1. TEST CROSS How to tell the difference between homozygous and heterozygous individuals that have dominant phenotypes © 2007 Paul Billiet ODWSODWS

2. Selfing or test cross? Plants can be selfed Homozygous plants (AA) are pure breeding Heterozygotes (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 selfed Animals require a test cross to be carried out (though these days a gene probe is more likely to be used). © 2007 Paul Billiet ODWSODWS

3. Coat colour in mice TraitsAllelesGenotypesPhenotypes GreyGGGGrey WhitegGgGrey ggWhite Grey 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 ODWSODWS

4. Homozygous Grey Phenotypes x White GenotypesGGgg GametesGGgg gg GGg G OffspringGreyWhite 100%0% © 2007 Paul Billiet ODWSODWS

5. Heterozygous Grey Phenotypes x White GenotypesGggg GametesGggg gg GGg ggg OffspringGreyWhite 50% © 2007 Paul Billiet ODWSODWS