. 70Incomplete Dominance

71. 71Another Example

72. 72CodominanceBoth the alleles can be expressedEg. Red cows crossed with white will generate Roan cows.Might seem to support blending theory.But F2 generation demonstrate Mendalian genetics (1:2:1)

73. 73CodominanceTwo alleles are expressed in heterozygous individuals.Example: blood type 1. type A = IAIA or IAi 2. type B = IBIB or IBi 3. type AB = IAIB 4. type O = ii

74. 74In humans, there are four blood types: A,B,AB and OBlood type is controlled by three alleles: A,B,OO is recessive, two O alleles must be present for the person to have type O bloodA and B are Codominant. If a person receives an A allele and B allele, their blood type is AB typeCrosses involving blood type often use an I to denote the alleles

75. 75Summary of Dominance RelationshipsXDominancecompleteXincompleteXcodominance

76. 76Try theseIn a plant species, if the B allele (blue flowers) and the b allele is (white flowers) are incomplete dominant (Bb is light blue).What offspring ratio is expected in a cross between blue flowered with light blue flowered plant?What would be the phenotypic ratio of the flowers produced by a cross between two light blue flowers?

77. 77LETHAL GENEA lethal gene causes death of individual in the appropriate genotype before they reach adulthood. Lethal are generally recessive, resulting in the death of the recessive homozygote. - Some lethals are dominant. In 1904 French geneticist Lucien Cuenot, discovered a recessive lethal affecting coat colour in mice.

78. 78LETHAL GENEHe observed that yellow colour is dominant;But while crossing 2 yellow mice, he got only 2:1 ratio.While he crossed yellow with recessive wild type (grey) found that all are yellow. Concluded all yellow mice were heterozygotes.

79. 79LETHAL GENELater it was suggested that homozygosity for yellow is lethal, and that these individuals died in utero, a fact observed by histological studies.

80. 80Recessive Lethal Alleles

81. 81Manx CatManx Cat – Tailless cat is another trait caused by an allele that has dominant effect in heterozygous and is lethal in homozygotes. The Manx and normal alleles are denoted by M and m respectively Mm X Mm

82. 82 Other example is achondroplasia, the most common form of dwarfism, with a normal length body trunk but shortened limbs.

83. 83Many genes have alleles that affect the rate of mortality but are not lethals. In general these are termed as deleterious or detrimental alleles.

84. 84PLEIOTROPY - Impact of a single gene on more than one characteristic - Sickle-cell disease- Most common inherited illness among black people- RBCs are sickle-shapedCan cause many problemsRBCs: Sickle-cell diseaseNormal RBCs

85. 85Sickle Cell AnemiaUnder conditions of low oxygen tension, hemoglobin S will precipitate, causing cells to sickle Mutations in same amino acid Some individuals die in childhood; Some individuals have mild symptoms HbA: V – H – L – T- P – G –G HbS: V – H – L – T- P – V–G

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87. 87Sickle cell anemia may be the result of a genetic mutation that happened in malaria-prone regions like Africa thousands of years ago. People with sickle cell trait may have been more likely to survive malaria epidemics - and because they survived when others did not, this allowed the trait to be passed down through generations.

88. 88PKU

89. 89Gene Interaction and EpistasisMendel was lucky that in his studies each character was governed by a single geneTypical Dihybrid ratio: 9:3:3:1The phenomenon of two or more genes governing the development of single character is known as Gene interaction. When one gene affects in anyway the expression of another gene, the phenomenon is called Epistasis.

90. 90Types of Gene Interaction1. Typical Dihybrid ratio for a single trait2. Duplicate gene action3. Complementary gene action4. Supplementary gene action5. Inhibitory gene action6. Masking gene action7. Polymeric gene actionAnd 8. Additive gene action.

91. 911.Typical Dihybrid ratio for a single traitSinglePeaRoseWalnutParents: Pea X Ross (PPrr) (RRpp) Walnut (PpRr)

92. 92A typical dihybrid ratio in the case of single character, Viz Comb shape in poultry. Genotype Ratio : 9:3:3:1Where, 9: Walnut3: Pea3:Rose1: Single

93. 932. Duplicate Gene Action:The non-floating habit in rice is controlled by two genes A1/A1; A2/A2Like that floating habit is controlled by a1/a1; a2/a2

94. 94Here, the presence of a single dominant allele of any of the two genes governing the trait produces the dominant phenotype (non-floating). The recessive genotype (floating habit) is produced only when the genes are in the homozygous recessive state. The genes that show duplicate interaction is called Duplicate gene action.

95. 95Duplicate Dominant Eptistasis (15:1)A/A;B/BA/A;B/bA/a;B/BA/a;B/bA/A;B/bA/A;b/bA/a;B/bA/a;b/bA/a;B/BA/a;B/ba/a;B/Ba/a;B/bA/a;B/bA/a;b/ba/a;B/ba/a;b/bA;BA;ba;Ba;bA;BA;ba;Ba;b15 : 1 Non FloatingFloating

96. 963. Complementary gene actionIn a sweet pea the development of purple flowers requires the presence of 2 dominant genes, C and R, e.g., CCRR.When either C or R alone present purple flowers cannot be produced; as a result white flowers are obtained e.g., ccRR Or CCrr Or ccrr

97. 97Parents: CCRR X ccrr (Purple) (White)Gametes: CR crF1 CcRr (Purple)

98. 98A/A;B/BA/A;B/bA/a;B/BA/a;B/bA/A;B/bA/A;b/bA/a;B/bA/a;b/bA/a;B/BA/a;B/ba/a;B/Ba/a;B/bA/a;B/bA/a;b/ba/a;B/ba/a;b/bA;BA;ba;Ba;bA;BA;ba;Ba;b9 : 7

99. 994.Supplementary Gene ActionIn this gene interaction, the dominant allele of one gene produces a phenotypic effect.The dominant allele of the other gene does not produce any phenotypic effect on its own;But when it is present with the dominant allele of the first gene, it modifies the phenotypic effect produced by the first gene.

