A testcross is a cross between an organism showing the dominant phenotype and a known homozygous recessive organism, i.e. one showing the recessive phenotype. This can be used to work out the genotype of the unknown organism based on the phenotypic ratios of the offspring. For example if there are no offspring expressing the recessive phenotype then the parent must be homozygous dominant. Alternately if there is a mixture of phenotypes within the progeny then the unknown parent must have been heterozygous.
- ↑ Hartl D. and Ruvolo M. (2012) Genetics 8th Edition, London: Jones and Bartlett Learning