3.4: Sex linkage review

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Key terms

Term	Meaning
Sex chromosome	One of two chromosomes that determines an organism's biological sex
Autosome	Chromosome that is not a sex chromosome
Sex-linked gene	Gene that is located on one of the two sex chromosomes
Carrier	Heterozygous individual that inherited a recessive allele for a genetic disorder but does not display symptoms of that disorder
Barr body	A condensed region in the nucleus of a cell, consisting of an inactivated X chromosome
Aneuploidy	Condition of having too many or too few chromosomes

Sex linkage

In humans, biological sex is determined by a pair of sex chromosomes: XX in females and XY in males. The other 44 chromosomes are autosomes.

Genes on either the X or Y chromosome are sex-linked traits. Genes found on the X chromosome can be found in either males or females, while genes found on the Y chromosome can only be found in males.

X-linked inheritance

There are many more X-linked traits than Y-linked traits because the Y chromosome is much shorter and fewer genes than the X chromosome.

Diagram of the human X and Y chromosomes. The X is much larger than the Y. The X and Y have small regions of homology at both tips, which allow pairing of the chromosomes during meiosis. The SRY gene is found on the Y chromosome, near the tip, just below the region of homology with the X chromosome.

X-linked genes have distinctive inheritance patterns because they are present in different numbers in females (XX) and males (XY).

Females have two X chromosomes, so she will have two copies of each X-linked gene, giving her the opportunity to be either homozygous or heterozygous for each sex-linked gene.

This illustration shows a Punnett square analysis of fruit fly eye color, which is a sex-linked trait. A red-eyed male fruit fly with the genotype X^{w}Y is crossed with a white-eyed female fruit fly with the genotype X^{w}X^{w}. All of the female offspring acquire a dominant W allele from the father and a recessive w allele from the mother, and are therefore heterozygous dominant with red eye color. All of the male offspring acquire a recessive w allele from the mother and a Y chromosome from the father and are therefore hemizygous recessive with white eye color. — Image from OpenStax, CC BY 4.0

X-linked disorders

X-linked human genetic disorders are much more common in males than in females. Since males only have one X chromosome, and therefore one copy of any X-linked genes, whatever allele the male inherits for an X-linked gene will be expressed.

An example of this is the blood-clotting disorder, hemophilia. Women who are heterozygous for hemophilia are carriers, and they usually don't display any symptoms themselves.

Sons of these women have a 550, percent chance of having hemophilia. Daughters have little chance of having hemophilia (unless the father also has it), and will instead have a 550, percent chance of being carriers.

A diagram shows an unaffected father with a dominant allele and an unaffected carrier mother with an x-linked recessive allele. Four figures of offspring are shown representing the various resulting genetic combinations: unaffected son, unaffected daughter, affected son, and unaffected carrier daughter. — Image from OpenStax, CC BY 4.0

X-inactivation

If males can survive with only one X chromosome, why doesn't it cause problems for women who have two X chromosomes?

As it turns out, for females, most of the genes in one of the X chromosomes is inactivated, forming a Barr body. This inactivation happens randomly during embryonic development.

Example:

A common example of X-inactivation is seen in cats. If a female cat is heterozygous for black and tan alleles of a coat color gene found on the X, two Xs (and thus, the two alleles of the coat color gene) will be inactivated at random in different cells during development.

Image of a tortoiseshell cat, illustrating the X-inactivation processes responsible for the different patches of color on its coat. The cat has a mix of black and tan patches of fur, some small and some large. The cat's genotype is $\text X^O\text X^o$, where the large _O_ stands for organge and the small _o_ stands for black. * The orange patch is made up of cells in which the X with the orange allele ($\text X^O$) is active, while the X with the black allele ($\text X^o$) is compacted into a Barr body.* The black patch is made up of cells in which the X with the black allele ($\text X^o$) is active, while the X with the orange allele ($\text X^O$) is compacted into a Barr body. — Image modified from Wikimedia, Public domain.

The result of this is a tortoiseshell coat pattern, made up of alternating patches of black and tan fur.

Sex chromosome aneuploidy

Aneuploidy, or disorders of chromosome number, are generally caused by nondisjunction. This occurs when pairs of homologous chromosomes or sister chromatids fail to separate during cell division.

Diagram depicting nondisjunction in meioisis I. One pair of homologous chromosomes fail to separate during meiosis I, leading to two abnormal cells as products of meiosis I: one cell with an extra chromosome and one with a missing chromosome. In meiosis II, the chromatid of the chromosomes are separated normally. This leads to production of two gametes with an extra chromosome (n+1 gametes) and two gametes with a missing chromosome (n-1 gametes).

Diagram depicting nondisjunction in meioisis II. Homologous chromosomes separate normally during meiosis I. However, the sister chromatids of one chromosome fail to separate during meiosis II, and instead move to the same pole of the cell and are segregated into the same gamete. In this case, the products of meiosis are two normal, euploid gametes (n), one gamete with an extra chromosome (n+1), and one gamete with a missing chromosome (n-1).

Individuals that have autosomal aneuploidy rarely survive to birth. However, due to the size of the X chromosome and because of X-inactivation, X chromosome aneuploidies tend to be much less harmful.

Diagram showing sex chromosomes and Barr body formation in human individuals with different sex chromosome genotypes. XX female: one active X, one Barr body. XY male: one active X, one Y, no Barr body. XXY male (Klinefelter syndrome): one active X, one Y, one Barr body. XXX female (triple X syndrome): one active X, two Barr bodies.

In Klinefelter syndrome, males have one or more extra X chromosomes, leading to a genotype of XXY. (Or in rare cases, XXXY or XXXXY!) Affected men may be infertile or develop less dense body and facial hair than other men.

Women affected with Triple X syndrome have an XXX genotype. Women with Triple X syndrome have female sex characteristics and are fertile (able to have children).

Women with Turner syndrome lack part or all of one of their X chromosomes (leaving her with just one functional X). People with this disorder develop as females, but often have short stature and may experience infertility and learning difficulties.

Common mistakes and misconceptions

Some people think that a recessive X-linked trait will show up more often in women because they have two X chromosomes. However, women are less likely to express recessive X-linked traits because there is potential for a "good" allele to mask a "bad" allele. On the other hand, if a male receives a "bad" allele from his mother, he has no chance of getting a "good" allele from his father (who provides a Y) to hide the bad one.
Codominance and X-inactivation are not the same. Although these two concepts may result in similar looking organisms, a heterozygous individual expressing a codominant trait will express both alleles fully and separately.

In X-inactivation, females express only one X chromosome in each cell, meaning that genes on the X chromosome are expressed singly instead of in a pair. Because the inactivated X chromosome is not the same in every cell, neighboring cells may express different proteins if different X chromosomes carry different alleles.

Contributors and Attributions

Khan Academy (CC BY-NC-SA 3.0; All Khan Academy content is available for free at www.khanacademy.org)

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