TEST CROSS

=Test Cross= If an individual displays the dominant characteristic (red feather in this example) they must carry at least one dominant allele (A_). However the other allele may also be the dominant form (A) or it may be the recessive form (a). The easiest way to determine whether this red individual has the genotype AA or Aa is to perform a test cross. This involves crossing the unknown individual (red) with the homozygous recessive one (aa). These individuals can easily be identified by their phenotype as they will have the recessive trait (they are blue). [image:http://i.imgur.com/ZdC7G1N.png] If the unknown individual is heterozygous (Aa) then we would expect roughly half the resulting offspring of a test cross to be red (Aa) and half to be Blue (aa). Alternatively if the individual is homozygous dominant (AA) we would expect all the resulting offspring to be red (Aa). Homozygous dominant individuals (AA) are sometimes called “pure breeding” for this reason. They always produce offspring that also display the dominant characteristic irrespective of the genotype of the other individual. "Pure breeding" can also, but less frequently, be used to describe homozygous recessive (aa) individuals who always produce offspring with the same trait when crossed with each other (aa x aa). To get a reliable result from a test cross you may need to produce a large number of offspring. If you get a few red offspring there is always a small chance the parent is actually heterozygous (Aa) and that as luck would have it was always the dominant allele (A) that had been passed on. However, if a large number of offspring have been produced and they all have the dominant characteristic (red feathers) it is far more likely that the parent is homozygous dominant (AA)