100. 100Parents: RRPrPr X rrprpr (Purple) (White)Gametes: RPr rprF1 RrPrPr (Purple)

101. 101Supplementary gene actionIn maize, the development of colour is governed by two completely dominant genes R and Pr. The dominant allele R is essential for (red) colour production Homozygous state of the recessive allele r (rr) prevent the production of red colour.Phenotypic ratio: 9:3:4 (Purple: Red: White)

102. 102RPrRprrPrrprRPrRprrPrrpr9 : 4 : 3 Modified ratio Still 1/16ths9:Purple3:Red4: White

103. 1035. Inhibitory gene actionOne dominant inhibitory gene prevents the expression of other dominant gene.E.g. , There are two varieties of white comb which are genetically different. One variety is called Leghorn represented by IICC, which carries a colour gene C, and other gene I, that inhibits the expression of the colour gene CThe other white var. called Wyandotte is recessive to the two genes viz., iicc and so it is white or colourless

104. 104When cross made btn 2 white var. Leghorn and Wyandotte, F1 hybrid also white, When this F1 allowed to self fertilize there was a segregation of 13:3 ratio of white and coloured respectively. Parents: IICC X iicc White (Leghorn) White (Wyandotte)Gametes: IC icF1 IiCc (White)

105. 105ICIciCicICIciCic13 : 3

106. 1066.Masking gene actionIn barely, seed colour is governed by two dominant genes B and Y. The allele B produces black colour, while Allele b produces white colourThe dominant allele Y produces YellowWhile, allele y produces white colour

107. 107Parents: BByy X bbYY (Black) (Yellow)Gametes: By bYF1 BbYy (Black)

108. 108A/A;B/BA/A;B/bA/a;B/BA/a;B/bA/A;B/bA/A;b/bA/a;B/bA/a;b/bA/a;B/BA/a;B/ba/a;B/Ba/a;B/bA/a;B/bA/a;b/ba/a;B/ba/a;b/bA;BA;ba;Ba;bA;BA;ba;Ba;b12 : 3 : 1

109. 1097. Polymeric Gene ActionIn Polymeric gene action, two completely dominant genes controlling a character produce the same phenotype when their dominant alleles are alone. But when dominant alleles of both the genes are present together, their phenotypic effect is enhanced as if the effect of the two genes were cumulative or additive.

110. 110Polymeric Gene ActionIn barley, two completely dominant genes A and B affect the length of awns (the needle like extensions of lemma).Dominant allele of the gene A or B alone (e.g., Aa bb, AA bb, BB aa, Bb aa) give rise to awns of medium length .Thus the phenotype of A is same as that of B.But when A and B together they produce long awn. Where as aa bb are awnless.

111. 111Parents: AABB X aabb (Long Awn) (Awnless)Gametes: AB abF1 AaBb (Long Awn)

112. 112AB Ab aB abABAbaBab9: Long Awn6: Medium Awn1: Awnless

113. 1138. Additive Gene actionIn additive gene action, each positive allele of the two genes governing a trait produces equal and identical effect on the character. Therefore the genes showing additive gene action are called multiple factors or more commonly Polygenes

114. 114Each polygene has two alleles: one allele of each polygene produces a +ve effect in the character governed by the gene. – Positive allele.The other allele of each gene has no effect on the character and is negative allele

115. 115 Seed Colour in tetraploid wheat is governed by two polygenes R1 and R2, and determining the colour, where as r1 and r2 do not produce any colour. Each R1 and R2, allele produces a small amount of colour. Therefore the total intensity of colour depends on the total number of positive alleles of the two genes present. Thus,R1R1R2R2 produces dark red colour; R1R1R2r2 generate medium dark red and R1r1R2r2 yield medium red and so on.

116. 116Parents: R1R1R2R2 X r1r1r2r2 (Red) (White)Gametes: R1R2 r1r2F1 R1r1R2r2 (Medium red)

117. 1171: Dark red4: Medium dark red6: Medium red4: Light red1: White

118. 118Summary of Epistasis

119. 119Multiple AlleleIt is generally accepted that gene has two alternative forms called Allele. Usually, one of the two alleles of a gene is dominant over the other, which is recessive e.g, Tall (TT)But in many cases, several alleles of a single gene governing the concerned trait and is known as Multiple alleles.

120. 120Both prokaryotes and Eukaryotes show multiple alleles.Examples:1. Human ABO2. Rh and other blood group3.Fur colour in rabbits and other mammals. 4. Drosophila eye colour

121. 121What is blood made up of?   An adult human has about 4–6 liters of blood circulating in the body. Among other things, blood transports oxygen to various parts of the body.The red blood cells contain hemoglobin, a protein that binds oxygen. Red blood cells transport oxygen to, and remove carbon dioxide from, the body tissues.The white blood cells fight infection.The platelets help the blood to clot, if you get a wound for example.The plasma contains salts and various kinds of proteins. 

122. 122What are the different blood groups?  The differences in human blood are due to the presence or absence of certain protein molecules called antigens and antibodies. The antigens are located on the surface of the red blood cells and the antibodies are in the blood plasma. Individuals have different types and combinations of these molecules. The blood group you belong to depends on what you have inherited from your parents.

123. 123There are more than 20 genetically determined blood group systems known today, but the AB0 and Rh systems are the most important ones used for blood transfusions. Not all blood groups are compatible with each other. Mixing incompatible blood groups leads to blood clumping or agglutination, which is dangerous for individuals.Nobel Laureate Karl Landsteiner was involved in the discovery of both the AB0 and Rh blood groups.

