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One problem that must be solved is how to tell the difference between homozygous and heterozygous individuals that have dominant phenotypes.
Plants could be selfed. Homozygous plants are pure breeding but heterozygotes will give 75% dominant and 25% recessive 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).
Example: Character: Coat colour in mice
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.
A test cross is also used to determine the proportions of gametes carrying different alleles.
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