19. In a heterozygous individual for a given gene, if a crossing over has occurred between the gene locus and the centromere of the chromosome, the segregation of the two alleles of the given gene will occur during meiosis at
(1) either anaphase I or anaphase II
(2) anaphase I only
(3) anaphase II only
(4) both anaphase I and II

Concept

• Start with a heterozygous gene $A/a$ on a pair of homologous chromosomes.

• Without crossing over, each homolog carries one allele along its entire length; when homologs separate at anaphase I, $A$ and $a$ go to different cells, so segregation occurs in meiosis I.

• If a single crossover occurs between the gene and the centromere, then after recombination:

  • Two chromatids are non‑recombinant (still whole‑chromatid $A$ or $a$).

  • Two chromatids are recombinant, each now carrying both alleles in different parts of the chromatid.

For those recombinant chromatids, the two alleles are now on sister chromatids rather than on homologs. Thus:

• For non‑crossover chromatids, $A$ vs $a$ segregate at anaphase I (homologous chromosomes separate).

• For crossover chromatids, segregation of $A$ vs $a$ happens only when sister chromatids separate at anaphase II.

Therefore, in this situation, segregation of the two alleles of that gene can take place either in anaphase I or in anaphase II, depending on which chromatid is being followed.

Option-by-option explanation

1. Either anaphase I or anaphase II – correct

   • Reflects the mixed behavior of recombinant vs non‑recombinant chromatids when crossing over occurs between gene and centromere.

2. Anaphase I only

   • True only if no crossover occurs between gene and centromere; all alleles segregate with homologs in meiosis I.

   • Once crossing over shifts alleles between chromatids, some segregate in meiosis II instead.

3. Anaphase II only

   • Would be true if all chromatids carried both alleles (e.g., special rearrangements), but here half are non‑recombinant and segregate earlier.

4. Both anaphase I and II

   • Suggests that the same pair of alleles is separating at both divisions, which is not accurate; each allele pair is resolved once, either at I or at II, not at both.

So, with a crossover between gene and centromere, segregation of the two alleles can happen in some cells at anaphase I and in others at anaphase II, making option (1) the best description.