Academic journal article The American Biology Teacher

The Y Chromosome

Academic journal article The American Biology Teacher

The Y Chromosome

Article excerpt

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It may seem strange to begin a paper about the Y chromosome with a discussion of recombination and mutation. But bear with me. Your patience will be amply rewarded.

For many years, scientists thought of recombination as a useful tool for constructing chromosome maps. Indeed, until DNA sequencing became widely available in the 1980s and 1990s, all chromosome maps were constructed using recombination frequencies; the more closely two genes are located on a chromosome, the less frequently they recombine. Yet recombination had been going on for several billion years before people started constructing chromosome maps. Why has this complicated and costly process survived billions of years of natural selection? The answer lies in the properties of mutation.

Mutation is a change in DNA. There are dramatic mutations, such as single base-pair changes that cause sickle cell anemia or severe combined immunodeficiency ("Boy in the Bubble" disease). However, most mutations are not nearly so devastating. Most mutations make no difference at all. They are either located in non-coding areas of the genome or are "silent" mutations within coding areas of genes. The next-most-common kind of mutation causes a small but slightly detrimental change to a protein. An organism with this kind of mutation will live but will be at a very slight selective disadvantage. Rarer still are the mutations that confer an adaptive advantage and are selected for.

What happens to the many slightly detrimental mutations that occur? Because of recombination, these mutations are weeded out over evolutionary time, hundreds or thousands of generations, because recombination events yield chromosomes that do not contain the mutated genes (Figure 1).

A gene that does not recombine will accumulate random mutations and eventually not code for a functional protein. Decay in genes that either don't recombine or no longer serve a function has been observed frequently. Most mammals have about 1000 genes that code for odor receptors, G proteins in cell membranes of cells in the nose. Each odor receptor has a unique shape and binds to a specific odorant. However, primates have only about 300 functional odor receptors. Additional odor receptors exist as "pseudogenes" in primate genomes. These stretches of DNA have recognizable "odor receptor" sequence but are clearly nonfunctional; there may be a stop codon in the middle of the gene. This loss of functional odor receptors occurred when primates developed excellent color vision. Because they relied on vision rather than smell to find food, there was no selection against individuals with fewer functional odor receptors.

Similarly, there are 15 known species of icefish. Adapted to living in frigid Antarctic oceans, these fish have no red blood cells. They have large gills and scaleless, highly vascularized skin that enables them to obtain and transport adequate oxygen directly from the cold, oxygen-rich waters. Icefish have no gene for alpha hemoglobin and only a partial, nonfunctional gene for beta hemoglobin (one hemoglobin molecule consists of two alpha chains and two beta chains). The ancestors of icefish had functional hemoglobin genes. These genes no longer function in icefish, so mutations in them are not selected against (Carroll, 2006).

Evolutionary History of the Y Chromosome

About 310 million years ago, there was no Y chromosome as we know it. At that time, when mammals were evolving from their common ancestors with reptiles and birds, what are now the X and Y chromosomes were a pair of autosomes similar to today's X chromosome. Sex in these ancestors was probably determined by the temperature at which the egg is incubated, as it is in many reptiles today. This can occur only in ectotherms, animals that do not maintain a constant body temperature. Otherwise, only males or only females would be produced, not a situation conducive to long-term reproductive success. …

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