Syntenic genes can assort independently. Explain this observation.

Define linkage disequilibrium. What is the physical basis of linkage, and what causes linkage equilibrium? Explain how crossing over eliminates linkage disequilibrium.

Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.

Previous genetic linkage crosses have determined that recombination frequencies are 6% for genes A and B, 4% for genes B and C, 10% for genes C and D, and 11% for genes D and E. The sum of these frequencies between genes A and E is 31%. Why does the recombination distance between these genes as determined by adding the intervals between adjacent linked genes differ from the distance determined by the test cross?

Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.

Assuming that organisms with any desired genotype are available, propose a genetic cross whose result could be used to determine which of the proposed genetic maps is correct.

On the Drosophila X chromosome, the dominant allele y⁺ produces gray body color and the recessive allele y produces yellow body. This gene is linked to one controlling full eye shape by a dominant allele lz⁺ and lozenge eye shape with a recessive allele lz. These genes recombine with a frequency of approximately 28%. The Lz gene is linked to gene F controlling bristle form, where the dominant phenotype is long bristles and the recessive one is forked bristles. The Lz and F genes recombine with a frequency of approximately 32%.

Using any genotypes you choose, design two separate crosses, one to test recombination between genes Y and Lz and the second between genes Lz and F. Assume 1000 progeny are produced by each cross, and give the number of progeny in each outcome category. (In setting up your crosses, remember that Drosophila males do not undergo recombination.)

On the Drosophila X chromosome, the dominant allele y⁺ produces gray body color and the recessive allele y produces yellow body. This gene is linked to one controlling full eye shape by a dominant allele lz⁺ and lozenge eye shape with a recessive allele lz. These genes recombine with a frequency of approximately 28%. The Lz gene is linked to gene F controlling bristle form, where the dominant phenotype is long bristles and the recessive one is forked bristles. The Lz and F genes recombine with a frequency of approximately 32%.

Can any cross reveal genetic linkage between gene Y and gene F? Why or why not?

Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.

The test cross Ae/aE x ae/ae  indicates the genes recombine with a frequency of 28%. If 1000 progeny are produced by this test cross, determine the number of progeny in each outcome class.