In Drosophila melanogaster , eyeless ( ey ) is a recessive trait in which the ey
ID: 36010 • 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.
I am confused on what a chromosome constitution is and for the actual cross, would this be the correct punnet square?
ey+ ey ey ey+ ey+ey+ ey+ey ey+ey ey eyey+ eyey eyeyExplanation / Answer
(ey+) -- Dominant allele -- wild type eye
(ey) -- Recessive allele --- tiny/absent eye
Male genotype is ey+ ey ey , let us number the same allele, ey+ ey1 ey2
In trisomic individual, three types of segregation can take place during gamete production, ey+ ey1 & ey2
ey2 ey+ & ey1 and lastly, ey1 ey2 & ey+
thus, trisomic individual has two different ways of producing ey+ ey gamete : ey+ ey1 & ey+ey2
Ignoring the labelling of gametes, the probabilities will be
2/6 probability of ey+ey gamete , 2/6 probability of ey gamete, 1/6 probability of ey ey gamete & 1/6 probability of ey+ gamete
Disomic parent - 1/2 ey gamete & 1/2 ey+ gamete
Therefore, the various combinations of fertilization of gametes can be obtained as follows: