Genetics linkage problems..take 2. Due Thursday start of class

1. Yellow vs grey body color in Drosophila is determined by alleles y and y+, vermillion vs wild type eyes by v and v+ and singed vs straight bristles by the alleles s and s+. All three loci are X linked. When heterozygous females were test crossed with yellow, vermilion, singled males, the following classes and numbers of progeny were obtained:

yellow, vermilion singed


yellow vermilion


yellow singed




vermilion singed






wild type


a. What is the order of the three genes? Construct a linkage map showing the genes in the correct order and the map distances.








b. What is the expected frequency of double crossovers assuming that crossing over occurs independently in the two chromosome regions? Comment on the observed number of double crossovers. Does it seem to match what you expect. Might be useful to read page 110 -111. This is also useful for the next problem!!!






Harder problems:

2. Genes A B and C in rabbits are linked and the map distances are given as follows:


A -------------(10 mu) ------- C ----------(15 mu)------------- B

Suppose these loci act in a biochemical pathway involved in coat pigment as follows:


Colorless ---------- Yellow ------------ Brown-------- Black


    1. Give the phenotype of the following genotypes: (1 point each)

a C b/a C b

A c b /a c b

A c B /A c B



B. A rabbit breeder has carefully produced a black rabbit with genotype:

A C B/a c b

What are the expected phenotypic ratios from a three point test cross involving this rabbit? (4 points)





C. Suppose there is no rabbit available for a test cross, but the breeder does manage to get another rabbit with A C B/a c b which she breeds with her first rabbit. This year for some reason, brown rabbits are in demand for Easter. What is the expected frequency of brown rabbits from this cross? ( 3 points)








Problem 3. A female Drosophila carries the recessive mutations a and b in repulsion (trans) on her X chromosomes. She is also heterozygous for an X linked recessive lethal allele l.

When she is mated to a true breeding normal male the following offspring are obtained:

Females: 1,000 + +

Males: 405 a +

44 + b

48 + +

2 a b


    1. What was the genotype of the female fly? ( 1 point)


    3. What was the genotype of the male fly (1 points)



    5. Why are all the females from this cross + +? ( 2 points)


    7. Why are there only 4 phenotypic classes for the males? (2 points)




E. Draw a chromosome map of the three genes in the proper order and with map distances. (4 points)