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Biology (Year 12)

Monohybrid Cross

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Ben Whitten

A monohybrid cross involves the fusion of gametes from two monohybrids, who are parents consisting of one dominant and one recessive allele, that differ in only one characteristic; i.e. only one gene is being investigated.

A Punnett square can be used to predict the genotypes and phenotypes of particular offspring (and in some cases to make predictions of the genotypes/phenotypes of the parents). There are different steps in working out a monohybrid cross. Context We are investigating the inheritance of one trait; albinism in guinea pigs. If a guinea pig is not an albino, then they are said to be either homozygous dominant (AA) or heterozygous (Aa); this means that albinism is an autosomal recessive trait, meaning that a guinea pig would need two recessive alleles (aa) to be albino. To begin, a key must be formed. In this case, the key would be;

  • A = non-albino

  • a = albino

The cross which will be performed in this case (a monohybrid cross) will be Aa × aa. The Punnett square which is formed is shown below.

Monohybrid cross for albinism

From this cross, we can see the genotype ratio and therefore the phenotype ratio. The genotype ratio is:

2 Aa : 2 aa = 1 Aa : 1 aa


50% Aa : 50% aa The phenotype ratio is:

2 non-albino : 2 albino = 1 non-albino : 1 albino


50% non-albino, 50% albino


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