Mendelian and non-Mendelian traits. (Answer in 150 words)

- Mendelian and non-Mendelian traits. (Answer in 150 words)
Introduction
A trait is Mendelian when a single nuclear locus, transmitted by segregation and independent assortment, predicts offspring phenotype ratios; much human variation does not behave this way.

Body
1. The Mendelian criterion: one locus, two or more alleles, clear dominance; a heterozygote cross gives 3:1, a testcross 1:1. Pedigrees show autosomal dominant, autosomal recessive or X-linked patterns — achondroplasia, cystic fibrosis, haemophilia A.
2. Departures at the nuclear locus: incomplete dominance and codominance (ABO, sickle-cell trait), multiple alleles, lethal alleles distorting ratios, pleiotropy in phenylketonuria, epistasis in the Bombay phenotype masking ABO, and reduced penetrance.
3. Extra-nuclear and polygenic inheritance: mitochondrial DNA is maternally transmitted, so LHON and MELAS descend only through mothers; imprinting separates Prader-Willi from Angelman; stature, skin colour and blood pressure are polygenic, continuous and not resolvable into ratios.

Conclusion
Mendel's rules remain the base model of transmission, but most anthropologically interesting human traits are quantitative, threshold or extra-nuclear.