20. If all four gametes AB, aB, Ab and ab are formed in equal probability. Then arrangement of chromosomes at metaphase-I of meiosis will be
Concept
Genotype: AaBb.
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Two homologous chromosomes carry:
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One homolog: A B
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The other homolog: a b
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Genes are unlinked on different homologous pairs (or far apart with free recombination), so all four gamete types AB, Ab, aB, ab should occur with equal probability (1/4 each).
At metaphase I:
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Homologous pairs align on the equatorial plate.
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The relative orientation of each pair (which pole each homolog faces) determines which combinations of alleles will go together into the same gamete after meiosis I and II.
The only arrangement that can yield all four combinations equally is where each homologous chromosome pair carries one complete combination (AB vs ab), and their orientation is random. This is what is depicted in option (1).
Why option (1) is correct
In diagram (1):
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One homolog has A–B; the other has a–b.
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At metaphase I, these two bivalents align so that the A‑B chromosome may go to either pole relative to other chromosomes.
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Independent assortment of the A vs a and B vs b homologs during anaphase I and II produces gametes:
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AB, Ab, aB, ab, each with probability 1/4.
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Thus, option (1) shows the correct metaphase‑I arrangement consistent with equal formation of all four gamete types.
Why the other diagrams are incorrect
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Arrangement (2)
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Puts A with a mixture of B/b on the same side, biasing which alleles travel together.
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This does not allow all four gamete types at equal frequency; some combinations would be over‑ or under‑represented.
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Arrangement (3)
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Both homologs carry the same B/b configuration relative to A/a, again restricting which allele combinations can form.
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Certain gametes (e.g., AB or ab) would be absent or reduced.
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Arrangement (4)
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Depicts only single chromatids, effectively like a haploid setup, not a proper metaphase‑I bivalent alignment.
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It cannot generate the full AaBb independent assortment pattern expected from a dihybrid.
Therefore, to obtain equal frequencies of AB, Ab, aB and ab gametes, the metaphase‑I arrangement must be that shown in figure (1).
