While human sex chromosomes are of type X or type Y, some organisms use different systems for sex differentiation.
Chromosomes are the structures in which DNA (deoxyribonucleic acid), the material that contains the genetic instructions for life, is packaged in each cell of an organism. Most human cells contain two sets of 23 single chromosomes, one set from each parent. One pair of chromosomes, called sex chromosomes, determines the sex of an individual. The two types of human sex chromosomes are X and Y; an individual with two X chromosomes is female, and an individual with an X chromosome and a Y chromosome is male.
X Chromosome
Both males and females have X chromosomes. X chromosomes in humans are estimated to contain between 900 and 1,400 genes in humans, a great deal more than the Y chromosome. In females, one X chromosome is randomly deactivated in all cells except egg cells. Females produce ova with only X chromosomes, while males produce sperm of both X and Y chromosomes.
Y Chromosome
Only males have Y chromosomes. Y chromosomes in humans are estimated to contain between 70 and 200 genes associated with male sex development, determination and fertility. Since Y chromosomes are passed down from father to son, they are frequently used for genealogical studies.
Sex Chromosome Abnormalities in Humans
When humans inherit a pair of sex chromosomes other than XX or XY, abnormalities can occur. Turner Syndrome, in which a female experiences abnormal physical development, such as short stature or a webbed neck, occurs when a female has only one X chromosome. Females with Triple-X syndrome have an additional X chromosome and are often taller than average. Males with Kleinfelter Syndrome have one or more extra X chromosome and often sterile. XYY syndrome occurs when males inherit an extra Y chromosome, often leading to above-average height and high levels of testosterone production.
Sex Chromosomes in Other Organisms
Several other sex-determination systems occur in other organisms. In some species of insects, sex is determined by the X-O system: females have two X chromosomes, while males have one X chromosome. The Z-W system, in which the female parent's ovum determines offspring's sex instead of the male parent's sperm, occurs in some insects, birds and fish. Sex determination for bees and ants occurs through the haplo-diploid system, in which fertilized eggs develop into females and unfertilized eggs develop into males.
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