Genetic crosses
There are many ways to calculate the ratios and probabilities of genetic crosses. For a simple autosomal single cross a punnett square is the standard approach when calculating the ratios. For a double cross or linked cross a forked line approach is better. This is based on possible alternatives rather than all fertilisation events.
Punnet squares may be used to determine potential genotypic and phenotypic ratios of offspring from parental genotypes.
Gregor Mendel first discovered the importance of punnet squares with the use of round or wrinkled seeds and yellow or green seeds of peas. From this, he was able to determine the importance of dominant and recessive alleles in genetics.
There are a number of prominent crosses which result in typical genotypic and phenotypic ratios, e.g. a homozygous dominant and a homozygous recessive cross would result in 100% of the offspring being heterozygous for the gene, however presenting the phenotypes for the dominant gene [1].
Another typical cross is a dihybrid cross which results in a 9:3:3:1 phenotypic ratio. Mendel represented this cross using peas.
A round yellow seed (WWGG) was crossed with a wrinkled green seed (wwgg) which results in the gametes WG and wg. The F1 progeny is then a round yellow seed (WwGg).
The resulting gametes from this are WG, Wg, wG and wg which are crossed and this results in 16 possible genotypes in the F2 progeny.
The genotypes of the F2 progeny are 1x WWGG, 2x WWGg, 2x WwGG, 4x WwGg, 1x wwGG, 2x wwGg, 1x WWgg, 2x Wwgg and 1x wwgg, which then results in a 9:3:3:1 phenotypic ratio with 9 round and yellow seeds, 3 wrinkled and yellow seeds, 3 round and green seeds and 1 wrinkled and green seed[2].