Humans have 46 chromosomes in each of their body cells - VCE - SSCE Biology - Question 3 - 2010 - Paper 1

The advantage of crossing over is that it increases genetic variability. During meiosis, homologous chromosomes exchange segments of genetic material. This recombination creates new combinations of genes, which can enhance the adaptability and survival of a species.

The male is phenotypically normal because the translocation involved is not disrupting any critical genes on the involved chromosomes. The normal functioning of other chromosomes compensates for the translocated chromosomes, allowing him to show no observable phenotypic effects.

The stage of meiosis shown in step R is Metaphase I. This is indicated by the alignment of homologous chromosome pairs at the metaphase plate, where they are positioned to be separated during the next phase.

The chance that a sperm will contain a normal chromosome 1 and a normal chromosome 2 is 50%. Since there are two types of possible gametes, one containing the normal chromosome 1 and 2 and another containing the translocated chromosome, the probability is calculated as follows:

$P = \frac{1}{2} = 0.5$

The reasoning for the 50% chance stems from the independence of assortment in meiosis. With the presence of the translocation, the male can produce two distinct types of gametes. Thus, when sperm are formed, half will have the normal chromosomes, leading to a probability of 50% for normal chromosome 1 and chromosome 2.