Chapter 4: Extensions of Mendelian Genetics

The study of heredity expands beyond Mendel’s initial findings to explore the complexities of allelic relationships and their influence on the physical expression of traits, collectively known as Extensions of Mendelism. Alleles do not always follow simple dominance patterns; for instance, incomplete dominance results in an intermediate phenotype in heterozygotes, as seen in snapdragon flower color, while codominance means both alleles are expressed equally and independently, exemplified by human MN and ABO blood groups. Genes often feature multiple alleles, creating an allelic series with a defined dominance hierarchy, such as the four alleles controlling rabbit coat color. Understanding gene function is crucial, as most genes specify a polypeptide product (which often forms functional enzymes) that impacts the phenotype. Mutations are categorized based on their impact: loss-of-function alleles are typically recessive (e.g., cinnabar in Drosophila), whereas gain-of-function or dominant-negative alleles are dominant (e.g., yellow-lethal in mice or Antennapedia in flies). Geneticists utilize the complementation test to determine if two independent recessive mutations are alleles of the same gene or if they reside in different genes, based on whether a wild-type or mutant phenotype is restored in the hybrid offspring. Phenotypes are heavily influenced by environmental factors, including temperature (e.g., shibire mutation) and diet (e.g., management of PKU), as well as internal biological factors like gender (e.g., pattern baldness). The relationship between genotype and phenotype can be imperfect due to incomplete penetrance (not all individuals with the genotype show the trait, like polydactyly) or variable expressivity (the degree to which the trait is expressed differs). When two or more genes govern a single trait, it involves gene interaction. A key interaction is epistasis, where an allele of one gene masks the effect of another gene, leading to modified Mendelian ratios, which helps geneticists map sequential steps in biochemical pathways (e.g., sweet pea flower pigmentation or squash fruit color). Conversely, pleiotropy describes a single gene influencing multiple, distinct phenotypic traits, as observed in PKU. Finally, the analysis of pedigrees extends to inbreeding, or consanguineous mating, which increases the frequency of homozygotes and can lead to inbreeding depression. The inbreeding coefficient (F) mathematically measures the probability that an individual carries two gene copies that are identical by descent from a common ancestor, providing a tool to quantify the intensity of inbreeding and determine the coefficient of relationship between relatives.