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 diversity. By exchanging genetic material between homologous chromosomes, crossing over creates new combinations of alleles, which can lead to variations in traits among offspring. This genetic diversity is crucial for the evolution and adaptability of species.

The male is phenotypically normal despite carrying a translocation because the translocation does not disrupt the genes required for normal functioning. If the genes involved in the translocation are not critical for the male's phenotype or are compensated by other alleles, then he can appear normal. Additionally, translocations can be balanced, meaning that there is no gain or loss of genetic material that would typically lead to phenotypic changes.

Step R of the diagram shows Metaphase I of meiosis, where homologous chromosome pairs are aligned at the metaphase plate before being separated into different cells.

The chance that a sperm will contain a normal chromosome 1 and a normal chromosome 2 can be calculated using the concept of independent assortment. Assuming that the translocation does not affect segregation, if there are 4 possible combinations of chromosomes formed during meiosis, with 2 normal and 2 translocated, the probability of obtaining one normal chromosome 1 and one normal chromosome 2 in a sperm is 1 in 4, or 25%.

The reasoning behind the probability stems from the fact that during meiosis, chromosomes segregate independently. Given the presence of normal and translocated chromosomes, we analyze the combinations that could arise from the male's sperm. Since only one combination provides both normal chromosomes, the likelihood is proportionate to the total possible chromosome combinations resulting from meiosis.