Genetics study guide

Monohybrid inheritance describes the pattern of inheritance of a single characteristic, or trait, that is determined by alleles at one gene locus. These traits are typically controlled by two alleles: a dominant allele and a recessive allele. Dominant alleles are expressed in the phenotype even if only one copy is present, while recessive alleles are only expressed when two copies are present. Genetic diagrams and Punnett squares are tools used to predict the genotypes and phenotypes of offspring from a genetic cross. A genetic diagram typically outlines the parental genotypes, the gametes they produce, and the possible offspring genotypes and phenotypes. Punnett squares provide a visual grid for combining gametes systematically. Family pedigrees are charts that track the inheritance of a trait through multiple generations within a family, using standardised symbols to represent individuals and their relatedness, as well as the presence or absence of the trait. They help to determine the mode of inheritance and predict the likelihood of the trait appearing in future generations.

Many of our observable characteristics, known as phenotypic features, are not determined by a single gene. Instead, they arise from the complex interplay of multiple genes, often in combination with environmental factors. This phenomenon is called polygenic inheritance or multifactorial inheritance. For example, human height, skin colour, and intelligence are influenced by numerous genes, each contributing a small effect, rather than being controlled by just one specific gene. This contrasts with single gene inheritance, where one gene pair largely dictates a specific trait, such as in genetic disorders like cystic fibrosis or Huntington's disease.