AP® Biology · Unit 5 · Topic 5.4
Sex-Linked Inheritance and Pedigrees
Genes on the X chromosome show up more often in sons because sons have only one X, so a single recessive allele has no second copy to hide behind. Pedigrees use this pattern to tell X-linked traits from autosomal ones.
A female has two X chromosomes, so a recessive allele on one X can be masked by a dominant allele on the other. A male has only one X (paired with a Y), so whatever allele sits on that lone X gets expressed, dominant or recessive, with nothing to cover it. That is why X-linked recessive conditions like red-green color blindness or hemophilia show up far more often in males: a female needs two copies of the rare allele to be affected, a male needs only one.
This asymmetry comes straight out of meiosis. A mother (XAXa) passes one of her two X chromosomes to each child with equal probability, so each son has a 1 in 2 chance of getting Xa and being affected, and each daughter has a 1 in 2 chance of being a carrier like her mother. A father (XAY) gives his Y chromosome to every son and his X chromosome to every daughter, which is why sons can never inherit an X-linked allele from their father, only from their mother. This is the fact that trips students up most, so it is worth fixing firmly: father-to-son transmission of an X-linked trait is impossible.
Reading a pedigree for X-linkage means looking for this exact signature: the trait skips generations by passing through unaffected carrier mothers, affects far more males than females, and never goes from an affected father to an affected son. Autosomal traits show no such sex bias and can pass father to son freely. This diagnostic reading connects directly to chi-square testing, where observed ratios in real pedigrees or crosses get checked against the 1:1 or 3:1 expectations that sex-linked and autosomal inheritance predict.
Key terms
- X-linked trait
- A trait controlled by a gene located on the X chromosome, giving it different inheritance patterns in males and females.
- Carrier
- A female who has one copy of a recessive X-linked allele and one dominant copy, so she is unaffected but can pass the allele on.
- Hemizygous
- Having only one copy of a gene instead of two, as males do for genes on the X chromosome.
- Pedigree
- A diagram tracking a trait through generations of a family, used to infer its pattern of inheritance.
Watch it explained
- How it worksHow does having only one X chromosome expose a recessive allele with nothing to mask it?
- How it worksWalk me through a cross between a carrier mother and an unaffected father for an X-linked trait.
- How it worksWalk me through reading a pedigree to decide whether a trait is autosomal or X-linked.
- Worked exampleFind the chance a son is affected when the mother is a carrier and the father is not affected.
- Common mistakeWhy doesn't a father ever pass an X-linked allele to his son?
Check yourself
End of topic.