In Chromosomes and Their Numbers you met the X and Y chromosomes. XX individuals are female and XY individuals are male. The X carries many genes that have nothing to do with being male or female; the much smaller Y carries few. Genes on the X are called X-linked.
Key idea
An XX individual has two copies of each X-linked gene; an XY individual has one. So one recessive X-linked allele shows in an XY individual. Write the alleles on the X: XA, Xa. The Y has no allele for these genes.
Why it works
A son gets his Y from his father, so his only X comes from his mother. A daughter gets one X from each parent. That is why X-linked recessive traits, like red-green color blindness, are much more common in males: one recessive allele is enough. A female needs two, and a female with one is a carrier with normal vision.
Worked example: A carrier mother
XAXa × XAY (a = color blindness). What are the chances for sons and for daughters?
- Mother's gametes: XA or Xa. Father's: XA or Y.
- Daughters get the father's XA: XAXA or XAXa. All have normal vision; half are carriers.
- Sons get the father's Y: XAY or XaY. Half are color blind.
Answer: Daughters: none affected (½ carriers). Sons: ½ color blind.
Worked example: Two ways to ask
In the same cross, what is the chance that a child is a color-blind son?
- Of the four equally likely boxes, only XaY is a color-blind son.
- Compare: "the chance a son is color blind" counts only the two son boxes, so it is ½.
Answer: ¼ (one box of four).
Worked example: A color-blind father
XAXA × XaY. Can a son be color blind?
- Sons get their X from the mother, who has only XA.
- Daughters get the father's Xa, so they are all carriers.
Answer: No son is color blind; every daughter is a carrier.
Common mistake
Two slips. One is giving the Y chromosome an allele (XAYa): the Y has no copy of X-linked genes. The other is thinking an X-linked trait passes from father to son. A father gives his son a Y, never his X, so he cannot pass an X-linked allele to a son.