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%
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