Punnett Square

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A Punnett Square.

Invented by Professor Reginald Crundall Punnett FRS (20 June 1875 - 3 January 1967)

A table used to predict the phenotype of the offspring of two known parent organisms. The punnett square can also be used to carry out a back cross. This is a technique where the phenotype of the offspring is used to determine the genotype of one or both of the parental organisms.

A punnett square can be used in order to assess the probability of offspring inheriting a particular trait from its parents. In general, a capital letter (e.g. A) indicates a dominant allele, whilst a lower case letter (e.g. a) indicates a recessive allele. The assumption is made that the likelihood of inheritance of each allele from each parent will be 0.5. For example, for a parent of genotype Aa, P(A)=0.5 and P(a)=0.5. If a parent is homozygous (AA or aa), the probability of inheriting the allele will be 1[1].


  1. http://anthro.palomar.edu/mendel/mendel_2.htm last accessed: 19/11/2015
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