Question

Chromosome IV in Drosophila is a very small chromosome and carries a tiny amount of genetic material. Fruit flies that are trisomic for chromosome IV have no apparent phenotypic abnormalities, and they retain their fertility. Among the genes on chromosome IV is one for which a recessive allele ey produces the "eyeless" phenotype. A male that is trisomic for chrosome IV and has the genotype + + ey is crossed to a diploid eyeless female with the genotype ey ey.

1) Assuming random segregation of chromosomes takes place during spermatogenesis and that all sperm are viable, what sperm genotypes are expected and in what proportions?

2) If these sperm are united with eggs from the eyeless female, what is the expected ratio of eyeless to normal-eyed flies among the progeny?

Answer 1

1. During meiosis, we have three chromosomes +, +, ey, when duplicating them in sister chromatids we will have the combination ++, ++, eyey, then multiple alignments are made to generate the metaphase, the pairs will be randomly joined and the odd will be left couple, two of the three arrangements will be added and will generate two sperm + ey and two +, on the other side 2 with ++ and 2 with ey will be produced, thus the genotypes will be the following:

1/3 +

1/3 hey

1/6 ++

1/6 hey

2.

to evaluate the probabilities of the genotype e and when crossing the spermatozoa already studied it will be the following

1/6 ++ hey

1/6 eyey

1/3 + eyey

1/3 + hey

With this result we can deduce that 1/6 will not have eyes and 5/6 are wild type, which supposes that there is no crossing and that wild flies have the genotype + eyey