Sex Chromosome Abnormalities

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Bailey, Regina. "Sex Chromosome Abnormalities." ThoughtCo, Sep. 15, 2017, thoughtco.com/sex-chromosome-abnormalities-373286. Bailey, Regina. (2017, September 15). Sex Chromosome Abnormalities. Retrieved from https://www.thoughtco.com/sex-chromosome-abnormalities-373286 Bailey, Regina. "Sex Chromosome Abnormalities." ThoughtCo. https://www.thoughtco.com/sex-chromosome-abnormalities-373286 (accessed October 24, 2017).
Y and X Sex Chromosomes
This is a computer representation of human sex chromosomes. The Y chromosome (left) is smaller than the X (right) chromosome. Credit: ALFRED PASIEKA/SCIENCE PHOTO LIBRARY/Getty Images

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 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 less 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 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 don't separate during anaphase I of meiosis I or sister chromatids don't separate during anaphase II of meiosis II.

Nondisjunction results in a number of abnormalities, including:

  • 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 chromosome.

The following table includes information on sex chromosome abnormalities, resulting syndromes, and phenotypes (expressed physical traits).

GenotypeSexSyndromePhysical Traits
Sex Chromosome Abnormalities
XXY, XXYY, XXXYmaleKlinefelter syndromesterility, small testicles, breast enlargement
XYYmaleXYY syndromenormal male traits
XOfemaleTurner syndromesex organs don't mature at adolescence, sterility, short stature
XXXfemaleTrisomy Xtall stature, learning disabilities, limited fertility