24. Of the following meiotic events
(A) formation of synaptonemal complex
(B) crossing over
(C) arrangements of chromosomes at the equatorial plate at Metaphase I
(D) separation of chromatids at Anaphase II
Which one(s) lead to variation?
(1) (B) only (2) (B) and (C)
(3) (A) and (D) (4) (B) and (D)
Understanding each event
The question asks which listed meiotic events lead to variation among gametes.
(A) Formation of synaptonemal complex – does not itself create variation
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The synaptonemal complex is a protein structure that forms between homologous chromosomes in prophase I, holding them aligned for recombination.
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It is structural and preparatory; by itself it does not change allele combinations. Variation comes from what happens while homologs are paired (crossing over), not from the scaffold itself.
(B) Crossing over – directly creates genetic variation
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Crossing over exchanges segments between non‑sister chromatids of homologous chromosomes in prophase I.
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This produces recombinant chromatids with new combinations of alleles that did not exist in either parent.
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It is one of the two major meiotic sources of genetic variation.
(C) Arrangement of chromosomes at metaphase I – directly creates variation
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At metaphase I, each pair of homologous chromosomes lines up randomly with respect to the spindle poles.
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This independent assortment means that maternal and paternal homologs are assorted into gametes in many different combinations.
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The random orientation of each pair produces a huge number of distinct gamete genotypes, so it is a second major source of variation.
(D) Separation of chromatids at anaphase II – mainly ensures correct segregation
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In anaphase II, sister chromatids (already recombinant or non‑recombinant from earlier steps) separate.
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This division distributes existing allele combinations to gametes but does not create new combinations; variation was already established by crossing over and independent assortment earlier.
Evaluating the options
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(B) only – incomplete, ignores the important contribution from independent assortment at metaphase I.
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(B) and (C) – correct – both crossing over and metaphase I alignment generate new allele combinations and thus genetic variation.
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(A) and (D) – synaptonemal complex formation and chromatid separation are necessary mechanics but do not themselves create new allele combinations.
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(B) and (D) – includes crossing over (correct) but wrongly pairs it with anaphase II instead of metaphase I.
Therefore, the meiotic events that lead to genetic variation are crossing over (B) and random arrangement of chromosomes at metaphase I (C), so the correct option is (2).
