AP Syllabus focus:
‘Sex-linked traits are determined by genes on sex chromosomes, producing characteristic inheritance patterns visible in pedigree data.’
Sex-linked inheritance explains why some traits appear more often in one sex than the other. Understanding how alleles on the X chromosome or Y chromosome are transmitted helps you interpret characteristic family patterns.
Chromosomal basis of sex-linked inheritance
Sex chromosomes carry genes not found (or not equally represented) on autosomes.
Human karyotype showing the 22 autosome pairs alongside the sex chromosomes, with an inset comparing XX (typically female) and XY (typically male). This visual reinforces that males and females differ in sex-chromosome composition, which changes how many copies of sex-linked genes they carry and therefore how traits can appear in pedigrees. Source
In many animals, including humans, females are XX and males are XY, so males and females differ in how many copies of sex-linked genes they carry.
Sex-linked trait: A trait determined by a gene located on a sex chromosome (usually X-linked), producing inheritance patterns correlated with biological sex.
X-linked genes: key idea
Most sex-linked traits discussed in AP Biology are X-linked because the X chromosome is large and gene-rich. Because males have only one X, they express whatever allele is present on that X.
Hemizygosity in males: an XY male has only one allele for most X-linked genes, so there is no “second allele” to mask it.
Females (XX): can be homozygous or heterozygous for X-linked alleles; heterozygous individuals may or may not show the trait depending on dominance.
Carrier: A heterozygous individual who has one recessive disease allele (commonly X-linked) but typically shows the dominant phenotype while able to pass the recessive allele to offspring.
Y-linked genes: key idea
Y-linked traits are caused by genes found only on the Y chromosome.
Only males can have Y-linked traits.
Transmission is father to all sons (and to no daughters), if the father is affected.
Recognising characteristic inheritance patterns (pedigree logic)
Sex-linked patterns are “visible in pedigree data” because inheritance depends on which parent carries the allele and the sex of the offspring.
X-linked recessive patterns
X-linked recessive traits are commonly tested because they create strong sex biases.
Pedigree illustrating a typical X-linked recessive inheritance pattern, where affected individuals are disproportionately male and females can appear as carriers. The diagram helps you visually track how an affected father passes his X chromosome to daughters (not sons) and how a carrier mother can produce affected sons. Source
More males affected than females, because a single recessive allele on the X is expressed in males.
No father-to-son transmission for X-linked traits (fathers give sons the Y, not the X).
Affected father: passes his affected X to all daughters (so daughters will be at least carriers if the mother is unaffected).
Carrier mother: can pass the recessive allele to:
sons (who may be affected)
daughters (who may become carriers or, more rarely, affected if they receive another recessive allele)
X-linked dominant patterns
X-linked dominant traits are less common but have distinctive pedigree signatures.
Affected father: transmits the trait to all daughters and no sons.
Affected mother (heterozygous): can transmit the trait to about half of sons and half of daughters.
Often appears in every generation if the allele remains in the family line.
Y-linked patterns
A Y-linked pedigree is typically straightforward:
Only males show the phenotype.
Every affected male has an affected father (unless there is a new mutation).
All sons of an affected male are affected.
Common reasoning pitfalls to avoid
Do not assume “sex-linked” means “sex-limited.” Sex-linked traits depend on chromosomal location (X or Y), not on traits expressed only in one sex due to hormones.
A female can have an X-linked recessive trait, but it generally requires two recessive alleles (one on each X), making it rarer.
A dominant phenotype in a pedigree is not automatically autosomal; check for father-to-son transmission and sex bias before deciding.
FAQ
Look for subtle asymmetries.
X-linked recessive often shows more affected males and no father-to-son transmission.
Autosomal recessive affects both sexes similarly and can show father-to-son transmission.
Small pedigrees can be ambiguous; additional relatives across generations improve confidence.
Pseudoautosomal regions are homologous segments present on both X and Y chromosomes that can recombine.
Genes located there can be inherited more like autosomal genes, so pedigrees may not show classic X-linked or Y-linked patterns.
If one X chromosome is preferentially inactivated in many cells, a heterozygous female may express more of one allele.
This can make carriers show mild symptoms or, rarely, a stronger phenotype than expected.
The Y chromosome contains far fewer genes than the X chromosome and many are involved in sex determination and spermatogenesis.
With fewer functional loci, fewer traits follow strict Y-linked inheritance patterns.
Yes, in limited cases.
Reduced penetrance, variable expressivity, or early lethality in some genotypes can make an allele present but not clearly expressed, creating an apparent “skip” in the pedigree.
Practice Questions
State one reason X-linked recessive traits are more common in males than females, and state one pedigree feature that supports X-linked inheritance. (2 marks)
Males are hemizygous for X-linked genes / only one X allele so recessive allele is expressed (1)
No father-to-son transmission for X-linked traits / affected fathers do not pass the trait to sons (1)
A pedigree shows an affected male, his unaffected partner, and four children: two sons (both unaffected) and two daughters (both unaffected). One daughter later has an affected son with an unaffected male. Using this information, explain why the trait is most consistent with X-linked recessive inheritance. (5 marks)
Affected father passes his X chromosome to all daughters, so daughters can be carriers (1)
Sons receive the father’s Y chromosome, so father-to-son transmission is not expected (1)
Unaffected daughters can still be heterozygous carriers for an X-linked recessive allele (1)
A carrier daughter can have an affected son if she passes the recessive X allele to him (1)
An unaffected male partner contributes a Y to sons, so the son’s phenotype depends on the maternal X (1)
