Genetic crosses

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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].

The Affect of Sex linked chromosomes in a Genetic Cross:

The most simple crosses deal with only two alleles for the same gene, thus resulting in usually a dominant and rececssive relationship, or a codominat relationship etc. However the cross of two alleles for a gene which is linked to a sex chromosome causes results which do not follow Mendel Rules. 

The best know example would be Hemophilia - the failure to form a blood clot to seal a internal or external wound. This gene has two allele XH and Xh. The dominant gene XH codes for a normal blood clot formation whereas Xh code for hemophilia(Brown, 2014). The 'X' in both represents the sex chromosome the gene is located on. Therefore this is a X linked gene. As a result son are twice more likely to suffer from hemophila than girls as they only have one X chromosomes.

If the mother is Xh Xh, a sufferer, and the father is XH Y , normal, all their sons will suffer from Hemophilia but none of their daugters will.

If the nother is a carrier, Xh XH, and the father is normal, XH Y, then their sons will have a 50% chance of contracting Hemophilia but again none of the daughter will.

These results are becasue the mother has the disease gene and will give her X chromosome to the son as the father must give the Y chromosome to have a baby boy as a  results the son has to express the genes on the single X chromosome he has. As such the father must give a X for there to be a baby girl to be born thus giving the daughter  the dominant gene making the daughters carriers instead of sufferers.


  1. Hartl, D.Ll and Ruvolo. M. 2012. Genetics; analysis of genes and genomes. Jones and Bartlett Learning. Page 83-86
  2. Hartl, D.Ll and Ruvolo. M. 2012. Genetics; analysis of genes and genomes. Jones and Bartlett Learning. Page 91

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