(a) Explain briefly how the process of meiotic division contributes to variation in a population (b) State Mendel's first and second laws of inheritance. (c...

Answer Details

(a) How meiotic division contributes to variation in a population

Meiosis introduces variation in two main ways:

• Crossing over: during the first meiotic division, homologous chromosomes pair and exchange segments (genes) at the chiasmata. This produces new combinations of alleles in the resulting gametes.
• Independent (random) assortment: the homologous chromosome pairs line up and separate independently of one another, so the chromosomes are shuffled and each gamete receives a different mixture of maternal and paternal chromosomes.

Because of these events, meiosis produces gametes that are genetically different from one another and from the parent cell. When these varied gametes fuse at random during fertilization, offspring with new gene combinations are formed, giving rise to variation within the population.

(b) Mendel's first and second laws

First law (Law of Segregation): the two members of a pair of alleles controlling a character separate (segregate) during the formation of gametes, so that each gamete carries only one allele of the pair.

Second law (Law of Independent Assortment): when two or more pairs of contrasting characters are considered together, each pair of alleles segregates and is distributed into the gametes independently of the other pairs.

(c) Cross of brown rat (BB) x white rat (bb) to the second filial generation

Let B = brown (dominant), b = white (recessive).

Parents: \(BB\) (brown) \(\times\) \(bb\) (white). Gametes: \(B\) and \(b\).

F1: all \(Bb\) - brown rats.

F1 selfed: \(Bb \times Bb\). Gametes: \(B\) and \(b\) from each.

F2 genotypes: \(1\ BB : 2\ Bb : 1\ bb\).

F2 phenotypes: 3 brown : 1 white, i.e. \(3:1\).