Test cross

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A Punnett square showing a typical test cross

In genetics, a test cross, first introduced by Gregor Mendel, involves the breeding of a dominant trait individual with a recessive individual, in order to determine the zygosity of the former by analyzing proportions of offspring with the recessive phenotype.

The genotype that an offspring has for each of its genes is determined by the allele inherited from its parents. The combination of alleles is a result of the maternal and paternal chromosomes contributed from each gamete at fertilization of that offspring. During meiosis in gametes, homologous chromosomes experience genetic recombination and segregate randomly into haploid daughter cells, each with a unique combination of maternally and paternally coded genes.[1]

Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous.


  1. ^ 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. 
  • Hopson, Janet L.; John H. Postlethwait (2008). Modern Biology. Austin: Holt Rinehart & Winston. ISBN 0-03-036769-7.