Expert: Walter Hintz - 2/14/2008

Question
This is a bit odd but if a breeder wanted to determine whether the black rabbit (use of only black and white genes, black is dominate) produced homozygous or heterozygous offspring. (all the offspring were black 23 of them).  What procedure would he use and how would it work.  Ive tried the punnet square but im not sure how to explain how it works.

Answer
When considering a dominant trait there is no way to tell if the animal is carrying  a recessive gene except by test crossing.  If two heterozygous black rabbits mate there is a 25% chance of getting a white rabbit but still it all depends upon chance. Obviously if all crosses result in black rabbits it would indicate that the parents are homozygous.  There is a test cross that would might however:  Mating a black and white rabbit.  If any white phenotypes occur then the black parent is heterozygous.