02test Cross
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TEST CROSSHow to tell the difference between homozygous and heterozygous individuals that have dominant phenotypes 2007 Paul Billiet ODWS
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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
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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
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2007 Paul Billiet ODWS
HomozygousGreyPhenotypesxWhiteGenotypesGGgg
GametesGGgg
ggGGgGgGGgGg
OffspringGreyWhite100%0%
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2007 Paul Billiet ODWS
HeterozygousGreyPhenotypesxWhiteGenotypesGggg
GametesGggg
ggGGgGgggggg
OffspringGreyWhite50%50%