In Drosophila melanogaster , eyeless ( ey ) is a recessive trait in which the ey
ID: 34025 • Letter: I
Question
In Drosophila melanogaster, eyeless (ey) is a recessive trait in which the eyes are tiny or absent. The ey locus is found on chromosome 4. Monosomy and trisomy for this very small chromosome are compatible with survival and fertility. A trisomic wild-type male with the genotype ey+ ey ey is crossed to an ey+ ey female. Assuming random segregation of chromosomes into gametes, determine the chromosome constitution and phenotypic ratios in the offspring.
Hint: Refer to Figure 8.21 for a description of chromosome segregation in a trisomic individual, then use the forked-line method/branching diagram to solve the problem.
Explanation / Answer
Cross between parents
ey+ ey ey (male) X ey+ ey (female)
Possible gametes produced by male
ey,
ey,
ey,
ey+
ey+
Possible gamets produced by female
ey+
ey
Offspring produced from this cross
eyey+
eyey,
eyey+
eyey
eyey+
eyey
ey+ey+
ey+ey
ey+ey+
ey+ey
Therefore, ratio of homozygous dominant : Homozygous recessive : heterozygous
= 2 : 5 : 3