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Gene Linkage
What structures actually assort independently?
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Gene Linkage
Gene Linkage
Thomas Hunt Morgan’s research on fruit flies led him to the principle of linkage.
Morgan discovered that many of the more than 50 Drosophila genes he had identified appeared to be “linked” together.
They seemed to violate Mendel’s principle of independent assortment.
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Gene Linkage
Morgan and his associates grouped the linked genes into four linkage groups.
Each linkage group assorted independently but all the genes in one group were inherited together.
Each chromosome is actually a group of linked genes.
According to these findings, how many chromosomes would you expect fruit flies to have?
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Gene Linkage
Morgan concluded that Mendel’s principle of independent assortment still holds true.
Chromosomes assort independently, not individual genes.
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Gene Maps
Gene Maps
Crossing-over during meiosis (during what phase????) sometimes separates genes that had been on the same chromosomes onto homologous chromosomes.
Crossover events occasionally separate and exchange linked genes and produce new combinations of alleles.
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Gene Maps
Alfred Sturtevant, a student of Morgan, reasoned that the farther apart two genes were, the more likely they were to be separated by a crossover in meiosis.
Recombination frequencies can be used to determine the distance between genes.
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Gene Maps
Sturtevant created a gene map showing the relative locations of each known gene on one of the Drosophila chromosomes.
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Gene Maps
Exact location on chromosome
Chromosome 2
0.0
Aristaless (no bristles on antenna)
13.0
Dumpy wing
48.5
Black body
54.5
Purple eye
67.0
Vestigial (small) wing
This gene map shows the location of a variety of genes on chromosome 2 of the fruit fly. The genes are named after the problems abnormal alleles cause, not the normal structure.
99.2
Arc (bent wings)
107.0
Speck wing
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Gene Maps
If two genes are close together, the recombination frequency between them should be low, since crossovers are rare.
If they are far apart, recombination rates between them should be high.
Alleles “A” and “E” are less likely to crossover together b/c they are farther apart from each other .
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10. Why Is This Important to Us?
The rate at which linked genes are separated and recombined can be used to produce a “map” of distances between the linked genes.
This information can be used to determine the relative position of genes on chromosomes.
Sturtevant’s methods have been used to construct genetic maps of many species, INCLUDING the Human Genome!!!
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Gene Maps
Exact location on chromosome
Chromosome 2
1.3
Star eye
31.0
Dachs (short legs)
51.0
Reduced bristles
55.0
Light eye
75.5
Curved wing
104.5
Brown eye
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11-5
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11-5
According to Mendel's principle of independent assortment, the factors that assort independently are the
genes.
chromosomes.
chromatids.
gametes.
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11-5
A chromosome is actually a group of
independent genes.
linkage groups.
crossed-over genes.
linked genes.
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11-5
Thomas H. Morgan is credited with the discovery of the principle of
segregation.
independent assortment.
gene linkage.
dominance.
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11-5
Linkage maps can be produced because the farther apart two genes are on a chromosome,
the less likely they are to assort independently.
the more likely they are to be linked.
the more likely they are to be separated by a crossover.
the less likely they are to be separated by a crossover.
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11-5
If two genes are close together on the same chromosome, they are more likely to
behave as though they are linked.
behave independently.
move to different chromosomes.
belong to different linkage groups.
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