The Chromosome Theory of Heredity Mutations Regulation of Gene Expression Objectives State the chromosome theory of heredity Explain how gene linkage affects inherited traits Describe the process of crossingover and explain how it increases genetic variety ID: 571342
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Slide1
Genes and Chromosomes
The Chromosome Theory of Heredity
MutationsRegulation of Gene ExpressionSlide2
Objectives
State the chromosome theory of heredity
Explain how gene linkage affects inherited traitsDescribe the process of crossing-over and explain how it increases genetic varietyDescribe gene mappingDescribe the process of sex determination and the patterns of inheritance for sex-linked traitsSlide3
Chromosome Theory of Heredity
Mendel’s work was incomplete because he never asked an important question: Where in the cell are the factors that control heredity? Where are the genes?Slide4
What do you see in this picture?
What do you recall about chromosomes?Slide5
Chromosome Theory and Heredity
Key terms
Chromosome theory of heredityLinked geneLinkage groupRecombinantSex chromosomeAutosomeX chromosome
Y chromosome
Sex-linkedSlide6
Genes and Chromosomes
Nucleus
contains chromsomeschromosome=threadlike structure in a cell that contains the genetic information that is passed on from one generation of cells to the nextSlide7
Genes and Chromosomes
Walter Sutton
1902, discovered gene locationChromosome theory of heredity genes are located on the chromosomes and each gene occupies a specific place on a chromosomeEach gene may exist in several forms or allelesEach chromosome contains just one of the alleles for each of its genes
Sutton’s development of the chromosome theory is an example of how the work of one scientist builds on the work of another scientistSlide8
Gene Linkage
Genes on a chromosome are linked together
They are inherited togetherLinked genes do not undergo independent assortment Linked genes=genes that are inherited in a groupSlide9
Gene Linkage
Thomas Hunt Morgan studied
drosphiliaEffects of gene linkageMorgan crossed purebred gray bodies and normal wings with purebred black bodies and small wingsGray (G) black (g) Normal wings (W) small wings (w)F1 should have been gray with normal wings (GgWw
)
When F1 crossed with black small-winged
drosphilia
(
ggww
) Morgan did not observe the expected results
Most gray-bodied
drosphilia
had normal wings and most black-bodied flies had small wings
Gene for body color and gene for wing size were somehow connected, or linked
They could not assort independentlySlide10
Gene Linkage
Linkage groups
Morgan studied more and more genesDiscovered genes fell into distinct linkage groups of genes that always tended to be inherited togetherThe linkage groups (chromosomes) assorted independently, but all genes on one group were inherited togetherBecause homologous chromosomes contain the same genes, there is one linkage group for every homologous pair of chromosomes(drosphilia
has four linkage groups, four pairs of
chromsomes
)
A cobra has 38 chromosomes. How many linkage groups would this make? Slide11
Tomorrow!
Crossing over in linked genes
Gene mappingSex linkageSlide12
Crossing-over
During prophase I of meiosis, homologous chromosomes may exchange sections of their
chromatids in a process called crossing-overIncreases genetic varietySlide13Slide14
Crossing-Over
Linkage groups explains some of the results of the
drosphilia crosses but does not provide a complete explanation83% have gene combinations like their parents17% have new gene combinationsRecombinants=individuals with new combinations of genesSlide15
Crossing-Over
If the genes for body color and wing size are linked, why aren’t they linked all the time?