124. 124Blood group AIf you belong to the blood group A, you have A antigens on the surface of your red blood cells and B antibodies in your blood plasma Blood group BIf you belong to the blood group B, you have B antigens on the surface of your red blood cells and A antibodies in your blood plasma

125. 125Blood group ABIf you belong to the blood group AB, you have both A and B antigens on the surface of your red blood cells and no A or B antibodies at all in your blood plasma.Blood group 0If you belong to the blood group 0 (null), you have neither A or B antigens on the surface of your red blood cells but you have both A and B antibodies in your blood plasma.

126. 126Multiple Allele

127. 127Rh factor blood grouping systemMany people also have a so called Rh factor on the red blood cell's surface. This is also an antigen and those who have it are called Rh+. Those who haven't are called Rh-. A person with Rh- blood does not have Rh antibodies naturally in the blood plasma (as one can have A or B antibodies, for instance). But a person with Rh- blood can develop Rh antibodies in the blood plasma if he or she receives blood from a person with Rh+ blood, whose Rh antigens can trigger the production of Rh antibodies. A person with Rh+ blood can receive blood from a person with Rh- blood without any problems.

128. 128Blood is frequently exchanged between mother and the fetus during childbirth. Thus, Rh-negative mothers often immunized by blood from Rh-positive fetuses (which may result when father is Rh-positive).Usually no ill effects are associated with exposure of the mother to the Rh-positive antigen during the first child birth. Subsequent Rh-positive children carried by the same mother, however, exposed to the antibodies produced by the mother against the Rh antigen. Such children may develop symptoms of hemolytic jaundice or anemia, a condition referred to as erythroblastosis fetalis. . The symptom may be mild or severe, even resulting in the death of the fetus.

129. 129The number of different genotype is possible to find out in multiple allelesIf n is the number of alleles of a gene, the number of different genotypes possible is n(n+1)/2, thus 2,3,4 or 5 alleles there are 3,6,10 and 15 possible genotype respectively. Note that although a large number of different alleles of a given gene may be present in a population or sp., only two of those alleles can be present in any diploid organism.

130. 130Alleles Homozygous Heterozygous Genotype1 1 0 12 2 1 33 3 3 64 4 6 105 5 10 156 6 15 21n n n(n-1)/2 n(n+1)/2

131. 131Coat colour in RabbitMultiple alleles involves coat colour in rabbits. Four alleles of rabbit coat colour (c) gene have been studied:C+ wild type or full colourCb “himalayan” white with black tips on the extremitiesCch “chinchilla” mixed colour & white hairsC albino These alleles show a gradation in dominance of C+ > Cb > Cch >C

132. 132Multiple allele in Drosophila eye colourWild type Drosophila have red eyes;But a vast array of eye-colour mutants have been studied extensively.Mutant alleles of one gene (white) result in flies with eye colour ranging from pure white through a series of intermediate colours up to nearly wild type red when present in homozygous condition. For e.g., the allele that produces the most extreme mutant phenotype was named white because the eyes of the flies homozygous for this allele are completely white (no coloration)

133. 133Other alleles of the gene were designated wa (white apricot)we (white eosin), wch (white cherry), wco (white coral), ww (white wine), wb (white blood), wcrr (white carrot), wcf (white coffee) and so on, to indicate that different levels of coloration of the eyes occurred in flies homozygous for each particular allele.

134. 134Codominance ProblemExample: homozygous male Type B (IBIB) x heterozygous female Type A (IAi)IAIBIAIBIBiIBi1/2 = IAIB1/2 = IBi IAIBIBi

135. 135Another Codominance Problem Example: male Type O (ii) x female type AB (IAIB)IAiIBiIAiIBi1/2 = IAi1/2 = IBi iIAIBi

136. 136Try These1. If a male has blood type B and a female has blood type A, what are the possible blood types in the offspring?2. Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?3. A child is type AB. His biological mother is also type AB. What are the possible phenotypes of his biological father?

137. 137Linkage Discovery of Linkage:In 1900, Mendel’s work was re-discovered, and scientists were testing his theories with as many different genes and organisms as possible.William Bateson and R.C. Punnett were working with several traits in sweet peas, notably a gene for purple (P) vs. red (p) flowers, and a gene for long pollen grains (L) vs. round pollen grains (l).

138. 138too manytoo fewobserve deviations from9 : 3 : 3 : 1 ratios derived from dihybrid crosses1 : 1 : 1 : 1 ratios derived from test crosses

139. 139It is universally accepted that genes are located in chromosomesDuring cell divisions each chromosome behaves as a unitTherefore, genes located in same chromosome would move together to the same pool during cell division.Linkage: The tendency of two or more genes to stay together during inheritance is known as Linkage. Because they are located on same chromosome.

140. 140Recombination During MeiosisRecombinant gametesParental gametes

141. 141Linkage in Drosophilawild-type femaleebony body, vestigial winged maleIn Drosophila Grey colour is dominant (G or b+) over black colour (g or b) and long wing (L or Vg+) over vestigial wing (l or Vg). The genes of these characters are linked together in same chromosome.

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144. 144Coupling and RepulsionIn Maize, Dominant gene C produces coloured seeds (C)Recessive gene produces colourless seeds (c)Another dominant gene governs full seeds (Sh)While recessive produce shrunken seeds (sh)Parents CCSh X ccshGamets C Sh c shF1 CcShsh

145. 145Test cross: When F1 are test crossed with recessive strain, out of 8,368 seeds 4,032 (48.2%) were coloured full; 4,035 (48.3%) were colourless shrunken; 149 (1.7%) were coloured shrunken; 152 (1.8 %) were colourless full.Here, the four phenotypic classes are not in the expected ratio of 1:1:1:1. The phenotypic classes coloured full and colourless shrunken have a much higher frequency than the expected 25%. These two character combinations are called Parental phenotype or parental types or Parental combinations.

