Errors in Chromosome Number

We are all products of a sperm fertilizing an egg. Humans have twenty-two pairs of chromosomes, plus the sex chromosomes. After meiosis, the sperm should have twenty-two chromosomes plus either the X or Y chromosome, and the egg should have twenty-two chromosomes plus an X chromosome (see figure 7.1). But sometimes, errors occur in the process of meiosis, and a given gamete is either missing a chromosome or has more than one copy of a particular chromosome. . When this happens, and the egg or sperm that has an aberrant number of chromosomes is involved in making a new individual, of course, that individual will have a wrong number of chromosomes. Because there are so many genes on each chromosome, such errors would be expected to be very deadly. A great majority of fetuses with abnormal chromosome numbers do not survive to birth. Thus, chromosomal abnormalities explain some cases of infertility and pregnancy loss. Indeed, up to 50 percent of first-trimester spontaneous abortions are due to chromosomal abnormalities. A small percentage of fetuses with chromosome abnormalities do survive to birth, and they account for a large proportion of babies with congenital malformations and mental retardation. The most common abnormality involving an abnormal chromosome number is Down's syndrome. This condition is due to an extra copy (for a total of three) of chromosome twenty-one, so it is also called trisomy 21. Individuals with Down's syndrome are typically short and have lower IQs than average, but they can live productive lives. Other relatively common cases of chromosomal abnormalities involve the sex chromosomes. Because the Y chromosome is not essential, and we only need one X chromosome, variations in sex chromosome numbers can occur without lethal consequences. Individuals with unusual sex chromosome numbers (such as XXX, XXY, XYY, and XO, where the individual has just one X chromosome) may be sterile or may show anomalies in secondary sexual characteristics that are generally not noticeable.

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