22. Two siblings who inherit 50% of the genome from the mother and 50% from the father show lot of phenotypic differences.
Which one of the following events during gametogenesis of the parents will maximally contribute to this difference?
(1) Mutation
(2) Recombination
(3) Independent assortment
(4) Environment

Core idea

Two siblings each get half their genome from the mother and half from the father, but not the same halves. During meiosis in each parent, recombination (crossing over) exchanges segments between homologous chromosomes, producing gametes with new allele combinations along each chromosome. This shuffling greatly increases genetic and thus phenotypic variation between siblings.

Option‑wise explanation

1. Mutation

   • New mutations in gametes do contribute variation, but their frequency per generation is relatively low.

   • Most differences between ordinary siblings are not from fresh mutations but from different combinations of existing parental alleles.

2. Recombination – correct

   • Crossing over in prophase I of meiosis swaps segments between homologous chromosomes, generating new haplotypes—novel combinations of alleles on the same chromosome.​

   • Together with independent assortment, recombination massively increases the number of possible gametes and is the major genetic source of phenotypic differences among siblings.

3. Independent assortment

   • Random orientation and segregation of whole homologous chromosome pairs also creates different allele combinations in gametes.

   • However, the question asks which single event “maximally” contributes; recombination creates new combinations within chromosomes on top of independent assortment and is generally considered the strongest generator of within‑family genetic diversity.​

4. Environment

   • Environmental factors strongly influence phenotype, but the question restricts us to “during gametogenesis of the parents,” which refers to events in meiosis, not post‑zygotic environmental effects.

   • Thus environment is not the correct choice here.

Answer: The event during parental gametogenesis that most strongly explains why siblings with the same 50% parental genome share differ so much phenotypically is recombination (option 2).