16.2.2—Genetic diagrams

A genetic diagram uses allele symbols, parental genotypes, gametes and a cross to show possible offspring genotypes and phenotypes. Probabilities describe expected proportions over many births, not a fixed order for one family.

Define allele meaning → write parental genotypes → list possible gametes → combine one gamete from each parent → state genotype and phenotype ratios → check that probabilities sum to 1.

The diagram makes segregation and fertilisation explicit. It also reveals whether a conclusion depends on complete dominance, independent assortment or another stated assumption.

A heterozygote cross Aa × Aa gives expected genotypes 1 AA : 2 Aa : 1 aa and a 3:1 phenotype ratio only if A is completely dominant and the loci behave as assumed.

A 3:1 ratio is not universal. Codominance, linkage, lethal alleles or environmental effects change the expected phenotype pattern.