18. In Drosophila males, where no recombination occurs, segregation of the two alleles of a gene occurs at which stage of cell division?
(1) Diplotene (2) Anaphase I
(3) Anaphase II (4) Mitotic Telophase
Concept
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Each allele of a gene resides on one of a pair of homologous chromosomes.
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In meiosis I, these homologs form bivalents and then move to opposite poles.
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This movement is the physical basis of Mendel’s law of segregation: the two alleles separate into different daughter cells during anaphase I.
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The fact that Drosophila males lack recombination does not change this; they still segregate homologous chromosomes normally.
Option-by-option explanation
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Diplotene
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Substage of prophase I when homologous chromosomes begin to separate and chiasmata (where present) become visible.
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No actual segregation to poles occurs yet; alleles are still in the same nucleus.
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Anaphase I – correct
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Homologous chromosomes of each bivalent are pulled to opposite poles.
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Because each homolog carries one allele, this is when the two alleles of a gene segregate into different cells.
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Anaphase II
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Separation of sister chromatids (equational division).
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By this stage each chromatid already carries only one allele, so allele pairs have already been separated in anaphase I.
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Mitotic telophase
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Occurs in mitosis, not in meiotic gamete formation.
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Mitosis maintains both alleles in each daughter cell; it does not produce haploid gametes or segregate allelic pairs.
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Therefore, in Drosophila males with no recombination, segregation of the two alleles of a gene occurs at anaphase I of meiosis.
