With the exception of identical twins which of the following

Solution

Ans. Correct option: B. Crossing over and random mating.

Crossing over during gametogenesis brings genetic variation in the resultant gametes. It results the gametes to have a blended (recombinant allelic combinations) genotype.

Moreover, the mating is always random under natural conditions in the sense that the fusion of egg and sperm is random. Or, the male and female gametes of all the possible genotypes mate randomly.

So, out of billions possible allelic combination in each of male and female gametes, there occurs mating of two unique gametes- each gamete is unique because it is one of the billions possible genotypes. So, the resultant zygote also has unique genotype because its allelic composition is the one among billions of possibilities.

The true reason of uniqueness of gametes and resultant zygotes arise from recombination during gametogenesis and random mating thereafter.

Option A. Incorrect: Non-random mating (or selective mating) reduces the possibility of getting a unique zygote. It increases the genetic similarity of the progenies.

The true reason of uniqueness of gametes and resultant zygotes arise from recombination during gametogenesis and random mating thereafter.

Option C. Incorrect: It does not specify recombination. If recombination does not occur, the genetic variation of resultant gametes becomes much limited.

Option D. Incorrect: It does not specify recombination and random mating. Inbreeding also reduces genetic variation among offspring