Test cross

Punnett squares showing typical test crosses and the two potential outcomes. The individual in question may either be heterozygous, in which half the offspring would be heterozygous and half would be homozygous recessive, or homozygous dominant, in which all the offspring would be heterozygous.

Under the law of dominance in genetics, an individual expressing a dominant phenotype could contain either two copies of the dominant allele (homozygous dominant) or one copy of each dominant and recessive allele (heterozygous dominant).[1] By performing a test cross, one can determine whether the individual is heterozygous or homozygous dominant.[1]

In a test cross, the individual in question is bred with another individual that is homozygous for the recessive trait and the offspring of the test cross are examined.[2] Since the homozygous recessive individual can only pass on recessive alleles, the allele the individual in question passes on determines the phenotype of the offspring.[3] Thus, this test yields 2 possible situations:

  1. If any of the offspring produced express the recessive trait, the individual in question is heterozygous for the dominant allele.[1]
  2. If all of the offspring produced express the dominant trait, the individual in question is homozygous for the dominant allele.[1]
  1. ^ a b c d Gai, J.; He, J. (2013), "Test Cross", Brenner's Encyclopedia of Genetics, Elsevier, pp. 49–50, doi:10.1016/b978-0-12-374984-0.01529-1, ISBN 978-0-08-096156-9, retrieved 2020-10-25
  2. ^ Griffiths JF, Gelbart WM, Lewontin RC, Wessler SR, Suzuki DT, Miller JH (2005). Introduction to Genetic Analysis. New York: W.H. Freeman and Co. pp. 34–40, 473–476, 626–629. ISBN 0-7167-4939-4.
  3. ^ Freeman, S; Harrington, M; Sharp, J (2014). "Using a Testcross to Confirm Predictions". Biological Science (Custom Edition for the University of British Columbia). Toronto, Ontario: Pearson Canada. p. 260.