Morgan proposed that linkages could be broken some of the timeIf two homologous chromosomes were positioned side by side, sections of the two chromosomes might cross, break, and reattach.This process would rearrange the genes on the chromosome and produce new linkage groupsSlide16
Gene Mapping
Further reasoned that crossing-over occurs at random along the linkage groups, and the distance between two genes determines how often crossing-over occurs between them
Close together crossing-over is rareFar apart crossing-over more common Slide17Slide18
Gene Mapping
Knowing the frequency with which crossing-over between two genes occurs makes it possible to map the positions of genes on a chromosome
Today we have detailed maps of Drosphilia that pinpoint the locations of more than 1500 different genesSlide19
Sex Linkage
1905 American biologist Nettie Stevens discovered that not every chromosome has a corresponding homologous chromosome
Discovered female mealworm contain 20 large chromsomes and male contain19 large and one small One of male chromosome pairs is not homologousThe pair has very different shapesSame thing was found in drosphiliaSlide20
Sex Linkage
These “mismatched”
chromosmes are the sex chromosomesfemale sex chromosomes=two matching sex chromosomes (XX)Male sex chromosomes=two dissimilar sex chromosomes(XY)Y chromosome=small and hook shapedThe other chromosomes, which are the same in both males and females, are called autosomesSlide21
Sex Linkage
Sex determination
The sex chromosomes in the male’s gametes determine the sex of the offspring
X
Y
X
XX
XY
X
XX
XYSlide22
Sex Linkage
Sex determination
When female gametes are produced, meiosis separates one of the X chromosomes into each egg cellIn the male, meiosis separates the X and Y chromosomes so that 50% carry X chromosome and 50% carry Y chromosomeWhen an X sperm fertilizes an egg a female is formedWhen a Y sperm fertilizes an egg a male is formedSlide23
Sex Linkage
Genes on sex chromosomes
Sex chromosomes also carry genes that affect other traitsSex-Linked a gene located on one of the sex chromosomesSlide24
Non-Disjunction
Occurs during meiosis
Some gametes contain extra chromosomesSome gametes are missing chromosomesSlide25
Klinefelter’s Syndrome
Male
XXY sex chromosomesSterileShow female characteristicsUnderdeveloped testesBreast development
Poor beard growthSlide26
Turner’s Syndrome
Female
X_Mental retardationSterileShort in stature
Underdeveloped ovaries
Increased chance of thyroid problemsSlide27
Tomorrow!
Gene Mapping Lab!Slide28
Board work 21
How are genes related to chromosomes?
How does crossing-over make genetic mapping possible?What are sex chromosomes? Autosomes?Why are the effects of recessive sex-linked alleles seen more often in males than in females?Slide29
MutationsSlide30
Mutations
Mistakes in duplicating genetic info and transmitting it to the next generation are rare, but they do occur
Mutations=change in the genetic material of the cellNot all are harmfulNo effectSlight effectHarmlessbeneficialSlide31
Mutations
Mutations in reproductive cells (germ cells)
Germ mutationsinheritableMutations that affect other cells of the bodySomatic mutationsCancer2 levelsChromosomal mutationsGene mutationsSlide32
Chromosomal mutations
Involve
Segments of chromosomesWhole chromosomesEntire sets of chromosomesResults in change in number or structure4 types
deletions
Duplications
Inversions
translocationsSlide33
Chromosomal mutations
Deletions
The loss of part of a chromosomeSlide34
Chromosomal mutations
Duplications
Opposite of deletion, segment of chromosome is repeatedSlide35
Chromosomal mutations
Inversions
Part of a chromosome becomes reversedSlide36
Chromosomal mutations
Translocations
Part of one chromosome breaks off and attaches to another, nonhomologous chromosomeSlide37
Chromosomal mutations
Nondisjunctions
Involve whole chromosomes or complete sets of chromosomesFailure of homologous chromosomes to separate normally during meiosisNot coming apart1 chromosome involveextra
copy in one cell and loss from another
More than 1dramatic increase in number, producing triploid (3N) or
tetraploid
(4N) organisms
Extra sets of
chromosomespolyploidy
Almost always fatal in animals
Plants are often larger and hardierSlide38
Gene mutations
Involve
Individual genesCauseChemical change that affects DNASlide39
Gene mutations
Point
mutationsaffect no more than a single nucleotideSlide40
Gene mutations
Insertion or deletion of nucleotide
Frameshift mutationscompletely change the polypeptide product produced by a geneSlide41Slide42
Board Work 22
Compare a chromosomal mutation and a gene mutation.