146. 146In the above e.g., it appears as if the 2 dominant genes C and Sh have a strong affinity for each other so that the phenotypes are greater than expected. This situation is referred to as Coupling phase.It is due to the presence of genes C and Sh in the same chromosome.

147. 147Similarly when coloured shrunken seeds (CC shsh) were crossed with colourless full (cc ShSh), F1 were coloured full (CcShsh). But when F1 test crossed with recessive strain (cc shsh) 47.9% were coloured Shrunken; 49.1% colourless full; 1.4% coloured full; 1.5% colourless shrunkenIn this case also the parental types were more frequent.It appears as if in this cross the dominant genes c and Sh dislike each other. This situation is referred to as Repulsion phase. It is due to the presence of dominant allele of one gene, e.g., C with the recessive allele of the other gene, e,g, sh in the same chromosome.

148. 148Complete and Incomplete linkageGene show a linkage because they are located on a chromosome. For e.g., C and Sh are present in one chromosome, while their recessive alleles c and sh are situated in the homologous chromosome. C shC shc Shc Sh

149. 149Each chromosome behaves as a unit during cell division.Therefore, C and Sh would move to one pole to while c and sh move to opposite pool. If this always happened, F1 generation (CcShsh) would produce 2 gametes viz., C Sh and c shWhen only parental character combinations are recovered in test progeny , it is called complete linkage.However, sometime, allele recombine to produce recombinant types like C sh, cShSuch a type are called Incomplete linkage.

150. 150Linkage is never 100%. No matter how tightly two genes are linked, if you observe enough individuals, you will find some recombinants.

151. 151Crossing OverRecombinant phenotypes are produced by recombinant gametes. These gametes, in turn, are produced due to crossing over. The exchange of homologous segments between nonsister chromatids of homologous chromosomes is known as Crossing over.Crossing over takes place during pachytene .

152. 152Centromere{sisterchromatids{sisterchromatidschiasmata

153. 153Crossing over occurs during meiosis of gametogenesis. The homologous chromosomes move towards each other and come to lie side by side.This phenomenon of pairing of homologous chromosomes is called synapsis.The paired homologous chromosomes are called bivalents. The homologous chromosome split into two chromotids

154. 154

155. 155Chiasmata mark the sites of recombination

156. 156Chiasmata mark the sites of recombination

157. 157chiasmata: visible manifestations of crossing overProposed by Janssens Prior to crossing over, the chromosomes of each bivalent get duplicated to form a tetrad. Crossing over occurs only between the nonsister chromatids of a tetrad. In this stage, the nonsister chromatid overlap with one another and form chiasma or point of contact

158. 158Recombination FrequencyRF =recombinant progenytotal progenyX 100%

159. 159Salient features of Crossing over1. Crossing over is the interchange of chromosomal segments between the homologous chromosome2. It occurs during meiosis or gametogenesis3. The crossing over occurs only between nonsister chromatids of the homologous chromosome4. The number of crossing over is depends upon the length of the chromosome. The longer the length, higher percentage of crossing over5. When the genes are located apart from one another the chances of crossing over is higher when the genes are closely located the chances for crossing over is lesser.

160. 160RECOMBINATIONproducts of meiotic recombination

161. 161Detection of recombination using a test cross

162. 162Kinds of crossing over:1. Single crossing over In this type, only one chiasma is formed. Only one chromatid of each chromosome is involved in single crossing over.2. Double crossing over Here two chiasma are formed. The chiasmata may be formed between the same chromatids or between different chromatids. Thus 2 or 3 or all the four chromatid may be involved. 3.Multipel crossing over: More than two chiasma are formed

163. 163Crossing over occurs in the Tetrad StageNeurospora crassa highly suited certain types of genetic studies. 1. After two haploid nuclei from cells of two different matting types fuse to form a diploid nucleus (similar like fertilization in higher org).2. The haploid products of meiosis, called ascospores, are maintained in linear order within an elongate tube like structure called ascus (plural asci). Thus each ascus contains all four products of a single meiotic event. Moreover all of the ascospore in each ascus can be analyzed genetically.

164. 1643. These haploid ascospores germinate and grow to produce multicellular mycelia (Singular mycelium) and all of these cells are also haploid. The genotype of each product can thus be determined without carrying out the test cross or other genetic manipulations. Because of the haploid state of the mycelium, the presence of recessive marker is never masked by dominant alleles. 4. Neurospora can be grown on a simple synthetic media

165. 165Neurospora crassa life cycle

166. 166Cytological basis of Crossing overNormally, the two chromosomes of any homologous pair are morphologically indistinguishable. Stern, Creighton and B. McClintock, however identified homologous that were morphologically distinguishable, that is they were not entirely homologous, Stern(1931)obtained a type of female Drosophila in which two X chromosomes are different from each other and also from other sets of chromosomes.He found that X chromosome in which a piece of Y chromosome is attached.The second X chromosome has been broken into two unequal segments and is shorten than the unbroken X chromosome

167. 167Both the chromosomes can be distinguished by microscopic examination. The broken C chromosomes contain a recessive gene (c) for carnation eye colour and the other chromosome contains dominant allele (C) red eyes. In addition, the broken X chromosome contains a dominant gene (B) for narrow bar eyes while, homologous contain recessive allele (b) for normal round eyes. Stern crossed this female having red bar eyes with a double recessive male having carnation round eyes.

168. 168He selected only females for his experiment. In the absence of crossing over, only two types of female gametes are produced:One type having broken X containing c and B genes;Other type having X with piece of Y attached containing C and b. But if crossing over occurred two More types of gametes are produced:One type having c and b in a normal size Xand the other type having C and B on a broken X and with a piece of YChromosome.

