Human genetics

The essentials

Gregor Mendel, working with pea plants, discovered the basic laws of inheritance (published 1866, rediscovered in 1900). Traits are controlled by genes, which come in alternative forms called alleles. Each person carries two alleles for each autosomal gene, one from each parent.

Key points

• Law of dominance: in a heterozygote, one allele (dominant) masks the other (recessive).
• Law of segregation: the two alleles separate during gamete formation, so each gamete carries one. A monohybrid cross Aa × Aa gives 3:1 phenotypes and 1:2:1 genotypes.
• Law of independent assortment: genes for different traits (on different chromosomes) are inherited independently; a dihybrid cross gives 9:3:3:1.
• Autosomal dominant traits appear in every generation (e.g. Huntington's disease); autosomal recessive traits can skip generations and appear from two carrier parents (e.g. albinism, sickle-cell anaemia, thalassaemia).
• Sex-linked (X-linked recessive) traits such as haemophilia and red–green colour blindness are far more common in males, who have only one X. A carrier mother passes the allele to half her sons on average.
• Y-linked (holandric) traits pass only from father to son.
• Sex-limited traits are expressed in only one sex though genes are in both (e.g. beard growth, milk production).
• Sex-influenced traits are expressed differently in the two sexes, e.g. pattern baldness: dominant in males, recessive in females.
• Monogenic traits are controlled by one gene; polygenic traits by many genes with small additive effects (height, skin colour), giving continuous variation; multifactorial traits result from many genes plus environment (e.g. diabetes, heart disease).

Exam tips

• Draw Punnett squares; work out ratios carefully.
• For X-linked questions, write X and Y separately for each parent.

Common mistakes

• Confusing sex-limited (one sex only) with sex-influenced (both sexes, different expression).