Name:
Test Crosses: When an organism has the dominant phenotype but the genotype is unknown (is
the organisms homozygous dominant or heterozygous?) a test cross is sometimes performed. By
crossing the organism with a homozygous recessive organism and looking at the outcome of the
offspring (phenotype) the genotype of the parent organism can be determined.
Problem 1: A florist knows the up-coming holiday will put a large demand on red roses (R). He
wants to make sure that his Red rose plant is PURE so that he doesn’t get a lot of white flowers
(r) when he pollinates the plant.
a. What possible genotypes might his red plant be?
b. What genotype should he cross his rose with to identify its genotype?
c. Draw out BOTH possible punnett squares below:
d. By looking at the offspring of the test cross, how will he tell if his plant is pure?
Problem 2: A dog breeder knows he can sell a beagle that is PURE for a “slim neck” for more
than he can sell a hybrid. A hybrid has a chance of producing the recessive “thick neck” beagle,
which is considered undesirable. He crosses his “slim neck” beagle with a “thick neck” beagle
and 8 puppies are born. 5 are “slim neck” and 3 are “thick neck”.
a. Can he determine if his beagle is pure or not?
b. If so, how? If not, why not?
Problem 3: A farmer wants to produce all yellow corn. He knows that sometimes corn comes
out yellow, sometimes it comes out white and sometimes it comes out with both yellow and
white kernels. He has heard that test crosses can help his case. To make sure that his plant can
only produce yellow kernels, he crosses his yellow plant with a yellow/white plant. He gets a
mixture of yellow and yellow/white offspring. He decides his plant is NOT pure for the yellow
allele.
a. Did the farmer have to perform a test cross to determine the genotype of
his corn? Explain?