169. 169carBF1 femaleAbnormality atanother locus ofX chromosomeAbnormality atone locus ofX chromosomeNocrossingoverCrossing overduring meiosisin F1 female Fertilizationby spermfrom carnationF1 malecarnation,barParental combinations ofboth genetic traits andchromosome abnormalitiescarnationnormalbarRecombinant combinationsof both genetic traits andchromosome abnormalitiesFertilizationby spermfrom carnationF1 malecarBcarBcar+B+car+B+carB+carB+carB+carBcar+B+carB+carB+carB+car+BcarB+car+Bcar+B+carB+carBStern’s: crossing over in Drosophila

170. 170Factors affecting crossing over1. High temperature increases the frequency of crossing over2. X-ray increases the frequency.3. The frequency crossing over decreases with increasing age in female Drosophila4. Crossing over is less frequent near centromeres and the tips of the chromosomes.5. Nutritional and chemical effect:

171. 171Linkage MappingEach gene is found at a fixed position on a particular chromosome. Making a map of their locations allows us to identify and study them better. In modern times, we can use the locations to clone the genes so we can better understand what they do and why they cause genetic diseases when mutated.Thus, the percentage of gametes that had a crossover between two genes is a measure of how far apart those two genes are.

172. 172Alfred Sturtevant, a student of Morgan’s was assigned a project of consolidating the early information on D. melonogaster. He conceived an approach in which recombination data would be used to describe the physical relationship of genes on a chromosome in a linear arrangement called linkage map or genetic map.

173. 173For example, here 11.26% of the offspring from the cross in Table were recombinant between genes pp and ll, making the map distance between them 11.26 map unit.

174. 174As pointed out by T. H. Morgan and Alfred Sturtevant, who produced the first Drosophila gene map in 1913. Morgan was the founder of Drosophila genetics, and in his honor a recombination map unit is called a centiMorgan (cM). A map unit, or centiMorgan, is equal to crossing over between 2 genes in 1% of the gametes.

175. 175Morgan and his students discovered a number of genes in D.melonogaster and examined their progeny, they found substantial variation in the frequency of recombination for different gene pair. GenesRecombination frequencyYellow (Y) – White (W)Yellow (Y) – Vermilion (V)White (W) – Vermilion (V)Vermilion (V)- Miniature(M)White (W) –Miniature(M)White (W) – rudimentary (r)Vermilion (V)– rudimentary (r)214/21.736 = 0.0101,464/4551 = 0.322471/1,584 = 0.297 17/573 = 0.0302,062/6116 = 0.337406/898 = 0.452109/405 = 0.269

176. 176Sturtevant constructed a map beginning with Y at 0.0 on left side and using the frequencies of recombination between adjacent genes. Y W V M r * * * * * 0.0 1.0 30.7 33.7 57.6

177. 177Three Point Crossing over

178. 178Three – Point Crossing over Assuming that 3 genes involved are arranged in a linear fashion; there are 3 possible order. Each having a different gene in the center. With 3 linked genes, there are 3 different recombination frequencies between pair of loci: between v and ct; between ct and cv; between v and cv.

179. 179 calculate distances: 2 genes at a time; e.g.: v  ct = [ ( 89 +3) / 1448 ] x 100

180. 180Drosophila ...

181. 181 # gene isolation ... # mapping 1st step… # tomato…map (ca. 1952)chromosome numbering

182. 182Interference RF is defined as the proportion of exchange between two linked genes. As we seen more than one recombinational exchange may occur on a chromosome. A standard way of describing the difference between the observed and expected numbers of double crossing over was first used by H.J.Muller.Interference is I = Observed frequency of double recombinants 1 - Expected frequency of double recombinants

183. 183Positive value indicated interference between recombination recombination events.While, values near zero suggesting no interference, that is that different recombinant events are independent to each other. For closely linked genes, the measure of interference may often be near one, while for widely separated genes on the same chromosome, I is often near zero.

184. 184Try this

185. 185Mapping: Locating genes along a chromosome

186. 186SEX DETERMINATION What determines Maleness and femaleness? Nature encompasses a vast array of diverse mechanisms of sex determination in different species. Generally in any organism, individual normally exhibit one of the two sex phenotype: female or male. In such sp, females produce female gametes (egg, ovules or macrospores) and males produce male gametes (sperm, pollen or microspores)Dioecious : Sp with separation of the sexes in different individuals are called dioecious – all the higher animals and some higher plants are e.g.Monoecious: Sp in which both male and female gametes are produced by each individual are monoecious,

187. 187In lower animals, the production of both eggs and sperms by the same organisms is more commonly called hermaphroditism, and individual organism producing both types of gametes are termed hermaphrodites.Although the two sex phenotypes are usually quite distinguishable in human and fruit flies, this is not universally the case. In lower eukaryotes, the two genetically distinct types of gametes are sometimes morphologically indistinguishable; this is called isogamy. E.g., green alga Chlamydomonas

188. 188Sex ChromosomesAt the cellular level the sex of an individual is determined genetically by the sex chromosomes.XX = female, XY = maleAll other chromosomes are called “autosomes” Thus, humans have 46 chromosomes, (44+2)

189. 189Human males are the heterogametic sex with two different sex chromosomes, (XY).Human females are the homogametic sex (XX).In other species sex determination differs: male birds ZZ female birds ZW

190. 190What is so different between the X and Y chromosomes? X- over 1000 genes identifiedY- 330 genes identified, many are inactiveOne gene on the Y is very important: SRY. The SRY gene is the primary determinant of sex.If SRY is present, testes develop in the early embryo. The testes secrete the hormone testosterone, which causes development as a male.If SRY is absent (no Y chromosome), ovaries develop instead of testes, and the embryo develops into a female.

