Chapter 13: Extensions and Deviations from Mendelian Genetics

Extensions and Deviations from Mendelian Genetics begins by exploring single-gene extensions, such as multiple alleles, where more than two allelic forms exist within a population, illustrated by the human ABO blood group system and Drosophila eye color loci. The text differentiates between modifications of dominance, specifically incomplete dominance, where heterozygotes display an intermediate phenotype like the coat color of Palomino horses, and codominance, where products of both alleles are simultaneously detected, as seen in MN blood groups. The concept of essential genes is introduced alongside lethal alleles, which can be recessive or dominant and result in organismal death, exemplified by the yellow coat color in mice and Huntington disease in humans. Significant attention is given to the variability of gene expression through the concepts of penetrance (the frequency with which a genotype manifests a phenotype) and expressivity (the degree of phenotypic expression), noting how these are influenced by the genetic background and environmental factors. The chapter details how internal environments, such as age and sex, lead to sex-limited and sex-influenced traits like pattern baldness, while external factors like temperature can alter phenotypes in Siamese cats. Moving beyond single genes, the summary explains maternal effect, where the nuclear genotype of the mother strictly determines the offspring's phenotype, as observed in the shell coiling of Limnaea peregra. Complex gene interactions are analyzed through the lens of epistasis, a phenomenon where one gene masks or modifies the expression of another, leading to modified Mendelian ratios such as 9:3:4 in recessive epistasis (Labrador coat color), 12:3:1 in dominant epistasis (squash fruit color), and 9:7 in duplicate recessive epistasis (sweet pea flower color). The complementation test is described as a method to determine if mutations occur in the same or different genes. Finally, the chapter covers extranuclear inheritance involving mitochondrial and chloroplast genomes, characterizing traits that follow non-Mendelian, uniparental (usually maternal) inheritance patterns, including the poky mutant in Neurospora, human mitochondrial disorders like Leber's hereditary optic neuropathy (LHON), and cytoplasmic male sterility in plants.