Genetics Practice 2: Beyond the Basics provides AP Biology students with essential exercises on incomplete dominance, co-dominance, lethal dominance, and sex-linked traits. This resource includes detailed Punnett squares and genotype-phenotype ratios to enhance understanding of genetic principles. Designed for high school biology curricula, it offers practice problems that prepare students for the AP exam. Topics covered include blood type inheritance, coat color in cattle, and dwarfism genetics. Ideal for reinforcing concepts and improving problem-solving skills in genetics.

Key Points

  • Explains incomplete dominance using radish color genetics.
  • Covers co-dominance with blood type inheritance examples.
  • Includes lethal dominant traits with achondroplasia scenarios.
  • Discusses sex-linked traits through hemophilia inheritance patterns.
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Name _____________________________ Period _________
AP Biology Date ______________________
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Developed by Kim B. Foglia • www.ExploreBiology.com • ©2008
GENETICS PRACTICE 2: BEYOND THE BASICS
Solve these genetics problems. Be sure to complete the Punnett square to show how you
derived your solution.
INCOMPLETE DOMINANCE
1. In radishes, the gene that controls color exhibits incomplete dominance. Pure-breeding red
radishes crossed with pure-breeding white radishes make purple radishes. What are the
genotypic and phenotypic ratios when you cross a purple radish with a white radish?
50% purple (RW)
50% white (WW)
R
W
W
RW
WW
W
RW
WW
2. Certain breeds of cattle show incomplete dominance in coat color. When pure breeding red
cows are bred with pure breeding white cows, the offspring are roan (a pinkish coat color).
Summarize the genotypes & phenotypes of the possible offspring when a roan cow is mated
with a roan bull
25% red (RR)
50% roan (RW)
R
W
R
RR
RW
W
RW
WW
25% white (WW)
CO-DOMINANCE
3. A man with type AB blood marries a woman with type B blood. Her mother has type O
blood. List the expected phenotype & genotype frequencies of their children.
25% type AB blood (I
A
I
B
)
50% type B blood (I
B
I
B
& I
B
i)
I
B
I
A
I
B
I
B
I
B
i
I
A
i
I
B
i
25% type A blood (I
A
i)
Name _____________________________ AP Biology
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4. The father of a child has type AB blood. The mother has type A.
Which blood types can their children NOT have? _______________________
5. A woman with type A blood and a man with type B blood
could potentially have offspring with what blood types? _______________________
6. The mother has type A blood. Her husband has type B blood.
Their child has type O blood. The father claims the child can’t
be his. Is he right? _______________________
7. The mother has type B blood. Her husband has type AB blood.
Their child has type O blood. The father claims the child can’t
be his. Is he right? _______________________
8. The mother has type AB blood. The father has type B blood.
His mother has type O blood. What are all the possibilities of
blood type for their children? _______________________
LETHAL DOMINANT
9. Achondroplasia (dwarfism) is caused by a dominant gene. A woman and a man both with
dwarfism marry. If homozygous achondroplasia results in death of embryos, list the
genotypes and phenotypes of all potential live-birth offspring
50% dwarfism (Aa)
25% normal (aa)
What is the expected ratio of dwarfism to
normal offspring?
A
AA
Aa
a
Aa
aa
67% dwarfism : 33% normal
SEX-LINKED
10. The genes for hemophilia are located on the X chromosome. It is a recessive disorder. List
the possible genotypes and phenotypes of the children from a man normal for blood clotting
and a woman who is a carrier. (HINT: You have to keep track of what sex the children are!)
50% females normal
50% females carrier
50% males normal
X
H
X
H
X
H
X
H
Y
X
h
X
H
X
h
X
h
Y
50% males hemophilia
EXTRA CREDIT: Remember those roan cows from question 2? They also have a second gene
for horn vs. hornless cattle. The allele for horns dominates the allele for hornless. If a bull and
cow are heterozygous for both genes, what are the probabilities for each possible phenotype?
I
A
_ x I
A
I
B
= no O
I
A
_ x I
B
_ = AB, A, B, O
No! I
A
_ x I
B
_ = AB, A, B, O
Yes! I
B
_ x I
A
I
B
= AB, A, B
I
A
I
B
x I
B
i = AB, A, B
Name _____________________________ AP Biology
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Solving it the short (probability) way:
R
RR
Rr
r
Rr
rr
h
H
HH
Hh
h
Hh
hh
Red, horn → RR x H_ = ¼ x ¾ = 3/16
Roan, horn → Rr x H_ = 2/4 x ¾ = 6/16
Red, no horn → RR x hh = ¼ x ¼ = 1/16
Roan, no horn → Rr x hh = 2/4 x ¼ = 2/16
White, horn → rr x H_ = ¼ x ¾ = 3/16
White, no horn → rr x hh = ¼ x ¼ = 1/16
Solving it the long way:
RH
Rh
rH
rh
RH
RRHH
red, horn
RRHh
red, horn
RrHh
roan, horn
RrHh
roan, horn
Rh
RRHh
red, horn
RRhh
red, no horn
RrHh
roan, horn
Rrhh
roan, no horn
rH
RrHH
roan, horn
RrHh
roan, horn
rrHH
white, horn
rrHh
white, horn
rh
RrHh
roan, horn
Rrhh
roan, no horn
rrHh
white, horn
rrhh
white, no horn
RRH_
red, horn: 3/16
RrH_
roan, horn: 6/16
RRhh
red, no horn: 1/16
Rrhh
roan, no horn: 2/16
rrH_
white, horn: 3/16
rrhh
white, no horn: 1/16
Usually your 9/16 if red showed
simple dominance to white
⎬
Usually your 3/16 if red showed
simple dominance to white
⎬
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FAQs

What is incomplete dominance in genetics?
Incomplete dominance occurs when the phenotype of a heterozygote is intermediate between the phenotypes of the two homozygotes. For example, in radishes, crossing pure-breeding red and white radishes results in purple radishes. This genetic phenomenon illustrates how alleles can blend rather than exhibit complete dominance, providing a unique visual representation of inheritance patterns.
How does co-dominance differ from incomplete dominance?
Co-dominance occurs when both alleles in a heterozygote are fully expressed, resulting in offspring with a phenotype that displays characteristics of both parents. An example is found in blood types, where a person with type AB blood expresses both A and B antigens. In contrast, incomplete dominance results in a blended phenotype, such as a pink flower from red and white parents.
What are the potential blood types of children from an AB and B blood type parent?
When a man with type AB blood marries a woman with type B blood, their children can inherit various blood types. The possible genotypes include type AB (IAIB), type B (IBIB or IBi), and type A (IAi). This genetic combination illustrates the principles of co-dominance and the inheritance of multiple alleles, which are crucial concepts in AP Biology.
What are the implications of lethal dominant traits like achondroplasia?
Lethal dominant traits, such as achondroplasia, can lead to embryo mortality if homozygous. In the case of two parents with dwarfism, the offspring's genotypes can include normal and dwarfism phenotypes, but homozygous achondroplasia (AA) results in embryo death. This genetic principle highlights the importance of understanding dominant inheritance patterns and their potential consequences on offspring viability.
How do sex-linked traits like hemophilia inherit?
Hemophilia is a recessive disorder linked to the X chromosome, primarily affecting males. In a cross between a normal man and a carrier woman, the potential offspring can be normal females, carrier females, normal males, or males with hemophilia. This inheritance pattern underscores the significance of sex-linked traits in genetics and their implications for family planning and genetic counseling.

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