Mendelian and non-Mendelian traits.

Mendelian traits in humans

A Mendelian trait is usually controlled by alleles at one locus. Those alleles separate when gametes form, so a clear pattern appears in a pedigree. Dominance, recessivity and sex-linkage change how the trait looks, but they do not cancel particulate inheritance.

• Gregor Mendel showed this with peas. William Bateson and Archibald Garrod then applied the same logic to humans, including inborn errors of metabolism.
• ABO blood groups, described by Karl Landsteiner, are a standard anthropological example. There are three alleles at one locus (I^A, I^B and i). A and B are codominant, and O is recessive, but transmission is still one-locus Mendelism.
• PTC tasting, oculocutaneous albinism, Huntington disease (autosomal dominant) and haemophilia A (X-linked recessive, famous in European royal pedigrees) are further human examples.
• Tay–Sachs disease is still Mendelian even though the homozygous child often does not reproduce. The locus follows Mendel; the fitness of the genotype does not.

Family study and Hardy–Weinberg teaching both start from this single-locus picture.

Non-Mendelian traits

Most characters that anthropologists actually measure are not garden peas.

• Polygenic and multifactorial traits: Stature and skin colour are the classic anthropological examples. Many genes of small effect, plus sunlight, diet and disease, produce a continuous curve rather than two neat classes. R. A. Fisher and later quantitative genetics explained this as many Mendelian loci acting together, which is why skin colour must never be taught as a 3:1 “racial” mark.
• Linkage: Genes on the same chromosome travel together until recombination. They do not assort independently.
• Mitochondrial inheritance: mtDNA comes from the mother. A father with a mitochondrial disorder does not pass it to his children. Leber hereditary optic neuropathy is a clinical example.
• Imprinting and epistasis: A gene may be silenced according to which parent it came from, or one locus may hide another. Simple dominance arithmetic then fails.
• Sickle-cell: The locus is Mendelian, but the heterozygote has a different cellular picture from either homozygote. The inheritance is Mendelian; the phenotype is not a simple dominant–recessive class.

Why the distinction matters

Mendelian analysis is the right tool for blood groups, many clinical loci and genetic counselling. Non-Mendelian architecture explains everyday human variation, complex disease and the old misuse of “race”. Single-factor, multifactor, lethal and polygenic inheritance are two readings of the same genome.