Normal Sex ChromosomesIn 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 are either X or Y. The female gametes or eggs however, 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.
Sex Chromosome AbnormalitiesA chromosome abnormality that causes individuals to have an abnormal number of chromosomes is called aneuploidy. If the zygote has an additional chromosome (three instead of two) it is trisomic for that chromosome. If the zygote is missing a chromosome it is monosomic. Aneuploid cells occur as a result of 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.
In sex chromosomes, nondisjunction results in a number of abnormalities. 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 expressed physical traits.
Sex Chromosome Abnormalities
|Sex Chromosome Abnormalities|
|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|