Introduction
Inheritance in biological anthropology explains how phenotypic and genotypic traits are transmitted across generations. While Gregor Mendel's work in 1866 established the basis of classical monogenic inheritance, human biological variability exhibits both simple single-locus patterns and complex, non-classical transmission mechanisms.
1. Mendelian Traits
Mendelian traits are discrete, qualitative characters governed by alleles at a single gene locus. They strictly adhere to Mendel's Laws of Segregation and Independent Assortment, presenting clear dominant-recessive inheritance that can be mapped predictably using Punnett squares or pedigree analysis.
- Autosomal Dominant Traits: Manifest even in heterozygous condition (Aa). Examples include Huntington's disease, achondroplasia, and brachydactyly.
- Autosomal Recessive Traits: Expressed phenotypically only in homozygous recessive state (aa). Examples include Albinism, Cystic Fibrosis, and Phenylketonuria (PKU).
- Sex-Linked Traits: Loci present on sex chromosomes, such as X-linked Hemophilia and red-green colour blindness.
2. Non-Mendelian Traits
Non-Mendelian traits deviate from classical monogenic phenotypic ratios due to complex multi-allelic, polygenic, or environmental interactions, exhibiting continuous quantitative variation.
- Codominance and Multiple Alleles: More than two alleles exist for a locus, and heterozygous phenotypes express both alleles simultaneously. The human ABO blood group system, discovered by Karl Landsteiner, features three alleles (IA, IB, i), where IA and IB are codominant with respect to each other and both dominant over i.
- Polygenic Inheritance: Quantitative traits regulated by the additive effect of multiple genes across multiple loci. Traits such as human skin pigmentation (demonstrated by Charles Davenport) and stature show continuous variation following a normal distribution curve.
- Pleiotropy: A phenomenon where a single genetic mutation influences multiple unrelated phenotypic systems. For instance, the HbS allele in sickle-cell anemia alters erythrocyte morphology while simultaneously providing heterozygote resistance against Plasmodium falciparum malaria.
- Cytoplasmic/Mitochondrial Inheritance: Maternal transmission of mitochondrial DNA (mtDNA) without paternal contribution, bypassing nuclear segregation. This serves as an indispensable tool for physical anthropologists in reconstructing phylogenetic lineages, such as tracing the universal maternal lineage known as Mitochondrial Eve.
Conclusion
While Mendelian traits are fundamental to clinical genetics and pedigree studies, non-Mendelian inheritance forms the cornerstone of modern physical anthropology, explaining polygenic adaptation, continuous clinal variation, and complex population evolutionary dynamics.