LINKAGE If the genes are situated in the same chromosome and are fairly close to each - PowerPoint Presentation

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LINKAGE

If the genes are situated in the same chromosome and are fairly close to each other, they tend to be inherited together. This type of coexistence of two or more genes in the same chromosome is known as linkage.

SUTTON’S VIEW ON LINKAGE:

Sutton predicted that the chromosomes are the bearers of the units of heredity or genes and since the number of hereditary units is much larger than the number of chromosomes, therefore each chromosome must contain a number of genes.

He further stated that since the chromosomes move as units during meiosis to the gametes, all the genes which are situated in the same chromosome will be linked together. As a result each species of animals and plants would have a specific number of chromosomes found in that species.

2.

COUPLING AND REPULSION HYPOTHESIS:

W. Bateson and Punnet in 1906 formulated the hypothesis of coupling and repulsion the unexpected F2 results of a dihybrid cross between a homozygous sweet pea (Lathyrus odoratus) having dominant alleles for blue flowers (BB) and long pollen grains (LL) with another homozygous double recessive plant with red flowers and round pollen grains (bbll).

The F1 individuals (BbLl) had blue flowers and long pollen grains. When they test crossed a F1 dihybrid, (blue long- BbLl) with it double recessive parent (red round, bbll), then besides getting the expected 1:1:1:1 test cross ratio, they received phenotypic ratio of 7:1:1:7.

Parent

Blue Long

x Red Round

BBLL

bbll

P1 gametes BL

↓

bl

F1

Blue Long

BbLl

Test

cross

F1 Blue Long X P1 Red Round

BbLl

X

bbll

Test cross progeny

Blue Long Blue Round Red Long Red Round

7 : 1 : 1 : 7

Test cross progeny: The 7:1:1:7 test cross ratio of this cross clearly indicated that there was a tendency in the dominant alleles to remain together. Similar was in case with recessive alleles. This deviation was therefore explained as “gametic coupling” by Bateson and Punnett.

Similarly it was observed that when two such dominant or recessive alleles come from different parents, they tend to remain separate. This was called repulsion.

To understand the phenomenon of repulsion we can consider another test cross performed by Bateson and Punnet.

In another test cross they took a sweet pea plant with blue flowers and round pollens (BBll) and another plant with red flower and long pollens (bbLL) and obtained the ratio of 1:7:7:1 in F2.

P1

Blue

Round

x Red

Long

BBll

bbLL

P1 gametes

Bl

↓

bL

F1 Blue Long

BbLl

Test cross F1

Blue

Long x

P1 Red Round

BbLl

bbll

Test

cross progeny

Blue Long Blue Round Red Long Red

Round

1

: 7

: 7

:

1

3. MORGAN’S VIEW ON LINKAGE:

Morgan stated that the pairs of genes of homologous parent tend to entre in same gametes and to remain together. Whereas same genes from heterozygous parent tend to entre in different gametes and remain apart from each-other.

He further stated that the tendency of linked genes remaining together in original combination is due to their location in same chromosome.

According to him the degree or strength of linkage depends upon the distance between the linked genes in the chromosome.

Morgan’s concept about the linkage developed the theory of linear arrangement of genes in the chromosomes which helped the cytogeneticists in the construction of genetic or linkage maps of chromosomes.

4. CHROMOSOME THEORY OF LINKAGE:

Morgon alongwith Castle formulated the chromosome theory of linkage which is as follows-

1.The genes which show the phenomenon of linkage are situated in the same chromosomes and these linked genes usually remain bounded by the chromosomal material so that they can not be separated during the process of inheritance.

2. The distance between the linked genes determines the strength of linkage. The closely located genes show strong linkage than the widely located genes which show the weak linkage.

3. The genes are arranged in linear fashion in the chromosomes.

KINDS OF LINKAGE:

T. H. Morgan and his co-workers by their investigations on Drosophila and other organisms have found two types of linkage- complete linkage and incomplete linkage.

1. COMPLETE LINKAGE:

It is the phenomenon in which parental combinations of characters appear together for two or more generations in a continuous and regular fashion. In this type of linkage genes are closely associated and tend to transmit together.

Example:

The genes for bent wings (bt) and shaven bristles (svn) of the fourth chromosome mutant of Drosophila melanogaster exhibit complete linkage.

2. INCOMPLETE LINKAGE:

The linked genes do not always stay together because homologous non-sister chromatids may exchange segments of varying length with one another during meiotic prophase.This sort of exchange of chromosomal segments in between homologous chromosomes is known as crossing over.

This linked genes which are widely located in chromosomes and have chances of separation by crossing over are called incomplete linked genes and phenomenon of their inheritance is called incomplete linkage.

Example:

The incomplete linkage have been reported in female Drosophila and various other organisms such as tomato, maize, pea, mice, poultry and man etc.

INCOMPLETE LINKAGE IN DROSOPHILA:

The wild type Drosophila has gray body and long wings (b+v+/b+v+), where alleles for gray b+ and long v+ dominate over the mutant alleles for black b and vestigial v. When a gray long fly (b+v+/b+v+) is crossed with a black-bodied and vestigial-winged fly (bv/bv), the F1 heterozygote (b+v+/bv) when test crossed with double recessive parent (bv/bv), instead of occurring of two class of phenotypes in the ratio of 1:1, occur four classes of phenotypes as shown in the figure

Parent

Gray Long

x

Black Vestigial

b+v+/b+v+

↓

bv/bv

F1 Gray Long

b+v+/bv

Test cross F1 Gray Long x Black Vestigial

b+v+/bv bv/bv

Test cross ratio

b+v+/bv: b+v/bv : bv+/bv : bv/bv

Gray Gray Black Black

Long

Vestigial Long Vestigial

41.5% 8.5% 8.5% 41.5%

The test cross results are clearly showing that parental combinations (gray-long and black-vestigial) are those expected from complete linkage and appeared in 83% cases. The other two (gray-vestigial and black-long) are new combinations and appeared in 17% cases. Thus, in 17% cases crossing over has occurred.

INCOMPLETE LINKAGE IN MAIZE:

In Zea mays (maize) a case of incomplete linkage between the alleles for colour and shape of the seed has been observed by Hutchison. When a maize plant with seeds having colour and full endosperm (CS/CS) is crossed with another plant having recessive alleles for colourless, shrunken seeds (cs/cs), F1 heterozygotes are found with the phenotype of coloured full and genotype of CS/cs. When F1 hybrid is test crossed with double recessive parent (cs/cs) four classes are obtained instead of two as shown in the figure

Parent

Coloured Full x Colourless Shrunken

CS/CS

↓

cs/

cs

F1

Coloured Full

CS/cs

Test cross F1

Coloured Full

x

Colourless Shrunken

CS/cs cs/csTest cross ratio CS/CS Cs/cs cS/cs cs/cs Full Shrunken Full Shrunken 48% 2% 2% 48%

7-57

The test cross results are clearly showing that parental combination of alleles (CS/CS and cs/cs) are those expected from complete linkage and appear in 96% cases, the other two are new combinations (Cs/cs and cs/cs) and appear in 4% cases. Thus, in 4% cases crossing over has occurred between linked genes.

SIGNIFICANCE OF LINKAGE:

The phenomenon of linkage has one of the great significance for the living organisms that it reduces the possibility of variability in gametes unless crossing over occurs.

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