Q15-11TYU - Campbell Biology

Question

Crossing over is thought to be evolutionarily advantageous because it continually shuffles genetic alleles into novel combinations. Until recently, it was thought that the genes on the Y chromosome might degenerate because they lack homologous genes on the X chromosome with which to pair up prior to crossing over. However, when the Y chromosome was sequenced, eight large regions were found to be internally homologous to each other, and quite a few of the 78 genes represent duplicates. (Y chromosome researcher David Page has called it a "hall of mirrors.”). Explain what might be a benefit of these regions.

Step-by-Step Solution

Answer

Humans have a single Y chromosome, which means they do not have a homologous chromosome for pairing during meiosis to repair any gene defect. 

Thus, the presence of duplicate genes that are homologous to each other helps to maintain the chromosome's integrity.

Step 1: Homologous chromosomes

The chromosomes that pair with identical DNA (deoxyribonucleic acid) sequences, length, loci, and centromere position are called homologous chromosomes. One of the chromosomes in the homologous pair is obtained from the father and the other from the mother.

During meiosis, the maternal chromosome of the homologous chromosome pair up with the paternal chromosome. Crossing-over occurs between the homologous chromosomes, which results in genetic recombination. 

Thus, recombination leads to genetic variation.

Step 2: Sex chromosomes of humans

All humans have 22 autosomes and a pair of sex chromosomes: X and Y. Females have a pair of X chromosomes, whereas males have one X and one Y chromosome. So, females' X chromosomes have a homologous pair. In contrast, the Y chromosomes of males are single, so they don't have a homologous chromosome.

The presence of a homologous chromosome allows crossing over. Apart from creating genetic recombination, the pairing of the homologous chromosomes helps correct the faults in the nucleotide sequences. 

It helps fill the gaps in case of missing nucleotide or restore the nucleotide in case of incorrect nucleotide in the sequence.

Step 3: Presence of duplicate genes on the Y chromosome

Duplicate genes on the Y chromosome homologous allow the same region to pair up during meiosis and allow crossing over. This would help rectify any errors present in the genes as the Y chromosome does not have any homologous pair.

This would help maintain the entire genetic sequence of the Y chromosome and thereby maintain the chromosome's integrity.