Chromosome iv in drosophila is a very small chromosome and carries a tiny amount of genetic material. fruit flies that are triso
mic 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 chromosome iv and has the genotype + + ey is crossed to a diploid eyeless female with the genotype eyey.
If I assume that random segregation of chromosomes takes place during spermatogenesis and all sperms are viable then I will take the male through meiosis. The three chromosomes are +, +, ey Then you double them into sister chromatids which is ++, ++, eyes. If I consider all possible alignments during metaphase as, the three homologous pairs and gather at the metaphase plate. Remember that any two of the three pairs will join together and odd pair out will line up at the metaphase plate with no partner pair. There are three different arrangements which are possible for the three homologous pairs. Two of the three possible arrangements will segregate and make two sperm with +ey and two sperm with +. The other arrangement will segregate and make two sperm with ++ and two sperm with ey. When meiosis is being done, then the sperm genotypes and their proportions are, 1/3+ey, 1/3+, 1/6++, 1/6ey
