Section 5.3: Problem Set 4: Linkage

1. Suppose that, in the pea plant, the flower-color gene is linked to the seed shape gene. The red (R) and white (r) flower-color alleles are codominant, with the heterozygote Rr having pink flowers.
The wrinkled seed allele (W) is dominant to the round seed allele (w).
A pea plant with white flowers and round seeds is bred to a plant of known genotype Rw/rW. This notation means we not only know the alleles at each locus, but also know that R and w are on one chromosome, and r and W on the other. The recombination frequency between these genes is 0.3.
What is the probability that an offspring they produce is pink-wrinkled; pink-round; white-wrinkled; white-round?

2. A pea plant with genotype RG/rg is bred with a plant of genotype rg/rg. With respect to flower-color and seed-color, they have four pink-green, four pink-yellow, four white-green, and four white-yellow offspring. Is there any evidence that the genes are linked?

3. In humans, the X-linked genes for color-blindness and hemophilia are linked, since they are both on the X-chromosome. Suppose that the recombination frequency is 0.26.
A man who is normal for both genes marries a woman who is a carrier for both color-blindness and hemophilia. Her genotype is Xb+Xh+/XbXh.
(a) what is the probability that their daughter is a carrier for color-blindness; a carrier for hemophilia; a carriers for both; not a carrier for either?
(b) what is the probability that their son is is color-blind; has hemophilia; has both traits; has neither?

4. Suppose that in the pea plant, the flower color, seed color, and seed shape genes are all linked. In addition to the notation in question 1, the green seed color allele (G) is dominant to the yellow seed color allele (g).
A plant that is RGW/rgw is bred with a plant that is rgw/rgw.
They have 100 offspring. For flower-color, seed-color, seed-shape, respectively, of these offspring

40 are pink green wrinkled
40 are white yellow round
7 are pink yellow round
6 are white green wrinkled
4 are pink green round
3 are white yellow wrinkled

(a) What are the recombination frequencies between flower and seed color; flower color and seed shape; seed color and seed shape?
(b) Is it reasonable that there are no pink-yellow-wrinkled or white-green-round offspring? What do you think the order of these loci on the chromosome might be?

5. A pea "pink-green" plant with genotype RG/rg is bred with a "white-yellow" plant of genotype rg/rg. Suppose that they are linked, and the recombination frequency is 0.2.
(a) What is the probability a pink-flowering offspring has yellow seeds?
(b) What is the probability a yellow-seed offspring has white flowers?
(c) Suppose 5 offspring are observed, what is the probability that exactly 2 are pink-flowering?
(d) Suppose 5 offspring are observed, what is the probability that at most 1 has yellow seeds?
(e) Suppose 5 offspring are observed, what is the probability that exactly one has pink flowers and yellow seeds?

6. Suppose that in question 4 the offspring were

22 are pink green wrinkled
21 are white yellow round
24 are pink yellow round
22 are white green wrinkled
3 are pink green round
2 are white yellow wrinkled
3 are pink yellow wrinkled
3 are white green round

(a) Are seed-color and seed-shape linked?
(b) Is flower color linked to seed-color or to seed-shape?
(c) Assuming flower color is not linked to seed-color and seed-shape, what is the probability a yellow-round offspring has pink flowers? What is the probability a yellow-wrinkled offspring has white flowers?
(d) Assuming the recombination frequency between seed-color and seed-shape is 0.1, what is the probability a pink-flowering yellow-seed offspring has round seeds? What is the probability a white-flowering wrinkled-seed offspring has green seeds?