Drosophila females heterozygous for each of three recessive autosomal mutations

Question

Drosophila females heterozygous for each of three recessive autosomal mutations with independent phenotypic effects (thread antennae [th], hairy body [h], and scarlet eyes [st]) were testcrossed to males showing all three mutant phenotypes. The 1000 progeny of this testcross were:

hairy, thread                              400

scarlet                                        398

wild type                                   57

hairy, thread, scarlet                  53

thread, scarlet                            45

hairy                                           44

thread                                         2

hairy, scarlet                               1

(A)What is the genotype of the female Drosophila? Show the arrangement of alleles on the relevant chromosomes.

(B)Indicate the distance between each of the three genes. Show the calculations.

Explanation / Answer

First, designate the alleles:

th+ = wild-type thread antenna

th = striped thorax

h+ = wild-type eyes

h = glassy eyes

st+ = wild-type body

st = coal-colored body

Designate the types of events that gave rise to each group of individuals and the genotypes of the gametes obtained from their mother. (Since the paternal gametes contain only the recessive alleles of these genes [th h st], they do not change the phenotype and can be ignored.)

Progeny

Number

Type of event

Genotype

hairy, thread

400

single crossover

h, th, st+

scarlet

398

single crossover

h+, th+, st

wild type

57

parental

h+, th+, st+

hairy, thread, scarlet

53

Parental

h, th, st

thread, scarlet

45

single crossover

h+, th, st

hairy

44

single crossover

h, th+, st+

thread

2

Double crossover

h+, th, st+

hairy, scarlet

1

Double crossover

h, th+, st+

There is one set of flies that have not undergone any recombination. There are two sets with single crossover, and one set with double crossover.

In order to determine the order of genes, compare a double crossover progeny with its most similar progeny, h+, th, st+ with h+, th+, st+. This shows that double crossover has occurred leaving the h+, st+ alleles the same with only change in th allele. This indicates that th allele is in the middle of the chromosome.

In order draw map, calculate the recombination frequencies between the center gene and each of the genes on the ends. For h and st, the non-parental combination of alleles are in class 3 and 4; therefore, RF = (57 + 53)/1000 = 110/1000 = 0.11 or 11%. Finding all the RF values, map can be drawn.

th+ = wild-type thread antenna

th = striped thorax

h+ = wild-type eyes

h = glassy eyes

st+ = wild-type body

st = coal-colored body

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