191. 191Genes on the Y chromosomeThere are three classes of genes on the Y.Genes shared with X chromosome define the pseudoautosomal regions (PAR)Genes similar to X chromosome genes are X-Y homologsGenes unique to the Y including SRY gene

192. 192What is SRY?SRY – Sex determining Region of YTDF – testis determining factorSRY – starts male development by - turning on testis-determining genes - turning off ovary-determining genes

193. 193Embryology of internal reproductive organs

194. 1941.Initiates the process that directs the indifferent gonads toward testis development 2. Activates Sertoli cells to produce Mullerian inhibiting hormone, causing Mullerian duct degeneration  

195. 195Stimulates Leydig cells to secrete testosterone, which then directs development of the Wolffian ducts towards epidiymides, vas deferens and seminal vesicles   - Testosterone conversion to dihydrotestosterone (DHT) - directs development of the urethra, prostate gland and Male sex organ.

196. 196 Y chromosome ( SRY region;TDF gene) is not present. - no TDF to tell it to form testis- gonadal tissue develops towards ovary formation - In the absence of testosterone – Wolffian duct system degenerates- In absence of MIH – Mullerian ducts continue to develop towards fallopian tubes, uterus, and upper vagina.

197. 197The different mechanisms of chromosomal sex determination may be grouped into five classes:1. XX female, XO male2. XO female , XX male3. XX female, XY male4. XX male, XY female5. Diploid (2n) female, haploid (n) male.

198. 1981. XX female, XO maleIn grasshoppers, protenor and many other insects belonging to Orthoptera, females have two X chromosomes, while males have only one X. Consequently, somatic cells of female have one chromosomes more than those of males. Here females are homogametic sex i.e., the sex producing only one type of gametes with respect to the sex chromosomes. In males, the single X chromosome remains unpaired and half of the sperms produced by males have one chromosome, while the other have none.

199. 1992. XO female; XX maleThis system of sex determination is known in some insects species only, e.g., Fumea. In such sp., females have only one X chromosome. As a result they produce two types of eggs, half having an X chromosome and other half having none. The males, on the other hand, have two X chromosomes; all the sperms, therefore have one X chromosomes. Here male – homogametic sexFemale – heterogametic sex.

200. 2003. XX- female; XY- maleThis system is most common among animals. It is found in humans, mice, most other mammals, etc. Male- XYFemale – XXY chromosome differs in morphology from the X chromosome, and generally smaller than X.

201. 2014. XY- female, XX- MaleThis system of sex determination operates in birds, reptiles, some insects (Silk warm). The scheme is essentially opposite to mammals.The bird sex chromosomes are called Z and W.In birds, a ZZ individual is male, and a ZW individual is female.

202. 202Diploid (2n) female, Haploid (n) maleThis system of sex determination is found mainly in Hymenoptera – honey bees, ants, termites, etc.In these sp, the somatic chromosome number of female is diploid, while that of males is only haploid. The unfertilized haploids develop male and called drones. These drones carry only half the number (16) of chromosomes of the female (32)

203. 203Environmental:Egg size: In the case of sea worm, Dinophilus, the size of eggs appears to determine the sex of animals developing them. Animal developed from eggs of relatively larger size are females, while those obtained from comparatively smaller sizes are males.

204. 204Association with females:The larvae of the sea worm Bonelia are sexually undifferentiated.These larvae that attach to the proboscis of female worms develops into males. In contrast, those larvae that do not attach to female remains free living female

205. 205Temperature: Some species have environmentally determined sex. Among reptiles, the temperature at which the eggs develop determines the sex. For example, in the turtles, eggs incubated at 30oC become female, while those incubated at lower temperatures become male.

206. 206Sex index or Genic balance theoryFormulated by Bridges.According to this theory, sex is determined by the relative number of X chromosomes and autosomes. Its actually the ratio between X chromosome and autosomes.Thus, Number of X chromosomeSex Index = Number of haploid sets of autosomesIf the sex index is 1 = female 0.5 = male intermediate = intersex Above 1 = meta female less than 0.5 = meta male

207. 207GynandromorphsIn Drosophila, some individuals show male characteristics in a part of their body, while their remaining parts show the female phenotype. Such a individuals are known as Gynandromorphs. Gynandromorphs are believed to arise from XX zygotes . During embryonic development, in one or more cells one of the two X chromosomes does not pass to any pole at anaphase and as a result, is lost.

208. 208Sex determination in PlantsThe mechanism of sex determination in plants are similar to animalsEnvironmental sex determination: In some plants, either sex determination is due to envt. Or it is greatly affected by envt. Equisetum plants grown under optimum conditions became female and while those grown under adverse conditions became male.In many other plants like, melons, cucumber, Cannabis etc, the sex of the flower is affected by many environmental factors such as temperature, day length and ethylene, GA3, etc. In general, a treatment with GA3 or ethylene promotes production of female flowers.(e.g, Cannabis)

209. 209Chromosomal sex determination in plantsMany dioecious plants do not show distinct sex chromosomes. But some of the sp, do show clear cut chromosome. (i) XX female, XY male: Asparagus, Spinach (ii) XY female, XX male: Fragaria elateriaIn Melandrium the Y chromosome determines maleness , while X chromosome specifies femaleness. A single Y chromosome is able to produce male flowers even in the presence of four X chromosomes (XXXXY), but such plants produce some bisexual flowers.

210. 210The Y chromosome of Melandrium is the largest chromosome of its genome. It has four functionally distinct segments. The first segment contains female suppressor and is located at one end of the Y chromosome. A deletion for this segment produce hermaphrodite flowers on XY chromosome.The second region, lying next to first, has the genes that promote maleness, i.e., initiate the development of the anthers. The third segment contains genes governing male fertility and anther maturation. A deletion of this segment produce male sterile plants with abortive anthers. The last segment located end of the Y chromosome, is homologous to a segment of X chromosome.

