What is a Sex-Linked Trait?
Sex-linked traits are inherited via genes located on the sex chromosomes, X or Y, rather than on the autosomes. Because males have only one X chromosome, X-linked recessive traits like red-green color blindness and hemophilia appear far more often in men than in women.
A sex-linked trait is controlled by a gene on the X or Y chromosome, so its inheritance pattern differs between males (XY) and females (XX) — most sex-linked disorders are X-linked recessive and show up more often in males.
- •Gene located on the X chromosome
- •Males (XY) need only one copy to show the trait
- •Females (XX) can be carriers without symptoms
- •Examples: color blindness, hemophilia, Duchenne muscular dystrophy
- •Gene located on the Y chromosome
- •Passed only from father to son
- •Never appears in females
- •Examples: some infertility genes, hairy ear rims
Step-by-step worked examples
A colorblind man (X^cY) and a homozygous normal-vision woman (X^CX^C) have children. What are the possible genotypes of their daughters and sons?
Father contributes X^c or Y. Mother contributes X^C only. Daughters: X^CX^c (all carriers, normal vision) Sons: X^CY (all normal vision)
A carrier woman (X^CX^c) and a normal-vision man (X^CY) have a son. What is the probability he is colorblind?
Mother's eggs: 1/2 X^C, 1/2 X^c Father's sperm: 1/2 X^C (→daughter), 1/2 Y (→son) Sons get Y from father + either X^C or X^c from mother P(colorblind son) = 1/2 (50%)
Hemophilia (X-linked recessive) affects 1 in 10,000 males in a population. Estimate the carrier frequency among females (Hardy-Weinberg-style q).
Male frequency = q (X-linked, one allele) = 1/10,000 = 0.0001 Carrier females ≈ 2pq ≈ 2×0.0001 = 0.0002 (about 1 in 5,000 women)
Flashcards
Quick quiz
Q1.A father is colorblind (X^cY) and the mother has normal vision and is not a carrier (X^CX^C). What fraction of their daughters will be carriers?
Q2.Which chromosome combination makes a male affected by an X-linked recessive trait with just one copy of the allele?
Q3.A carrier mother (X^CX^c) and unaffected father (X^CY) have a son. What is the probability the son is affected?
Q4.Which trait pattern is passed only from father to son?
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Common mistakes
Sex-linked traits always come from the mother. — Correct: Both parents can pass X-linked alleles; fathers pass their single X to all daughters.
A carrier female always shows symptoms. — Correct: Carriers are typically heterozygous and unaffected because the dominant normal allele masks the recessive one.
Sex-linked means the same as sex-limited. — Correct: Sex-linked = gene is ON a sex chromosome; sex-limited = gene is autosomal but expressed only in one sex (e.g., milk production).
Fathers pass X-linked traits to their sons. — Correct: Fathers give sons a Y chromosome, not an X, so sons never inherit their father's X-linked alleles.
FAQ
What is sex-linked inheritance?
It's the inheritance pattern of genes located on the X or Y chromosome, causing traits to appear at different rates in males and females.
What is an example of a sex-linked trait?
Red-green color blindness and hemophilia are classic X-linked recessive examples, appearing far more often in men.
How do you calculate sex-linked trait probabilities?
Use a Punnett square with X and Y chromosomes as alleles, tracking which parent contributes each sex chromosome to sons and daughters.
Why are X-linked recessive traits more common in men?
Men have only one X chromosome, so a single recessive allele is expressed; women need two copies to show the same trait.