Test Cross

A test cross involves mating an unknown genotypic individual with a known homozygous recessive

  • This is because recessive alleles will always be masked by the presence of dominant alleles
  • Hence the phenotype of any offspring will reflect the genotype of the unknown parent

Testing an Unknown Dominant Phenotype

Test crosses can be used to determine whether a dominant phenotype is homozygous or heterozygous

  • If the unknown parent is homozygous dominant, all offspring will express the dominant phenotype
  • If the unknown parent is heterozygous, half the offspring should be dominant and half recessive

test cross 1

Testing for Gene Linkage

Test crosses can also be used to determine if two genes are linked or unlinked by mating with a known heterozygote

  • If there is an equal ratio of the four potential phenotypes, the two genes are likely unlinked (independent assortment)
  • If there are two phenotypes in high amounts and two phenotypes in low amounts (recombinants), the two genes are likely linked
  • A chi-squared test for association can be used to determine the statistical likelihood of each scenario

test cross 2

Test crosses require large numbers of offspring to produce reliable data for meaningful conclusions

  • With the advent of genetic screening and genome mapping, test crosses have become less commonly used