A test cross is when you breed an individual of an unknown genotype with a homzoygous recessive individual (rr). You are trying to find out whether the unknown individual is RR or Rr because it displays the dominant phenotype. By carrying out many test crosses over a long period of time then you can use the offspring of the unknown individual to determine its phenotype. If any offspring display the recessive phenotype, then the unknown parent individual must be Rr, as both parents must have passed on a recessive allele for an rr offspring to be produced. If, over a long period of time, no recessive offspring have been produced, then the unknown parent individual is most likely to be RR. However, because the allele the parent passes on is random, it is possible that for every offspring produced its recessive allele was never randomly selected and passed on but if a lot of test crosses have been done, this is unlikely. The most important thing is that you can't be 100% sure of the unknown individuals genotype if no recessive offspring are produced. Here is a picture of the two punnet squares you would use (comparing the offspring of an RR and Rr individuals)
Thank you so much
Hi Maryangel!
A test cross is when you breed an individual of an unknown genotype with a homzoygous recessive individual (rr). You are trying to find out whether the unknown individual is RR or Rr because it displays the dominant phenotype. By carrying out many test crosses over a long period of time then you can use the offspring of the unknown individual to determine its phenotype. If any offspring display the recessive phenotype, then the unknown parent individual must be Rr, as both parents must have passed on a recessive allele for an rr offspring to be produced. If, over a long period of time, no recessive offspring have been produced, then the unknown parent individual is most likely to be RR. However, because the allele the parent passes on is random, it is possible that for every offspring produced its recessive allele was never randomly selected and passed on but if a lot of test crosses have been done, this is unlikely. The most important thing is that you can't be 100% sure of the unknown individuals genotype if no recessive offspring are produced. Here is a picture of the two punnet squares you would use (comparing the offspring of an RR and Rr individuals)
Hope this helps!