A level Biology exam revision resources written by A level Examiners


The genetic crosses up to this point have looked at characteristics where there are two forms of the gene (alleles) present. Some characteristics are controlled by genes which have more than two forms. The clearest example of this is blood groups in humans. The characteristic of blood group is determined by a gene which is denoted by a capital letter I. There are three alleles of this gene which are denoted using capitals A, B and O.

Blood group determination is very interesting because alleles A and B are codominant and allele O is recessive. The pattern of genotypes and phenotypes is shown in the table 2, below.

Table 2 : The pattern of genotypes and phenotypes
The pattern of genotypes and phenotypes

One example of a genetic cross involving blood group inheritance is shown in the diagram below.


* Note it is normal to always write in dominant alleles before recessive.

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