211. 211Sex Linked InheritanceChromosomes are the carriers of genes. Genes are located on both the autosomes and sex chromosomes. Genes onAutosomes – somatic charactersSex chromsomes- sex charactersHowever, certain genes present in the sex chromosomes control the somatic characters. The characters which are controlled by such genes are called sex linked character and transmission of such characters from one generation to next is called sex linked inheritance

212. 212Discovery and types of Sex linked inheritanceWas first discovered by Thomas H. Morgan in 1910 on Drosophila melanogaster.It can be classified into three types depending upon the chromosomes (X or Y) having sex linked genes. 1. X – Linked inheritance2. Y - Linked inheritance3. XY - Linked inheritance

213. 2131. X- Linked inheritanceCertain sex linked genes are located only on X chromosomes and their alleles are absent in Y chromosomes. These genes are called X – linked genes;Their mode of inheritance is called X-linked inheritanceThis pertains to the inheritance of those characters which are controlled by genes located in the non-homologous part of X-chromosomesExamples: Colour blindness, Hemophilia – human Eye colour in Drosophila

214. 214Y – Linked inheritanceThe sex linked genes are located on Y chromosomes only are called Y linked genes. The Y linked genes are confined only male (human).Hence they are also called Holandric genes (holo – whole; andros –male)These genes are transmitted directly from the father to the son Example: Hairy ear rims in Men

215. 215XY Linked inheritanceCertain sex linked genes are located on both X and Y chromosomesThey are called XY linked genes and their mode of inheritance is called XY linked inheritance.

216. 216General patterns of Sex linked InheritanceX – Linked Recessive:1. Usually more males than females are affected2. No offspring of an affected male are affected, making the trait skip generations in pedigree. But an exception to this pattern occurs in the rare instance when the affected male mate with a female carrier, producing a affected female offspring.X – Linked Dominant:1. Affected male produce all affected female offspring and no affected male offspring2. Approximately half the offspring of affected females are affected, regardless of their sex.

217. 217Y – Linked Inheritance:1. Traits are always passed from father to son2. Only males are affected. Kinds of Sex Linkage:1. Diagenic: when the characters pass from father to grandson through his daughter Father → Daughter → Grandson2. Diandric: When the characters pass from mother to grand-daughter through her son Mother → Son→ Granddaughter

218. 2183. Hologenic: when characters go directly from female to female, i.e., from mother to daughter and then to granddaughter. Mother → Daughter → Granddaughter4. Holandric: When the characters are pass from male to male, i.e, from father to son and then to his grandson. Father → Son → Grandson

219. 219Sex linkage in Drosophila:Normal wild type of fruit fly has red eye (RR). In the population of red eyed flies, white eyed (recessive) appeared as mutation (rr). When white eyed male was mated with red eyed female F1 flies had red eyesIn F2 on an average of 3 flies had red eyes and 1 had white eyes

220. 220This results reveal white eye is due to recessive gene. But if F2 individuals are classified on the basis of their sex as well as eye colour, a peculiar picture is emerged. All the F2 female had red eyes, but half of the males had red eye and the other half had white eyes. It appears that eye colour of a fly in F2 depended on its sex

221. 221But when white eyed female flies were mated with red eyed males, half the flies were red-eyed and the remaining half had white eyed. In F2 generation, both male and female flies showed the ratio of 1 red: 1 white. Morgan correctly reasoned that white eye gene was located on X chromosome

222. 222Colour blindnessWe have 3 color receptors in the retinas of our eyes. They respond best to red, green, and blue light.Each receptor is controlled by a gene. The blue receptor is on an autosome, while the red and green receptors are on the X chromosome (sex-linked).

223. 223Features of Colour blindnessColour blindness is a sex linked character discovered by WilsonIt is a hereditary disease and the affected person cannot distinguish green and red colour. The red blindness is called Protonopia, these persons cannot see red colourWhile, green blindness is called deuteronopia, such a persons cannot see green colour. Colour blindness is a recessive character, represented by ccThe genes are for colour blindness is located on X chromosome. It is common in male but rare female.Colour blindness follows criss- cross inheritance as transmitted from father to grandson through daughter. It is never transmitted from father to son

224. 224XCXC - Normal femaleXCXc - Carrier femaleXCY - Normal MaleXcY - Affected maleXcXc - Affected female

225. 225

226. 226Hemophilia Hemophilia is another sex-linked trait.Discovered by John Cotto.Hemophilia is characterized by delayed blood clotting. People with hemophilia can easily bleed to death from very minor wounds. Otherwise called as “Bleeders disease”Hemophilia is sex linked recessive character (hh) and the genes are located on X chromosomesIt is common in men and rare in women

227. 227This disease is appeared as a mutant in Queen Victoria and from her it was transmitted to her descendants.“Royal disease”XH XH - Normal femaleXH Xh - Carrier femaleXH Y - Normal MaleXh Y - Affected maleXh Xh - Affected female

228. 228Sex linked in CatSex linkage has been noticed in Tortoise shell cats. This involves the inheritance of coat colour. Tortoiseshell cats have patches of black and orange fur. Almost all tortoiseshells are female. Heterozygous for the X-linked coat color gene, one allele black and the other allele orange.When a black coloured female is matted with yellow coloured male, tortoise shell female is obtained.Even reciprocal also tortoise are female ♀ ♂XB XB X Xb Y (Black) (Yellow) XBXb (Tortoise shell)

229. 229Incomplete Sex LinkageThe genes located on homologous regions of sex chromosomes do not inherit together because crossing over occur may occur in these region.So these genes are incompletely sex linked genes and their mode of inheritance are incomplete sex inheritance.Xeroderma pigmentosum is an abnormal condition of skin in man, in which the skin becomes extremely sensitive to light.Even low light can produce pigmentation. In extreme cases, develops cancerous growth in body. Retinitis pigmentosa is another case of incomplete sex linkage caused by the interaction of a number of dominant and recessive genes. In this case, the affected person develops night blindness leading to the formation of pigmentation in retinal wall.

