- Key People:
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- Clarence E. McClung
sex chromosome
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What are sex chromosomes?
Sex chromosomes are chromosomes that determine whether an individual is male or female. In humans and other mammals, these are designated as X and Y. Females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
During meiosis, the sex chromosome pair separates, passing on either an X or a Y chromosome to separate gametes. Females produce egg cells that normally carry a single X chromosome. Eggs fertilized by X-bearing sperm become females (XX), while those fertilized by Y-bearing sperm become males (XY).
The X chromosome carries many more genes than the Y. Genes found only on the X chromosome are called sex-linked. Recessive sex-linked traits are more frequent in men because they only have one X chromosome.
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How do sex chromosomes differ from other chromosomes?
Sex chromosomes and autosomes differ in their function and composition. Sex chromosomes, either X or Y, determine an individual's sex, while autosomes are all other non-sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes.
Females typically have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The X chromosome is larger and carries more genes than the Y chromosome. Genes found on the X chromosome control sex-linked traits. The Y chromosome primarily carries genes that determine maleness. Unlike autosomes, sex chromosomes do not carry an identical set of genetic information.
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What roles do the X and Y chromosomes play in determining biological sex?
In humans and other mammals, sex chromosomes determine whether an individual is male or female. Females have two X chromosomes (XX), while males typically have one X and one Y chromosome (XY).
The larger X chromosome carries many genes, some of which influence sex-linked traits such as red-green colorblindness and hemophilia. Males are more likely to express recessive sex-linked traits because they have only one X chromosome. The Y chromosome is smaller and carries fewer genes, mainly related to male development, including those that determine the development of the testes in the fetus.
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How are sex chromosomes passed from parents to children?
In humans and other mammals, sex chromosomes determine whether an individual is male or female. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
During meiosis, the XY pair in males separates, passing either an X or a Y chromosome to separate sperm cells. Females, possessing two X chromosomes, normally pass on a single X chromosome in each egg cell.
An egg fertilized by an X-bearing sperm results in a female (XX), whereas an egg fertilized by a Y-bearing sperm results in a male (XY). The X chromosome carries many more genes than the Y chromosome. Genes located on the X chromosomes exhibit sex-linkage, also known as crisscross inheritance.
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What are some conditions that can happen if there are extra or missing sex chromosomes?
Variations in the number of sex chromosomes can lead to several conditions. These conditions include:
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Turner Syndrome: Females with Turner syndrome have only one X chromosome (45,X). This can cause short stature, a webbed neck, and incomplete development of secondary sex characteristics, often leading to infertility.
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Klinefelter Syndrome: Males with Klinefelter syndrome have an extra X chromosome (47,XXY). This condition is associated with increased height and infertility. Partial breast development (gynecomastia) can also occur.
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Trisomy X: Females with trisomy X have an extra X chromosome (47,XXX). This condition is often clinically mild, but some females may experience menstrual irregularities or sterility.
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47,XYY Syndrome: Males with 47,XYY syndrome have an extra Y chromosome. This condition is associated with tall stature, and there may be mild learning disabilities.
These sex chromosome abnormalities arise from unequal sharing of sex chromosomes during meiosis. The symptoms associated with these conditions are generally less severe than those associated with autosomal abnormalities.
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How do sex chromosomes affect traits besides determining biological sex?
Sex chromosomes, such as X and Y in humans, primarily determine an individual's sex, but they also influence other traits. Genes located on the X chromosome, which do not have counterparts on the Y chromosome, lead to what is known as sex-linked inheritance. Because males have only one X chromosome, they are more likely to express recessive traits linked to the X chromosome, such as red-green color blindness and hemophilia.
Females, with two X chromosomes, can be carriers of these traits without expressing them, unless they inherit the affected X chromosome from both parents. Additionally, abnormalities in the number of sex chromosomes can result in conditions like Turner syndrome (45,X) or Klinefelter syndrome (47,XXY), affecting various aspects of development and fertility. Genetic factors on the Y chromosome can also affect behaviors like aggression.
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sex chromosome, either of a pair of chromosomes that determine whether an individual is male or female. The sex chromosomes of human beings and other mammals are designated by scientists as X and Y. In humans the sex chromosomes consist of one pair of the total of 23 pairs of chromosomes. The other 22 pairs of chromosomes are called autosomes.
Individuals having two X chromosomes (XX) are female; individuals having one X chromosome and one Y chromosome (XY) are male. The X chromosome resembles a large autosomal chromosome with a long and a short arm. The Y chromosome has one long arm and a very short second arm. This path to maleness or femaleness originates at the moment of meiosis, when a cell divides to produce gametes, or sex cells having half the normal number of chromosomes. During meiosis the male XY sex-chromosome pair separates and passes on an X or a Y to separate gametes; the result is that one-half of the gametes (sperm) that are formed contains the X chromosome and the other half contains the Y chromosome. The female has two X chromosomes, and all female egg cells normally carry a single X. The eggs fertilized by X-bearing sperm become females (XX), whereas those fertilized by Y-bearing sperm become males (XY).
Unlike the paired autosomes, in which each member normally carries alleles (forms) of the same genes, the paired sex chromosomes do not carry an identical complement of genetic information. The X chromosome, being larger, carries many more genes than does the Y. Traits controlled by genes found only on the X chromosome are said to be sex-linked (see linkage group). Recessive sex-linked traits, such as hemophilia and red–green colour blindness, occur far more frequently in men than in women. This is because the male who inherits the recessive allele on his X chromosome has no allele on his Y chromosome to counteract its effects. The female, on the other hand, must inherit the recessive allele on both of her X chromosomes in order to fully display the trait. A woman who inherits the recessive allele for a sex-linked disorder on one of her X chromosomes may, however, show a limited expression of the trait. The reason for this is that, in each somatic cell of a normal female, one of the X chromosomes is randomly deactivated. This deactivated X chromosome can be seen as a small, dark-staining structure—the Barr body—in the cell nucleus.
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The effects of genes carried only on the Y chromosome are, of course, expressed only in males. Most of these genes are the so-called maleness determiners, which are necessary for development of the testes in the fetus.
Certain differences of sex development (DSDs; formerly called intersex disorders), in which an individual’s anatomical sex and genetic constitution do not match what is considered the typical binary of male or female, involve the sex chromosomes. Turner syndrome, Klinefelter syndrome, and 45,X/46,XY gonadal dysgenesis generally are considered to be examples, although the categorization of these conditions remains controversial. Other categories of DSDs are associated with abnormalities in hormone function, mutations in genes on autosomal chromosomes, and other variations. These categories include 46,XX DSD, involving female virilization (in which a female acquires features associated with male hormones); 46,XY DSD, involving abnormalities in testicular differentiation or testosterone biosynthesis or action; XX, sex reversal, in which the individual is phenotypically male; XY, sex reversal, in which the individual is phenotypically female; and ovotesticular disorder, characterized by the presence of tissues from both the ovaries and the testes.
See also X trisomy; XYY-trisomy.