Sex Chromosome Abnormalities

Sex chromosome abnormalities occur as a result of chromosome mutations brought on by mutagens (like radiation) or problems that occur during meiosis. One type of mutation is caused by chromosome breakage. The broken chromosome fragment may be deleted, duplicated, inverted, or translocated to a non-homologous chromosome. Another type of mutation occurs during meiosis and causes cells to have either too many or not enough chromosomes. Alterations in the number of chromosomes in a cell can result in changes in an organisms phenotype or physical traits. Normal Sex Chromosomes In human sexual reproduction, two distinct gametes fuse to form a zygote. Gametes are reproductive cells produced by a type of cell division called meiosis. They contain only one set of chromosomes and are said to be haploid (one set of 22 autosomes and one sex chromosome). When the haploid male and female gametes unite in a process called fertilization, they form what is called a zygote. The zygote is diploid, meaning that it contains two sets of chromosomes (two sets of 22 autosomes and two sex chromosomes). The male gametes, or sperm cells, in humans and other mammals are heterogametic and contain one of two types of sex chromosomes. They have either an X or a Y sex chromosome. However, the female gametes or eggs contain only the X sex chromosome and are therefore homogametic. The sperm cell determines the sex of an individual in this case. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male. X and Y Chromosome Size Difference The Y chromosome carries genes that direct the development of male gonads and the male reproductive system. The Y chromosome is much smaller than the X chromosome (about 1/3 the size) and has fewer genes than the X chromosome. The X chromosome is thought to carry around two thousand genes, while the Y chromosome has less than one hundred genes. Both chromosomes were once about the same size. Structural changes in the Y chromosome resulted in the rearrangement of genes on the chromosome. These changes meant that recombination could no longer occur between large segments of the Y chromosome and its X homologue during meiosis. Recombination is important for weeding out mutations, so without it, mutations accumulate faster on the Y chromosome than on the X chromosome. The same type of degradation is not observed with the X chromosome because it still maintains the ability to recombine with its other X homologue in females. Over time, some of the mutations on the Y chromosome have resulted in the deletion of genes and have contributed to the decrease in the size of the Y chromosome. Sex Chromosome Abnormalities Aneuploidy is a condition characterized by the presence of an abnormal number of chromosomes. If a cell has an additional chromosome (three instead of two), it is trisomic for that chromosome. If the cell is missing a chromosome, it is monosomic. Aneuploid cells occur as a result of either chromosome breakage or nondisjunction errors that happen during meiosis. Two types of errors occur during nondisjunction: homologous chromosomes dont separate during anaphase I of meiosis I or sister chromatids dont separate during anaphase II of meiosis II. Nondisjunction results in some abnormalities, including the following: Klinefelter syndrome is a disorder in which males have an extra X chromosome. The genotype for males with this disorder is XXY. People with Klinefelter syndrome may also have more than one extra chromosome resulting in genotypes which include XXYY, XXXY, and XXXXY. Other mutations result in males that have an extra Y chromosome and a genotype of XYY. These males were once thought to be taller than average males and overly aggressive based on prison studies. Additional studies, however, have found XYY males to be normal.Tuner syndrome is a condition that affects females. Individuals with this syndrome, also called monosomy X, have a genotype of only one X chromosome (XO).Trisomy X females have an additional X chromosome and are also referred to as metafemales (XXX). Nondisjunction can occur in autosomal cells as well. Down syndrome is most commonly the result of nondisjunction affecting autosomal chromosome 21. Down syndrome is also referred to as trisomy 21 because of the extra chrom osome. The following table includes information on sex chromosome abnormalities, resulting syndromes, and phenotypes (expressed physical traits). Genotype Sex Syndrome Physical Traits XXY, XXYY, XXXY male Klinefelter syndrome sterility, small testicles, breast enlargement XYY male XYY syndrome normal male traits XO female Turner syndrome sex organs don't mature at adolescence, sterility, short stature XXX female Trisomy X tall stature, learning disabilities, limited fertility Sex Chromosome Abnormalities