Crossing over

1. For this question, let's return to the deep ocean viper fish with linked genes. Recall the assumptions for these fish: some can produce biochemical lights along the length of their bodies and some cannot. Also, some have very large eyes while some have almost no eye at all. Further, these two traits are each controlled by a single locus with the production of lights (L) and large eyes (E) being the dominant alleles for each. Lastly, assume that these genes are linked. The chromosomes of the male and female viper fish of our problem are pictured below.


Male Female



Now, suppose that crossing over occurs between the homologs of the female. What gametes will she produce?





Did the female produce a greater variety of gametes before or after crossing over occurred?




What are the possible genotypes of their offspring (choose all that are correct)?




What are the possible phenotypes of the F1 generation (again, choose all that are correct)?




2. In the domestic cat the single locus genes for deafness and white color are linked. Assume that normal hearing (D) is dominant to deafness and that white (W) is dominant to any other color. A female heterozygous for both genes (with one dominant and one recessive allele for the two traits on each of the homologous chromosomes) mates with a male homozygous white and heterozygous for deafness. If crossing over occurs in the female, what percentage of each genotype will be found in the offspring?

WWDD:
WWDd:
WWdd:
WwDD:
WwDd:
Wwdd:
wwDD:
wwDd:
wwdd:




What percentage of the offspring will have each of the following phenotypes?

White / Normal hearing:
White / deaf:
Other color / Normal hearing:
Other color / deaf:





3. Gryphons are mythological animals with the head and shoulders of an eagle and the hindquarters of a lion. For the sake of this problem, they come in two colors: black (B), which is a dominant allele, and golden (b), which is the recessive allele. Their wings can have normal feathers (N), which are dominant, and iridescent feathers (n), which are recessive. These two genes are linked in gryphons. Two gryphons heterozygous for both traits mate. The female has one each of the dominant and recessive alleles on the homologs. The male has both dominant alleles from the two traits on one homolog and both recessive alleles on the other homolog. Assuming crossing over occurs in both gryphons, what is the genotypic ratio of the F1 generation (BBNN: BBNn: BBnn: BbNN: BbNn: Bbnn: bbNN: bbNn: bbnn)?




What is the phenotypic ratio of the F1 generation (Black with normal feathers : Black with iridescent feathers : Golden with normal feathers : golden with iridescent feathers)?




Can you tell from the genotypic and phenotypic ratios that these two traits are linked?



4. Let's return to the mice we discussed under the "linked genes" section. Recall that white fur (W) is dominant to white fur with black spots and that black eyes (B) are dominant to pink. Before, we crossed a female that was homozygous dominant for fur color and heterozygous for eye color with a male that was heterozygous for both traits. In the male, each homolog has one dominant allele and one recessive allele from the two traits. Assume that crossing over occurs in the male. Without crossing over, this cross produced offspring that were 75% white/black eyes and 25% white/pink eyes. Will the percentages of the phenotypes in the F1 generation be different after crossing over occured in the male?




What percentage of the F1 generation have pink eyes?




When no crossing over was involved, the F1 generation had 25% each of the WWBb, WWbb, WwBB, and WwBb genotypes. With crossing over, will the variety of genotypes in the F1 be the same?




If the number of genotypes are not the same, are there more or fewer after crossing over occured?





5. Assume that feather color in the great horned owl is linked with the length of their feathered ear tufts. Also assume that dark feathers (D) are dominant to pale feathers and long ear tufts (L) are dominant to short. Two owls that are heterozygous for both traits are mated. The male's parents had genotypes of DDll and ddLL while the female's parents had genotypes of DDLL and ddll. Which alleles are linked in the male and female?




Male:
Female:

Assume that crossing over occurs in the female. How many of each of the genotypes will be produced in the F1 generation (assume 16 offspring)?

DDLL:
DDLl:
DDll:
DdLL:
DdLl:
Ddll:
ddLL:
ddLl:
ddll:




How many of each of the phenotypes will be produced in the F1 generation (this time assume 8 offspring)?

Dark feathers / Long ear tufts:
Dark feathers / Short ear tufts:
Pale feathers / Long ear tufts:
Pale feathers / Short ear tufts:



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