Chapter 4: Extensions of Mendelian Genetics

Allelic variations at a single locus are explored first, including incomplete or partial dominance, where heterozygotes display a blended, intermediate phenotype (e.g., pink snapdragons) leading to identical 1:2:1 genotypic and phenotypic ratios. In contrast, codominance describes the distinct, simultaneous expression of both alleles in the heterozygote (e.g., the MN blood group). The existence of multiple alleles in a population, such as those controlling the human ABO blood type, introduces complex dominance hierarchies. Essential genes are linked to lethal alleles, which often act recessively to cause death (e.g., the yellow coat allele in mice is dominant for color but recessive lethal for survival), thereby modifying expected Mendelian ratios. Moving beyond single-gene effects, the text emphasizes gene interaction and epigenesis, where multiple gene pairs influence a single characteristic. A prime manifestation is epistasis, where one gene masks or modifies another, generating various modified F2 ratios typically expressed in sixteenths, such as the 9:3:4 ratio (recessive epistasis, e.g., mouse coat color) or the 12:3:1 ratio (dominant epistasis, e.g., squash fruit color). Complementation analysis provides a tool to determine if two mutations causing a similar phenotype are alleles of the same gene or belong to different genes. Conversely, pleiotropy occurs when a single gene mutation results in multiple, seemingly unrelated phenotypic effects throughout the organism (e.g., Marfan syndrome). Sex-related inheritance patterns are detailed, including X-linkage for genes lacking counterparts on the Y chromosome, rendering males hemizygous and leading to characteristic crisscross inheritance (e.g., human color blindness). Additionally, autosomal genes may display sex-limited inheritance (expression confined entirely to one sex, e.g., cock feathering) or sex-influenced inheritance (heterozygous phenotype differs between sexes, e.g., pattern baldness). The final phenotype is recognized as a product of genotype-environment interaction, modulated by penetrance (the frequency of expression) and expressivity (the degree or range of expression). Other influential factors include environmental conditions (e.g., temperature-sensitive conditional mutations), position effects on gene location, and genetic anticipation, where age of onset decreases and severity increases in subsequent generations due to unstable DNA repeats (e.g., Myotonic dystrophy).