TEST CROSS How to tell the difference between homozygous and heterozygous individuals that have dominant phenotypes.

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TEST CROSS How to tell the difference between homozygous and heterozygous individuals that have dominant phenotypes

Selfing or test cross? Plants can be selfed (crossed with themself!) 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 to determine phenotype © 2007 Paul Billiet ODWS

Coat colour in mice Traits Alleles Genotypes Phenotypes Grey G GG White g Gg gg 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 ODWS

Homozygous Grey White GG G Gg 100% 0% Phenotypes x White Genotypes GG gg Gametes G g Gg Offspring 100% 0% © 2007 Paul Billiet ODWS

Heterozygous Grey White Gg G g 50% Phenotypes x White Genotypes Gg gg Gametes G g Offspring 50% © 2007 Paul Billiet ODWS

Understanding Pedigree Charts