Bio A – Mendelian Genetics
Name:
Extension Genetics Practice problems:
ALL parts must be completed. Zero credit is given for the “answer” alone.
Part I: Incomplete dominance
Problem 1: In Japanese four o’clock flowers, color is inherited by genes that show incomplete
dominance. In such flowers, a cross between a homozygous red (R) flower and a
homozygous white (W) flower will always result in pink flowers. A cross is made between
two pink flowers. What is the probability for each phenotype?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Problem 2: In chickens, feather color is incompletely dominant. A white rooster mated with a
black hen creates chicks with grey feathers. What phenotypic AND genotypic ratios of the
offspring would you expect from the mating of a grey rooster and a black hen?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Draw a picture of the result using colored pencils
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Bio A – Mendelian Genetics
Name:
Problem 3: Curly hair is not completely dominant to straight hair. If a female who has curly
hair and a male who has straight hair were to reproduce, what would be the genotypes and
phenotypes of the offspring?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Part II: Co-dominance
Problem 1: In shorthorn cattle, the hybrid between red and white is roan. Roan means that the
animal has BOTH red hairs AND white hairs. If a roan is bred with a white, what will the
phenotypic ratio be?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
Draw a picture of the result using colored pencils:
c. Answer:
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Bio A – Mendelian Genetics
Name:
Problem 2: Blood type in humans is determined by the presence of 2 of 3 possible alleles. The
alleles are IA, IB and iO. IA is completely dominant to iO. IB is completely dominant to iO. IA and
IB are co-dominant with each other. Suppose a female who is homozygous for Type A blood
marries a male who is heterozygous for type B blood. Their first child is type AB and their
second child is type O. Do both these children belong to the same father? Explain.
a. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Sex Linked Traits: Gender is humans and various other organisms is determined by a specific
pair of chromosomes called sex chromosomes. All other chromosome pairs are called autosomes.
In humans, the sex chromosomes are designated as X and Y. Females have two X chromosomes
while males have X and Y. In gamete formation, the eggs of the female always contain an X
chromosome while there are two types of sperm produced by the male. If the egg is fertilized by
an X containing sperm, the offspring will be female and if it is fertilized with a Y containing
chromosome it will be male.
1. What are autosomes?
2. What sex chromosomes do males have? Females?
3. Draw and complete a Punnett Square showing the possible crosses between a male and
female. Give the phenotype of the offspring (gender).
4. A mating pair of fruit flies produces 300 offspring. Approximately how many of them
will be male and how many will be female?
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Bio A – Mendelian Genetics
Name:
Traits determined by genes located on the X chromosome are said to be sex-linked. One of the
most common in humans is color blindness. Hemophilia is another sex-linked trait in which the
blood does not clot properly. These sex-linked traits are more common in males because the X
chromosomes carry an assortment of genes while the Y chromosomes have very few genes.
Therefore, a gene on the X chromosome of the male will be expressed whether it is dominant or
recessive. When such a gene for an undesirable sex-linked trait is present in a female, there is
usually a dominant normal allele on the other X chromosome, producing the normal phenotype.
Although such females appear normal, they are carriers of the trait and may pass on this gene to
their offspring.
5. What are two sex-linked traits in humans?
6. What is a carrier? Can males be a carrier?
Part III: Sex-linked Problems
Problem 1: The ability to see color is determined by a gene found on the X chromosome. When
this gene is defective, it causes colorblindness. A colorblind person can not distinguish between
certain colors, such as red and green. The good news is that the “normal” allele is dominant to
the colorblind allele. Suppose a Colorblind Female (XcXc) marries a normal male (XCY). Can
they have any male children who can see color? Can they have any female children who can see
color? NOTE: look at how the genotypes are written. Use a lower case c for the recessive
colorblindness gene and write it as a superscript on the X chromosome. The normal dominant
gene is capital C and is underlined so that you can tell it is a capital letter. The Y chromosome
does not have a superscript because there is no gene on the Y chromosome.
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
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Bio A – Mendelian Genetics
Name:
Problem 2: Hemophilia is a disease where the blood does not clot very well. The mutated gene
that causes this disease is found on the X chromosome. Again, the mutation is recessive and the
normal allele is dominant. Suppose you have a Homozygous normal female and a male with
hemophilia. What is the probability of getting a healthy child (male or female)? What fraction
of the male children will be healthy?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
Problem 3: A normal female marries a color blind male. What are the chances the offspring will
be colorblind if they are females? What are the chances the offspring will be colorblind if they
are males?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
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Bio A – Mendelian Genetics
Name:
Problem 4: A carrier female marries a normal male. How many of the male offspring can be
expected to be color-blind? How many of the male offspring can be expected to have normal
vision? How many of the female offspring can be expected to be carriers?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (3 parts)
Problem 5: A man whose mother is colorblind marries a woman with normal vision. What is
the genotype of the husband and wife? What percent of their offspring can be expected to be
colorblind? What percent of the male offspring can be expected to be colorblind? What percent
of their offspring can be expected to be carries?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (3 parts)
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Bio A – Mendelian Genetics
Name:
Part IV: 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?
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Bio A – Mendelian Genetics
Name:
V. Dihybrid Crosses
Problem 1: In mice the gene for coat color as two forms. The allele for colored coat is
dominant to the allele for albino. There are two forms for the gene controlling whiskers,
as well, straight is dominant to bent.
Cross two mice that are heterozygous for both these genes.
a. What is the genotype of each mouse?
b. What possible gametes can each mouse make (remember each gamete
contains one allele for EACH trait being considered)?
c. Draw out the Punnett square for this cross
d. What proportion of the offspring are albino?
e. What proportion would have bent whiskers?
f. What is the phenotypic ratio of ALL the offspring? (be sure to include the
traits!)
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