1(a) Mendelian and Non-Mendelian Traits (10 Marks)
Introduction
The study of inheritance is central to biological anthropology, as it explains how traits are transmitted across generations. Gregor Johann Mendel (1865), through his experiments on Pisum sativum (pea plant), laid down the Law of Segregation and Law of Independent Assortment, which formed the basis of Mendelian inheritance. Later, the rediscovery of his work in 1900 by Hugo de Vries, Carl Correns and Erich von Tschermak established genetics as a science.
Body
Mendelian Traits
Traits inherited according to Mendel’s laws.
- Discrete Expression – Mendel (1865) demonstrated monogenic inheritance where traits appear in predictable ratios (3:1, 9:3:3:1).
- Sickle Cell Anemia – First described by James Herrick (1910); Linus Pauling (1949) identified its molecular basis. J.B.S. Haldane (1949) showed its role in balanced polymorphism, explaining resistance to malaria in endemic regions.
- Blood Group Systems – Karl Landsteiner (1901) discovered the ABO system, following codominant inheritance. Later, Landsteiner and Wiener (1940) discovered the Rh factor, demonstrating Mendelian dominant–recessive transmission.
- Anthropological Studies – Earnest Hooton (1937) used Mendelian traits like ear lobe attachment in racial classification studies, linking them to human variation.
Non-Mendelian Traits
Inheritance patterns that do not follow Mendel’s ratios.
- Polygenic Inheritance – Ronald A. Fisher (1918) integrated Mendelian principles with biometrics, showing that multiple genes contribute to continuous variation.
- Skin Colour – Explained by Charles Davenport (1913, Heredity in Relation to Eugenics) and refined by Hulse (1962) as a polygenic trait influenced by both genes and environment, studied in anthropology as an adaptation to UV radiation.
- Codominance – ABO blood group (Landsteiner, 1901) where A and B alleles are both expressed in AB phenotype.
- Sickle Cell Trait – Studied by E.A. Beet (1949) and A.C. Allison (1954), demonstrating codominance: heterozygotes express both normal and abnormal hemoglobin.
- Pleiotropy – Antoine-Bernard Marfan (1896) described Marfan syndrome, where one gene affects multiple systems (skeletal, cardiovascular, ocular).
- Mitochondrial Inheritance – Douglas C. Wallace (1980s) demonstrated maternal transmission of mitochondrial DNA, widely used in anthropological genetics for studying human migrations and evolutionary history.
- Epistasis – Term introduced by William Bateson (1909). Example: Bombay Blood Group, discovered in Mumbai by Bhende (1952), where H-gene deficiency masks ABO expression; significant in Indian anthropological studies.
Contemporary Studies
Susan Strome et al. (2024): In their work “Clarifying Mendelian vs non-Mendelian inheritance”, they examined misconceptions about inheritance patterns. They argued that traits such as incomplete dominance, codominance and sex-linked inheritance still obey Mendel’s laws of segregation and independent assortment. They highlighted that these cases only deviate from expected phenotypic ratios and urged educators to improve public understanding of genetics.
Aruna Kawadkar et al. (2022): In their study “Inheritance Pattern and Association Studies of Some Human Mendelian Traits among Different Communities from Nagpur, India”, they investigated traits such as earlobe attachment, tongue rolling, widow’s peak, Morton’s toe, hitchhiker’s thumb, and eye/hair color across Indian communities. Using descriptive and observational methods, they documented how these traits vary in frequency between groups, providing anthropological insight into genetic diversity.
C. Bonilla, Guadalupe Herrera and M. Sans (2023): In their paper “What can Mendelian randomization contribute to biological anthropology?” they introduced Mendelian Randomization (MR) as a methodological innovation for anthropological research, using genetic variants as instrumental variables to strengthen causal inference in environmental adaptation, nutrition and life history theory.
J.B.S. Haldane (1949): “Natural selection maintains sickle-cell heterozygosity where malaria is endemic — nature’s balance between harm and advantage.”
Conclusion
While Mendelian traits highlight simple one-gene–one-character inheritance, non-Mendelian traits reflect the complexity of human genetics, showing how multiple factors interact. Together, they form the foundation of human genetics and physical anthropology, aiding in understanding both normal variation and disease inheritance.
Thinkers Mentioned
- Gregor Mendel
- James Herrick
- Linus Pauling
- J.B.S. Haldane
- Karl Landsteiner
- Ronald A. Fisher
- William Bateson
Key Terms
- Polygenic Inheritance
- Codominance
- Pleiotropy
- Epistasis
- Mitochondrial DNA
- Balanced Polymorphism
