Sex chromosomes and X-linked traits

Autosomes come in matching pairs in both sexes. One pair of chromosomes is different: in many species it differs between males and females and decides which sex an individual becomes. Genes on that pair are inherited in their own lopsided way.

Sex chromosomes and how they set sex

Sex chromosomes are a distinctive pair of chromosomes that differ between males and females and determine an individual’s sex. Animals use several systems. Most involve one special pair of chromosomes, but not all do.

A few details make these systems easy to tell apart:

• In the X-Y system, the Y chromosome causes maleness. It carries a gene called SRY that switches development onto the male pathway, so a person with XXY develops as male, and a person with a single X and no Y develops as female.
• In the X-O system, the Y doesn’t matter. A fly with one X and two sets of autosomes (a ratio of 0.5) is male whether or not it has a Y. Two X chromosomes (a ratio of 1.0) make a female.
• In the Z-W system, the pattern is flipped from mammals: the male has two matching chromosomes (ZZ) and the female has two different ones (ZW).
• In the haplodiploid system, there are no sex chromosomes at all. Male bees develop from unfertilized eggs, so they are haploid.

Most flowering plants make male and female parts on the same plant. In the species with separate male and female plants, sex is often set by X and Y chromosomes, with XY males and XX females.

Check yourselfA person has two X chromosomes and one Y. Would you expect that person to develop as male or female?

Male. In mammals, the Y chromosome’s SRY gene switches development onto the male pathway, however many X chromosomes are present.

X-linked genes

In humans, the X chromosome is large and carries far more genes than the small Y. Genes found on the X but not the Y are X-linked genes. Genes found on the Y but not the X are Y-linked, and there are few of them. Any gene found on one sex chromosome but not the other is a sex-linked gene, and in practice the term usually means X-linked.

A male has only one X, so for an X-linked gene he has a single copy. He can’t be homozygous or heterozygous, since those words describe two copies. Instead he is hemizygous: he carries just one copy of the gene.

Why recessive X-linked traits show up in males

Hemophilia A is an X-linked recessive disease in which blood doesn’t clot properly, so even a small injury can cause dangerous bleeding. The recessive allele codes a defective clotting protein. Write the normal X as Xᴴ and the hemophilia X as Xʰ.

Here is a cross between a carrier mother (XᴴXʰ) and an unaffected father (XᴴY):

Half the sons have hemophilia. None of the daughters do, because each daughter also gets her father’s Xᴴ, which masks the recessive allele. Half the daughters are carriers, though, who can pass the allele to their own sons. A son always gets his X from his mother and his Y from his father.

Genes like these also gave the chromosome theory strong support. Breeding experiments with fruit flies showed that an eye-color gene sits on the X chromosome: the trait was inherited exactly the way the X is.

Check yourselfWhy are X-linked recessive diseases more common in males than in females?

A male has only one X, so a single recessive allele produces the disease. A female needs the recessive allele on both of her X chromosomes, which is much less likely.

X-linked or autosomal? Reading the pedigree

An X-linked recessive trait and an autosomal recessive trait can both appear in children of unaffected parents. What sets the X-linked one apart is how it follows the sexes, because a son’s only X comes from his mother.

Check yourselfAn affected woman and an unaffected man have four sons, and two of them are unaffected. Could the trait be X-linked recessive?

No. An affected woman is XʰXʰ, and every son gets his only X from her, so all her sons would be affected. Unaffected sons point to an autosomal recessive trait.