Exercise - Crossing Over & Recombination

Problems

1. Ebony (black body, e) and apterous (vestigial wings, vg) are recessive traits, present on different chromosomes. An ebony female with wild-type wings (whose parents were true breeding for wild-type wings) is crossed with an apterous male with wild-type (brown) body color (whose parents were true breeding for wild-type body color).

a. What are the genotypes of the parental flies?

b. List the genotype(s) and phenotype(s) you expect to see in the F1 offspring. Explain how you arrived at your answer.

2. A cross is performed between an ebony female fly with wild-type eyes and a male fly with wild-type body color and sepia (brown) eyes(s) male fly. Ebony (e) and sepia (s) are recessive traits. All of the F1 offspring have the wild-type phenotypes.

a. What are the genotypes of the parental and F1 flies? Explain your reasoning.

The F1 flies are crossed, and the following results are obtained:

48 wild-type body color and eyes
26 ebony body color, wild-type eyes
24 wild-type body color, sepia eyes
2 ebony body color,sepia eyes

b. Examine the ratios of F1 phenotypes. Is this expected for traits that are not linked? Explain.

c. What is the recombinant genotype? Explain.

d. What is the frequency of recombination? You must show your work!

e. Does this recombination frequency estimate map distance directly? Why or why not?

3. In maize, three recessive genes (z, xt, and cm) are linked on chromosome 3. A homozygous plant for the recessives is crossed with a wild-type plant. The F1 is crossed to get an F2 generation with the following results:

a. What are the parental, single recombinant, and double recombinant genotypes?

b. What is the order of the three genes on chromosome 3? Explain your answer.

c. What is the coefficient of coincidence for the double crossovers in this scenario? Explain your answer and show your work!

d. What does the coefficient of coincidence tell you about interference in this chromosomal region?