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.
Practice Questions
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.
