Sex-linked inheritance and linkage

Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes. Females have XX, males have XY.

Traits inherited through genes on the sex chromosomes are called sex-linked.

Diseases caused by recessive genes on the X chromosome, color blindness and hemophilia, are more common in males. The reason: the Y chromosome has no allele for them. Hemophilia is inherited as an X-linked recessive trait.

T. Morgan carried out experiments on the fruit fly Drosophila and discovered the law of linkage: genes located on the same chromosome are inherited together.

Linkage is disrupted by crossing over. Crossing over: the exchange of segments between homologous chromosomes in prophase I of meiosis.

Inheritance of sex: mother XX × father XY.

Father's gamete	Mother's gamete X	Mother's gamete X
X	XX (daughter)	XX (daughter)
Y	XY (son)	XY (son)

The ratio is 1:1.

Key facts

• Females are homogametic (XX), males are heterogametic (XY). The sex of a child is determined by the father's gamete.
• XᵈY × XᴰXᴰ: daughters are carriers (XᴰXᵈ), sons are healthy (XᴰY).
• The frequency of crossing over is measured in morgans: 1 morgan = 1% crossing over. The farther apart the genes are, the more crossing over occurs.
• A trait linked to the Y chromosome: hair growth on the ear rim (only from father to son).
• In birds it is the opposite: males are ZZ, females are ZW.