Download Punnet Squares - Practice Problems

Punnet Squares - Practice Problems
Monohybrid Crosses
1. In Elves, curled toes (T) is dominant to flat toes (t). If an Elf that is heterozygous were crossed with an
Elf that is homozygous recessive, what possible offspring could there be in the F1 generation? Show your
work.
2. Achondroplasia, a form of dwarfism, is inherited as a dominant condition (A). What is the probability of
two heterozygous dwarfs having normal height children? Show your work.
3. In guinea pigs, black fur is dominant (B) to white fur (b). A black guinea pig mates with a white guinea
pig and all black offspring are produced. Is the black guinea pig heterozygous or homozygous dominant
(most likely)? Use two Punnet squares to show your answer.
4. If two black guinea pigs mate and produce a liter of 9 black and 3 white offspring, what is the genotype of
the parents? Show your work.
5. Diabetes has been found to be inherited (in some cases) through a recessive allele (d). What is the chance
of a heterozygous father and a diabetic mother having a diabetic child? Show your work.
Dihybrid Crosses
1. In Jimsonweed, purple flower (P) is dominant to white (p), and spiny pods (S) are dominant to smooth (s).
In a cross between a Jimsonweed homozygous for white flowers and spiny pods and one homozygous for
purple flowers and smooth pods, what are all possible phenotypes and genotypes of the offspring? Show
your work.
2. In flying reindeer, being able to fly (F) is dominant to not flying (f). Black noses (B) are dominant to
shiny red noses (b). Rudolph’s father is heterozygous for both traits; his mother is homozygous recessive
for both traits. What is the chance of obtaining a special reindeer like Rudolph (can fly with a red nose)?
3. In humans, normal skin color (A) is dominant over albino (a). A diabetic albino man marries a normal
woman whose mother was an albino and whose father was diabetic. What are the genotypes of the man and
the woman? What proportion of their children would be expected to be both non-diabetic and have normal
color? Show your work.
Other Patterns
Incomplete Dominance
1. Incomplete dominance is seen in snapdragons. The allele that causes red flowers (R) is not completely
dominant over the allele that causes white flowers (W). When a plant is heterozygous for the trait of flower
color (RW), pink flowers result. Cross two pink snapdragons, and provide the genotype and phenotype of all
offspring. Show your work.
2. Using the information from #1, cross a red snapdragon with a pink snapdragon. Show your work.
3. In certain cats, tail length is determined by a gene that demonstrates incomplete dominance. The allele that
causes a long tail (T) is not completely dominant over the allele that causes no tail (t). If a cat is
heterozygous for this trait (Tt), then the cat will have a short tail. What is the probability that the offspring
will be manx cats (no tail) if a short – tailed cat is bred with a manx cat (no tail)?
Codominance
The best example of this is the human blood group. In human blood, 3 alleles exist (IA, IB, and i) which can
result in four different phenotypes (A, B, AB and O). The alleles for A and B exhibit codominance with one
another but are both dominant to i. Being homozygous i (ii) leads to blood type O.
1. A woman homozygous for type B blood marries a man who is heterozygous type A. What will be the
possible genotypes and phenotypes of their children?
2. A man with type O blood marries a woman heterozygous type B blood. What will be the possible
genotypes and phenotypes of their children?
3. A type A woman whose father was type B marries a type B man whose mother was type A. What will be
their children’s possible genotypes and phenotypes?
4. What is the probability that a couple whose blood types are AB and O will have a type A child? Type B?
Type AB? Type O?
Sex – linkage
1. Color blindness is the inability to distinguish between certain colors. Red-green color blindness is caused
by an X-linked recessive allele, Xcb. In order for a female to be color blind, what genotype would she have
to be? What about a colorblind male? Remember, females have the genotype XX and males are XY.
2. What is the probability of a color blind female giving birth to a color blind male?
3. What would be the possible genotypes of the parents of a colorblind female?
4. A normal vision male and a normal vision female have a male child which is colorblind. What are the
genotypes of the parents?
5. Male pattern baldness is a recessive sex-linked trait on the X chromosome (Xb). A woman, whose father
had male pattern baldness, marries a man with this trait. What is the probability that any son born will have
pattern baldness?
6. Hemophilia is a disease caused by a recessive allele (Xh) found on the X chromosome. A normal man
marries a woman that is heterozygous for the trait. Give the genotypes and phenotypes of all possible
offspring.