TEST CROSSHow to tell the difference between homozygous and heterozygous individuals that have dominant phenotypes 2007 Paul Billiet ODWS

Selfing or test cross?Plants can be selfedHomozygous plants (AA) are pure breedingHeterozygotes (Aa) will give 75% dominant (AA and Aa) and 25% recessive (aa) phenotypes in their offspring

Animals are not usually hermaphroditic so they cannot be selfedAnimals require a test cross to be carried out (though these days a gene probe is more likely to be used).

2007 Paul Billiet ODWS

Coat colour in miceGrey mice could have one of two different genotypes, GG or Gg.If they are crossed with a white mouse (gg) these genotypes will give two different results

2007 Paul Billiet ODWS

TraitsAllelesGenotypesPhenotypesGreyGGGGreyWhitegGgGreyggWhite

2007 Paul Billiet ODWS

HomozygousGreyPhenotypesxWhiteGenotypesGGgg

GametesGGgg

ggGGgGgGGgGg

OffspringGreyWhite100%0%

2007 Paul Billiet ODWS

HeterozygousGreyPhenotypesxWhiteGenotypesGggg

GametesGggg

ggGGgGgggggg

OffspringGreyWhite50%50%