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Chapter 15: Q15-11TYU (page 313)

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.鈥�). Explain what might be a benefit of these regions.

Short Answer

Expert verified

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 by step solution

01

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.

02

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.

03

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.

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Most popular questions from this chapter

Which one of Mendel鈥檚 laws describes the inheritance of alleles for a single character? Which law relates to the inheritance of alleles for two characters in a dihybrid cross?

Neither Tim nor Rhoda had Duchenne muscular dystrophy, but their firstborn son does. What is the probability that a second child will have the disease? What is the probability if the second child is a boy? A girl?

Butterflies have an X-Y sex determination system that is different from that of flies or humans. Female butterflies may be either XY or X0, while butterflies with two or more X chromosomes are males. This photograph shows a tiger swallowtail gynandromorphy, which is half male (left side) and half female (right side). Given that the first division of the zygote divides the embryo into the future right and left halves of the butterfly, propose a hypothesis that explains how nondisjunction during the first mitosis might have produced this unusual looking butterfly.

Assume that genes A and B are on the same chromosome and are 50 map units apart. An animal heterozygous at both loci is crossed with one that is homozygous recessive at both loci. What percentage of the offspring will show recombinant phenotypes resulting from crossover? Without knowing these genes are on the same chromosome, how would you interpret the results of this cross?

Assume you are mapping genes A, B, C, and D in Drosophila. You know that these genes are linked on the same chromosome, and you determine the recombination frequencies between each pair of genes to be as follows: A-B, 8%; A-C, 28%; A-D, 25%; B-C, 20%; B-D, 33%.

  1. Describe how you determined the recombination frequency for each pair of genes.

  2. Draw a chromosome map based on your data.
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