230. 230Sex Limited TraitsSome genes fail to produce their phenotypic effects in the presence of certain hormones and are described as sex limited. In human being several characters are restricted with particular sex. For e.g, facial hair in male only (beard )

231. 231Sex-Influenced TraitsIn contrast to Sex limited gene where one expression of a trait is limited to only one sex only; sex influenced genes are those whose dominance is influenced by the sex of the bearer.Such characters are called Sex influenced traits. Good examples: male pattern baldness in humans and horns in sheep.Pattern baldness is found in both sexes, but is rarer in females, but it is dominant in males and recessive in females. Thus, male heterozygotes are bald but female heterozygotes have normal hair.

232. 232The Adams family and Baldness

233. 233Dominant sex influenced traitAutosomal genes responsible for horns in some breeds of sheep may behave differently in the presence of male and female sex hormones. In suffolk sheep neither sex is horned, and the genotype is “hh”. Among the heterozygous F1 progeny crosses between these breeds, horned males and hornless female are formed.Because both the sex are genotypically alike (h+h), the genes behave as dominance in male and recessive in females, i.e., only one allele is required for male but the allele must be homozygous expression in female. GenotypeMalesFemalesh+h+h+hhhHornedHornedHornlessHornedHornlessHornless

234. 234Deleterious recessive Sex Linked Genes in Human1. Lesch –Nyhan Syndrome: Characterized by excess of uric acid, is inherited through sex linked recessive gene. This means that mother contributes the X chromosome with defective gene to a male zygote.Half of the male children of carrier mothers may be expected to inherit this disease. These are deficient for the enzyme hypoxanthine-guanine phosphoribiosyltransferase.(HPRT)Infant who receive this disease appears normal at birth and for several months.By about 10 months of age, they become abnormal and mental retardation, and patient may literally bite of his own fingers and lips.

235. 2352. Duchene type Muscular Dystrophy: Is also depends on sex linked recessive gene. If mother is carrier, about half of the male children are expected to be affected. Can be identified by chromosome study.It affects male before they reach teens, with muscular deterioration .Muscles of leg and shoulders become stiff and the children usually become paralyzed and crippled during their middle or late teens. Virtually all die before age of 21.

236. 2363. Hunter syndrome: It is characterized by mental retardation, coarse features, hirsutism (abnormal hairiness), and a characteristic broad nose and a large protruding tongue. Symptoms appear in childhood, and a chemical procedure for diagnosis has been developed.Mucopolysaccharides (Containing an amino sugars as well as uronic acid) accumulate in skin cells of these patients.Stained with O-toluidine blue, will results in pink in colour.

237. 237EXTRA CHROMOSOMAL INHERITANCEGenes controlling the phenotypes of eukaryotes are contained in nucleus. This was certainly the case for the genes studied by Mendel, and for most, if not all.“However, the nucleus is not the whole story”Several important phenotypes of eukaryotes are controlled by genes in the cytoplasm. The genes that are present on the cytoplasm are called Plasmagenes. Plasmagenes also transmit character from one generation to next This is known as cytoplasmic or extrachromosal inhreitance

238. 238The first good example of cytoplasmic inheritance emerged from Carl Corren’s studies of the four –O’ Clock plant and from Erwin Baur’s work on Pelargonium in 1909. Both Mirabilis and Pelargonium frequently have variegated leaf colour, showing patches of green and white.However, certain strains of both the sp can be either all green or all white (though the white strains do not survive very long).

239. 239Non-Mendelian inheritanceIn Mendel expt., it made no difference which parent carried which genotype. For eg., yellow seed (male) X green (female)Will give the same result as reverse.But with leaf colour with Mirabilis it made a great difference: When Correns crossed any male (white or green or variegated) with green female shows all progeny were green.Similar with white female.This pattern of inheritance is called Maternal inheritance

240. 240This behavior violates Mendel’s law in 2 ways:1. Usual Mendelian ratio of phenotypes were not observed2. There was a difference between reciprocal crossHow do we explain this phenomenon?

241. 241The situation become much clearer when we realize the colour in the plant is due to chlorophyll in the chloroplast. Genes are governing in the chloroplast and not in the nuclear gene.But you can ask why such maternal parent dependence?This is because the maternal parent contributes most of the cytoplasm

242. 242Kappa particles in Paramecium Sonnerborn found that there are two strains of paramecium They are killer and sensitiveKiller strain produces a toxic substances “Paramecin” that kills the other type. The production of paramecin in killer is type is controlled by certain cytoplasmic particles is known as “Kappa” particle (KK). The sensitive strains lack these kappa (kk). Kappa particles can pass from one generation to next. They transmitted through the cytoplasm.

243. 243When killer KK conjugate with non-killer kk, the ex-conjugants are Kk.But the development of a particular type depends on the duration of cytoplasmic exchange.In a normal case of conjugation the nuclear material alone is exchanged and as a result each conjugate gives rise to the organism of its own type.i.e., killer exconjugant produce killer while, non-killer will produce non-killer type.

244. 244But sometime the conjugation period is prolonged In such case apart from nuclear material, cytoplasmic material are also exchanged. During this cytoplasmic exchange the kappa particles present in the cytoplasm of the killer type will enter into the non killer type and convert it into a killer type.

245. 245Shell Coiling in LymneaThe snail Limnaea shows two types of coiling namely dextral (towards right side) and sinistral (towards left side).The dextral coiling is due to dominant alleles “DD” while the sinistral coiling due to recessive allele “dd”The experiments of Boycott indicate that the coiling is determined by the gene of the mother and not by the individuals own gene.If a dextral female crossed with sinistral male, all F1 offspring develop dextral shell like mother

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