Genetics Study Guide

Many observable characteristics, or phenotypic features, in living organisms are not determined by a single gene. Instead, most complex traits, such as human height or skin colour, are influenced by the interaction of multiple genes. This is known as polygenic inheritance. Each individual gene involved in a polygenic trait contributes a small, additive effect to the final phenotype, leading to a continuous range of variation within a population rather than distinct categories. Environmental factors can also interact with these multiple genes, further modifying the expression of the phenotype. This understanding moves beyond simple Mendelian genetics, which often focuses on traits controlled by a single gene with dominant and recessive alleles.

Alleles are distinct versions of a gene that determine specific characteristics. Humans, like many organisms, inherit two alleles for each gene, one from each parent. The combination of these alleles, known as the genotype, dictates the observable characteristic, or phenotype. Differences in inherited traits arise because individuals receive varying combinations of these alleles. For instance, if an individual inherits two identical alleles (homozygous), they will express the trait associated with that allele. If they inherit two different alleles (heterozygous), one allele might be dominant and mask the effect of the recessive allele, or both might contribute, leading to a blended or co-expressed trait. This vast array of allele combinations across a population leads to the diversity of characteristics observed.