Genetic markers

The essentials

Genetic markers are inherited traits or DNA sequences that vary between individuals and can be easily identified. Because they follow simple inheritance and are not changed by environment, they are used to compare populations, trace migrations, test parentage and identify individuals.

Key points

• ABO blood group (Karl Landsteiner, 1900): controlled by one gene with three alleles, IA, IB and i. IA and IB are codominant; both are dominant over i. Genotypes: A (IAIA or IAi), B (IBIB or IBi), AB (IAIB), O (ii). It is a classic example of multiple alleles and codominance.
• Distribution: blood group B is relatively frequent in India and Central Asia; group O is very common among many Native American groups; group A is relatively frequent in parts of Europe. Frequencies vary gradually from region to region.
• MN blood group: two codominant alleles M and N, giving types M, MN and N.
• Rh system: Rh-positive (D antigen present) is dominant over Rh-negative. An Rh-negative mother carrying an Rh-positive foetus may form antibodies that can harm a later Rh-positive baby (haemolytic disease of the newborn); this is preventable with anti-D injection.
• DNA markers: mitochondrial DNA (inherited only through the mother), Y-chromosome markers (father to son), microsatellites (short tandem repeats) and SNPs (single nucleotide polymorphisms). They help trace human origins and migrations and are used in DNA fingerprinting.

Exam tips

• Practise blood-group crosses; e.g. IAi × IBi can produce all four groups.
• Remember: mtDNA traces maternal lines; Y-DNA traces paternal lines.

Common mistakes

• Thinking a child of two group-O parents can be group A; ii × ii gives only O.
• Assuming the Rh problem arises in the first pregnancy; it usually affects later pregnancies.