Core concept

Mendel’s law of segregation says the two alleles of a gene separate so each gamete gets only one. Cytologically:

• Each allele sits on one of a pair of homologous chromosomes.

• In meiosis I, homologous chromosomes pair and then move to opposite poles.

• This movement separates the alleles into different daughter cells.

• Meiosis II only separates sister chromatids of the already segregated chromosomes.

So, when there is no crossing over, the key segregation of allele pairs happens in meiosis I.

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Option-by-option explanation

1. Meiosis‑I – correct

   • During anaphase I of meiosis I, homologous chromosomes (and therefore the two alleles) segregate to opposite poles.

   • This is the cytological basis of Mendel’s law of segregation.

2. Meiosis‑II

   • In meiosis II, sister chromatids of each chromosome separate.

   • By this stage, each chromatid carries only one allele for each gene (thanks to meiosis I), so the key allele segregation has already occurred.

3. Mitosis

   • Mitosis keeps chromosome number and allele combinations unchanged; each daughter cell gets both alleles if the parent was diploid.

   • It does not explain Mendelian segregation of alleles into gametes.

4. Cleavage

   • Cleavage is post‑fertilization embryonic cell division and does not involve reduction division or fresh allele segregation.

   • It simply partitions the already formed zygotic genome.

Therefore, in the absence of crossing over, the pair of alleles segregates during meiosis I, making option (1) correct.