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Honors Genetics Review
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Introduction:
How do organisms come to look and act the way they do? How are characteristics passed from generation to
generation? Genetics, the study of inheritance, attempts to answer these and other questions. Through the
genetics problems presented in this packet we will investigate several simple forms of inheritance. While
completing genetics problems, you will be asked to carry out step-by-step methods which will include reading
the problem, interpreting the information given, setting up a "Key", “working the crosses”, and finally
presenting and interpreting the results of the crosses.
In order to readily converse in the language of heredity you will first need to know and understand the follow
terms. Using your Biology book (Ch. 9) and class notes, define the words below in the space provided.
BASIC GENETICS VOCABULARY
Allele:
Genotype:
Phenotype:
Homozygous:
Heterozygous:
Dominant:
Recessive:
True-breeding:
Karyotype:
Genetics Problems
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Genetics Problems Set #1
MONOHYBRID CROSS (Simple dominance)
1. In humans the ability to roll one's tongue is dominant to the inability to do so.
b. Explain, in terms of genotype, how it is possible for a "roller" man and a "roller" woman to produce a roller
son, then a non-roller son, then a non-roller daughter.
2. Albinism (as in an "albino") in corn is caused by a recessive allele, and is normally lethal because the plant
cannot manufacture food without chlorophyll. It is possible experimentally to keep albino plants alive with
special feeding techniques. Show the expected offspring from a cross between an albino plant and a normal
plant that is heterozygous. What are the genotypic and phenotypic ratios?
3. You are a farmer and know that short plants are recessive to tall plants. You crossed tall plants with short
plants and the all the offspring were tall plants. What are the genotypes of the original tall and short plant?
Justify your answer.
4. In domestic swine there is a dominant allele which produces a white belt around the body. The recessive
allele produces uniform body color. One farmer wants to produce only belted hogs, and another wants only
uniformly colored animals. Which farmer would have an easier time producing a true-breeding herd?
Explain why.
5. Consider blue eyes in humans to be recessive to brown eyes. Show the expected children of a blue-eyed
woman with a brown eyed man whose mother had blue eyes. Include all ratios.
a. State what you know (and don't know) about the brown-eyed man's father genotype and phenotype
based on the information given here.
6. A woman has a rare eyelid abnormality called ptosis, which makes it impossible for her to open her eyes all
the way. The condition is caused by a single dominant allele. The woman's father had ptosis but her
mother was normal. Her father's mother also had normal eyelids.
a. What are the genotypes of the woman, the woman’s parents, and the woman’s father’s mother?
b. If the woman marries a man with normal eyes, what is the probability her children will have ptosis?
Genetics Problems Set #2
DIHYBRID CROSS (Simple dominance)
1. Carrion beetles lay their eggs in dead animals and then bury them in the ground until they hatch. Assume
that the preference for fresh meat (F) is dominant to the preference for rotted meat (f) and that the tendency
to bury the meat shallow (A) is dominant to the tendency to bury the meat deep (a). Suppose a female
carrion beetle homozygous dominant for both traits mates with a male homozygous recessive for both traits.
What is the genotypes of all the F1 generation offspring? If a male and female from the F1 mate, what would
the phenotypic ratio be of the F2? Answer this without doing a punnett square.
2. About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The ability
to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of
having two recessive alleles (t). Albinism is a trait with normal pigment being dominant (A) and the lack of
pigment being recessive (a). A normally pigmented woman who cannot taste PTC has a father who is an
albino taster. She marries a man homozygous recessive for pigment and heterozygous for PTC . What is the
phenotypic ratio for this cross? Show with a punnett square.
Genetics Problems
2
Genetics Problems Set #3
SEX-LINKED TRAITS
1. HEMOPHILIA
Most people’s blood clots normally. Hemophilia is a blood disorder which causes blood not to clot properly.
The allele for hemophilia is recessive to the allele for normal blood clotting. These alleles are located on the
human X chromosome. The Y chromosome is a genetic "blank" for this trait; it contains no allele for the
blood-clotting gene.
A number of cases of hemophilia have occurred among male descendants in the family of England's
Queen Victoria. She, however, did not have hemophilia. (Queen Victoria lived during the time of
Charles Darwin.)
a. Show ALL the genotypes of normal parents that would produce a male descendant with hemophilia.
b. Show ALL the possible genotypes required of parents to produce a hemophiliac daughter.
c. Explain what's wrong with the statement "The hemophiliac man's father was also a hemophiliac, so
he must have inherited the disease from his father."
2. RED-GREEN COLOR BLINDNESS
Red/green color-blindness in humans is X-linked.
a. A woman has normal vision, but her father is color-blind. Is she necessarily a carrier (heterozygous)?
Explain.
b. Diagram all genotypes and phenotypes of parents and expected children (predict one boy and one
girl) in a family where the woman has normal vision and no family history of color blindness, and the
man is color-blind.
Genetics Problems Set #4
CODOMINANCE/INCOMPLETE DOMINANCE
1. The snap dragon plant has three genotypes for flower color expressed as three different phenotypes:
red flowered plants, white flowered plants, and pink flowered plants. This is a incomplete dominant
trait.
a. Show with a punnett square the expected offspring of a cross between two pink-flowered plants.
Include genotypic and phenotypic ratios.
b. Show with a punnett square the expected offspring of a cross between a red-flowered plant and a
pink-flowered plant. Include genotypic and phenotypic ratios.
c. Is it possible to do a test cross on this type of dominance? Explain.
2. In shorthorn cattle the R allele, when homozygous, produces animals with red hair and the W allele, when
homozygous, produces cattle with white hair. An animal with a heterozygous genotype is roan in color,
meaning its coat contains both red hairs and white hairs.
a. Show with a punnett square the expected offspring of a cross with a roan cow and a red hair cow.
Include genotypic and phenotypic ratios.
b. Which would be easier to produce, red, roan or white haired cattle? Explain your answer.
c. This is an example of what kind of dominance? How do you know? Explain.
Genetics Problems
3
Genetics Problem Set #5
Pedigrees
The following pedigree is of the autosomal dominant disease called Huntington’s disease. This
disease results in the slow deterioration of physical and mental abilities. Experts say 1 in every
10,000 people suffer from this disease. This onset of this disease does not usually happen until
early adulthood. Answer the following questions based on the pedigree below.
1. What genotype will produce a normal person without the disease?
2. If you have the disease, is it a guarantee your children will have the disease too? Explain
3. Identify the genotypes of the labeled individuals above?
The following is a sex-linked recessive disease. Answer the questions based on the pedigree below.
4. If I didn’t tell you this is a sex-linked pedigree, how could you determine the type of pedigree?
Explain.
5. Which individuals in this pedigree are carriers?
6. Identify the genotypes of the individual #1-2, 5-6, 8-9, 12, 16-17, and 22. ?
Genetics Problems
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