Download Dihybrid Practice Problems:

Dihybrid Practice Problems:
In mice, the ability to run normally is a dominant trait. Mice with this trait are called
running mice (R). The recessive trait causes mice to run in circles only. Mice with this trait
are called waltzing mice (r). Black hair (B) is dominant over brown hair (b). For each of
the problems complete the following information.
a. Parental genotypes _____________________ X _______________________
b. Gametes for each: ______________________ ,
parent
_______________________
c. Draw the Punnett square. Note the size of the Punnett square is determined by
the number of gametes.
d. Phenotypic ratio: _______________________________
1. Cross a heterozygous black, heterozygous running mouse with a homozygous black,
homozygous running mouse.
2. Cross a homozygous black, homozygous running mouse with a brown, heterozygous
running mouse.
3. Cross a brown, waltzing mouse with a brown, waltzing mouse.
4. Cross a heterozygous black, homozygous running mouse with a brown, waltzing
mouse.
5. Cross a brown, heterozygous running mouse with a homozygous black, heterozygous
running mouse.
Waltzing Mouse video: http://www.youtube.com/watch?v=Y6Q9uJCN-nQ&feature=related
Genetics Practice Problems (These are not dihybrids. All of the crosses below are
monohybrid crosses).
1.
Cystic fibrosis is the most common fatal genetic disorder among Caucasians. Individuals who are
homozygous recessive for the defective cystic fibrosis gene have very thick mucus in their lungs
clogging their airways. Individuals with cystic fibrosis die at a young age, usually before reaching
adulthood. A heterozygous man marries a heterozygous woman. Based on this information,
answer the questions below:
a. Do the man and woman have the disease? Explain
b. What are the chances that their baby will have cystic fibrosis?
c. What are the chances that their baby will be a carrier for cystic fibrosis?
2. Sickle-cell anemia is a genetic disorder that causes red blood cells to be defective. This genetic
disorder is most common in individuals of African descent. If a person is homozygous for the
sickle-cell trait, the wall of their red blood cells are defective. A heterozygous person has some red
blood cells that are defective and some that are normal. A person who is homozygous for the
normal trait has all normal blood cells. If two heterozygous parents have a child, what are the
chances that they will have a baby with all normal red blood cells?
3. About 85% of Americans have the protein called Rh on their blood cells, making them Rh+. The
Rh+ blood type is caused by a dominant gene (B). Homozygous recessive individuals do not have
the Rh protein and have the Rh-blood type. An Rh- (bb) woman marries a heterozygous man (Bb).
Based on this information, answer the questions below.
a. What would the genotypic ratio of their children be?
b. What would the phenotypic ratio of their children be?
4.
Challenge problem: in orchids, red flower color (RR) is incompletely dominant to white (WW).
Heterozygous flowers (RW) are pink. Wild Bill, the rogue botanist, has one pink flower and some
white flowers. He wants to get a pure strain of red orchids. Unfortunately the pure red orchids
(RR) are only found in the wild and it is illegal to collect them. How can Wild Bill get a pure strain
of red orchids without breaking the law?