What is a somatic mutation? How does it differ from a germ mutation?How does nondisjunction result in chromosomal mutations?Slide43
Regulation of Gene Expression
Gene interactions
Incomplete dominanceCodominancePolygenic inheritanceGene expression in prokaryotesOperonRepressor
Gene activation
Gene expression in eukaryotesSlide44
Regulation of Gene Expression
As biologists have intensified their studies of gene activity, it has become clear that interactions between different genes and between genes and their environment are critically important Slide45
Gene Interactions
Dominance
How genes interact with each otherRemember….A gene is a section of DNAcodes for a polypeptideDominant allele codes
codes
for a specific polypeptide that works, recessive for one that does not workSlide46
Gene Interactions
Incomplete dominance
Inheritance in which an active allele does not entirely compensate for an inactive alleleSlide47
r
r
RRr
Rr
R
Rr
Rr
R
r
R
RR
Rr
r
Rr
rr
R=red
r
=
white
F1 generation
All pink
Red carnation
White carnation
Pink carnation
Pink carnation
F2 generation
1 red
2 pink
1 whiteSlide48
Gene Interactions
Codominance
Condition in which both alleles of a gene are expressedWritten as capital letters with subscripts or superscriptsEx: B1 and B2 or R and R’
Seen in many organisms
cattle=red hair is
codominant
with white hair (H
R
H
W
)
Look roan or pinkish white
Chickens=black feather are
codominant
with white feathers (F
B
F
W
)
Erminant
chickens (speckled black and white)Slide49Slide50
Gene Interactions
Polygenic Inheritance
A trait that is controlled by two or more genesMany traits are produced by the interaction of many genes…polygenicShape of your noseColor and markings on an animal’s coatSlide51
Gene Expression in Prokaryotes
The genes of a single organism cannot be activated ate the same time
Make many molecules it did not needWaste energyMust be able to produce the product of a gene quickly and in adequate amountsSlide52
Gene Expression in Prokaryotes
When the product of a gene (a specific protein) is being actively produced by a cell, we say that the gene is being expressed
Within a single organism, some gene are rarely expressed, some are constantly expressed, and some are expressed for a time and then turned offHow does a cell “know” when to make a protein and when not toTurn off and turn on?Slide53
Gene Expression in Prokaryotes
The
OperonGenes and regions of DNA that operate together; consists of a gene cluster and regions involved in the regulation and expression of that clusterConsists ofOperator=region of chromosome near the cluster of genes in an operon to which the repressor binds when the
operon
is “turned off”
Promoter=region of chromosome next to the operator in an
operon
to which RNA polymerase binds at the beginning of transcriptionSlide54
Gene Expression in Prokaryotes
The
OperonThe gene cluster in the operon studied by Jacob and Monod produces enzymes that break down lactoseBacteria does not produce enzymes in large amounts unless lactose is presentLactose induces production of enzymes to break down lactose for use as foodThis
operon
system
inducer
because it induces the production of enzymes
Enzymes not produced in the absence of lactoseSlide55
Gene Expression in Prokaryotes
The Repressor
When repressor nears the operator it attaches itself to the operator so that it sits between the promoter and the genesPosition blocks the access of RNA polymeraseSlide56
Gene Expression in Eukaryotes
Inducers induce the activation of genes
Bind directly to DNA and either start or increase transcription of particular genesmRNA produced during transcription may be altered before it is used to make protein during translationThe presence of DNA sequences that do not code for proteinExons=sequences that are complementary code for protein “expressed”
Introns
=segments that are not complementary and do not code for protein “intervening”Slide57
Board work 23
How do gene interactions affect gene expression?
Compare incomplete dominance and codominance.What is a polygenic trait?“Mutations in
introns
are less likely to affect
phenotype
than
mutations in
exons
.? Defend or refute this statement.