17.  In Drosophila melanogaster males, homologous chromosomes pair and segregate during meiosis but crossing over does not occur. At which stage of meiosis does segregation of 2 alleles of a gene take place in these individuals?
(1) Zygotene           (2) Diakinesis
(3) Anaphase I        (4) Anaphase II

Concept

• Each allele of a gene resides on one of a pair of homologous chromosomes.

• Mendel’s law of segregation is physically realized when these homologous chromosomes are pulled to opposite poles during anaphase I.

• In Drosophila males, even though no crossing over occurs, homologs still pair and then segregate normally, so each resulting cell gets only one allele for each gene at anaphase I.

Option-by-option explanation

1. Zygotene

   • Substage of prophase I where homologous chromosomes begin to pair (synapsis).

   • Alleles are still together in paired homologs; no segregation yet.

2. Diakinesis

   • Late prophase I with maximal chromosome condensation and chiasmata visible (in species with crossing over).

   • Chromosomes are preparing for division, but alleles have not moved to opposite poles.

3. Anaphase I – correct

   • Homologous chromosomes of each pair separate and migrate to opposite poles.

   • Since each homolog carries one allele, this is when the two alleles of a gene segregate into different daughter cells.

4. Anaphase II

   • Separation of sister chromatids, not homologous chromosomes.

   • By this point each chromatid already carries just one allele, so segregation of allelic pair has already occurred earlier in anaphase I.

Thus, in Drosophila melanogaster males (no crossing over, but normal pairing and segregation), the two alleles of a gene separate during anaphase I of meiosis.