Mendelian ratios express the proportions of different genotypes in the offspring of parents of particular combinations of genotypes.

• What proportions of different genotypes result from a cross between an AA male and an AA female?

• After meiosis, all the males gametes contain the A gene and all the female gametes also contain the A gene. They combine to produce AA offspring. The Mendelian ratio is therefore 100% AA offspring.

• What proportions of different genotypes result from a mating between an AA homozygote and an Aa heterozygote?

Again, all the AA individual's gametes contain a single A gene. When a heterozygote reproduces, half its gametes contain an A gene, and half an a. The pair will produce AA:Aa offspring in a 50:50 ratio.

• What proportions of different genotypes result from a cross between two heterozygotes?

Both male and female produces half a gametes and half A gametes: if we consider the female gametes (eggs, or ovules), half of them are a, and half of them will be fertilized by a sperm, and half by A sperm; the other half are A, and half of them will be fertilized by a sperm and half by A sperm. The resulting ratio of offspring is: 1/4 AA: 1/2 Aa: 1/4 aa.

These ratios follow from Mendel's laws of heredity: the law of segregation and the law of independent assortment.

The Mendelian ratios, in which paired diploid genes segregate into haploid gametes and the gametes of different individuals then combine at random, is the basis of all the theory of population genetics.

Gregor Mendel (1822 - 1884) is pictured opposite.

Do Mendelian ratios work for more than one genetic locus?