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```Complex Punnett Square Problems
Part I: Complete the following problems. List the parent genotypes, draw and fill in a
Punnett square, and then list the offspring genotypes and phenotypes.
1. A homozygous dominant brown mouse is crossed with a heterozygous brown
mouse (tan is the recessive color).
2. Two heterozygous red flowers (white flowers are recessive) are crossed.
3. A homozygous tall plant is crossed with a heterozygous tall plant (short is the
recessive size).
4. A heterozygous white rabbit is crossed with a homozygous black rabbit.
Part II: Working Backwards
Sometimes we only know about the offspring and we want to learn about the parents.
You should have noticed some things that will help you figure this out. For example,
when both parents are heterozygous the phenotypic ratio always comes out 3 to 1. If
one parents is homozygous recessive and the other is heterozygous, the ratio always
comes out 1 to 1. Keep this in mind when solving these problems.
5. In pea plants, yellow seeds are dominant to green seeds. A pea plant with yellow
seeds is crossed with a pea plant with green seeds. The resulting offspring have
about equal numbers of yellow and green seeded plants. What are the genotypes
of the parents? Make a Punnett square to prove it.
6. In another cross, a yellow seeded plant was crossed with another yellow seeded
plant and it produced offspring which were 25% green. What are the genotypes of
the parents? Make a Punnett square to prove it.
7. In humans, brown eyes are dominant to light eyes (blue or green). Make a Punnett
square to show how two brown eyed parents could have a blue eyed child.
Part III: Test Cross
When an organism has the dominant phenotype, its genotype can be either
heterozygous or homozygous dominant, you can’t tell just by looking at it. We call
the heterozygous individual a carrier because he is carrying the recessive trait but you
cannot tell by looking at him, and he can pass that onto his offspring. This is why
recessive traits often appear to skip generations.
In order to find out if the individual is homozygous or heterozygous, we must do a
test cross using a homozygous recessive individual.
8. You find a wild, black mouse. Explain how you would determine the genotype of
this mouse by drawing two Punnett squares. (black fur is dominant to white fur).
9. You have 24 offspring, 23 with black fur and 1 with white fur. What was the
genotype of the wild mouse?
10. If you have only 3 offspring and they are all black, can you tell what the
genotype of the mouse is? Explain why or why not.
Part IV: Incomplete Dominance
In Four o’clock flowers, the red and white alleles for flower color are both equal,
therefore neither dominates over the other color in the heterozygous individual. We
call this condition incomplete dominance because you have a blending of the two
alleles to create a third color, pink. The genotype for a red flower will be RR, the
genotype for a white flower will be rr and the genotype for a pink flower will be Rr.
11. Predict the offspring if you crossed a red Four o’clock flower with a pink flower.
12. Predict the offspring if you crossed two pink Four o’clock flowers.
Part V: Codominance and Multiple Alleles
Some traits are coded for by multiple alleles and both are present at the same time.
Unlike incomplete dominance, there is not any blending, the two alleles are actually
both seen together and therefor we will use different letters for the different alleles. In
humans, there are four types of blood: type A, type B, type AB, and type O. The
alleles A and B are codominant to each other and the O allele is recessive to both. So
a person with the genotype AA or AO will have type A blood.
a. What possible genotypes will produce type A blood? __________________
b. What possible genotypes will produce type B blood? __________________
c. What possible genotype will produce type O blood? __________________
d. What possible genotype will produce type AB blood? __________________
13. You are type O and marry a person with type AB. Complete a Punnett square for
this cross and list the possible genotypes and phenotypes of your offspring.
14. In the 1950’s, a young woman sued film star Charlie Chaplin for parental support
of her illegitimate child. Charlie Chaplin’s blood type was already on record as
type AB. The mother of the child had type A and her son had type O.
a. Complete a Punnett square for the possible cross of Charlie and the
mother.
b. The judge ruled in favor of the mother and ordered Charlie to pay child
support. Was the judge correct in his decisions based on blood typing
evidence, explain why or why not?
15. Suppose a newborn baby was accidentally mixed up in the hospital. In an effort to
determine the parents of the baby, the blood types of the baby and two sets of parents
were determined.
Baby – type O
Mrs. Brown – type B Mr. Brown – type AB
Mrs. Smith – type B Mr. Smith – type B
a. Draw a Punnett square for each couple, you might need more than one square
for each couple.
b. To which parents does the baby belong?
**Bonus**
If you know your blood type, use that information and what you know
about your parent’s blood types to determine for a fact the blood
In humans sex is determine by the twenty third pair of chromosomes known as “sex
chromosomes”. If you have two x-shaped (XX) chromosomes you are destined to be
a female. If you have an x and a Y-shaped (XY) chromosomes you are destined to be
a male. Since the X and Y chromosomes carry different information, any genes found
on the X chromosomes are referred to as sex-linked genes. Therefore, women will
have two alleles for these genes because they have two (XX) chromosomes. On the
other hand, men have only one allele for each of these genes because they have only
one X chromosome (XY).
16. Hemophilia is a sex-linked trait. A person with hemophilia is lacking certain proteins
that are necessary for normal blood clotting. Hemophilia is caused by a recessive allele so
use “N” for normal and “n” for hemophilia. Since hemophilia is sex-linked, a woman will
have two alleles (NN or Nn or nn) but a man will have only one allele (N or n).
A woman who is heterozygous (a carrier) for hemophilia marries a normal man:
a. What are the genotypes of the parents?
b. Make a Punnett square for this cross.
c. What is the probability that a male will have hemophilia?
d. What is the probability that a female will have hemophilia?
17. Can a color blind female have a son that has normal vision? Color blindness is caused
by a sex-linked recessive allele. Use N = normal vision and n = color blind
18. Use what you have learned to explain how certain traits appear to skip generations.
19. Use what you have learned to explain why men are more affected by sex-linked traits
than women.
20. What is the difference between regular dominance, co-dominance and incomplete
dominance?
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