Genetics Problems
DUE DATE: SEE Chapter 8 Checklist
Overview:
The classical approach to studying basic genetics is to solve simple word problems using a Punnett Square. These are described in your text, or if you need more background information on how to use a Punnett Square, GOTO http://en.wikipedia.org/wiki/Punnett_square . I have also posted tutorials on how to solve genetics word problems on our BB site.
Grading & Deliverable:
Do any 5 of the following problems for 5 points each. Points earned on this problem set will be applied to your Class Participation grade.
You will be graded on whether your answer is correct and how well you presented and analyzed your work. Whenever a written response is called for, you are responsible for convincing me that you understood the question and that you understand your answer.
Be sure to show all your work using Punnett Squares (see models for these at end of problem sheet) and label your answers carefully. You may copy the tables at the end of this write-up and insert them into the problems sheet to answer questions, then just type your answers and submit your final deliverable electronically through the Assignment Upload in the Genetics Problems folder. Alternatively, you can do the work by hand and scan your answers, but you must copy the image into a Word Document – do not submit a pdf file.
1. In humans, the allele for albinism is recessive to the allele for normal skin pigmentation.
a. What is the probability that a child of a heterozygous mother and father will be an albino?
b. If a child is normal, what is the probability that it is a carrier of the recessive albino allele?
2. In certain pea plants, the allele T for tallness is dominant over t for shortness.
a. If a homozygous tall and a homozygous short plant are crossed, what will be the phenotypes and genotypes of the offspring?
b. If both individuals are heterozygous, what will be the phenotypic and genotypic ratios of the offspring?
3. Smoos are strange animals with one of three shapes: round; cuboidal; or pyramidal; If two cuboidal smoos mate, they always have cuboidal shape offspring. If two pyramidal smoos mate, they always produce pyramidal offspring. It two round smoos mate, they produce all three kinds of offspring. Assuming only one gene locus is involved, answer the following questions.
a. How is smoo shape determined? (i.e., dominant-recessive; incomplete dominance; etc.) Explain your answer.
b. What would be the phenotypic ratio if a round and cuboidal smoo were to mate?
4. Color blindness is an X-Linked trait. A color-blind woman married a man with normal vision. They have ten children à six boys and four girls.
a. How many are expected to be non-color blind?
b. How many are expected to be color blind?
5. In one experiment, Mendel crossed a pea plant that bred true for green pods with one that bred true for yellow pods. All the F1 plants had green pods. Which form of the trait (green or yellow pods) is recessive? Explain how you arrived at your conclusion and use a Punnett Square to confirm your analysis.
6. Assume that a new gene has been identified in mice. One of the alleles specifies yellow fur color. A second allele specifies brown fur color. Suppose you are asked to determine whether the relationship between the two alleles is one of simple dominance, or co-dominance. What types of crosses would give you the answer? On what types of observations would you base your conclusions?
7. At one gene location on a human chromosome, a dominant allele controls whether you can curl the side of your tongue upward. People homozygous for the recessive allele cannot roll their tongue. At a different location, a dominant allele controls whether earlobes are attached or detached. These two gene pairs assort independently.
So, suppose a tongue-rolling woman with detached earlobes marries a man who has attached earlobes and can’t roll his tongue. Their first child has attached earlobes and can’t roll his tongue.
a. What are the genotypes of the mother, father, and child?
b. What is the probability that a second child will have detached earlobes and won’t be a tongue roller?
8. The ABO blood-typing system has been used to settle cases of disputed paternity. Suppose, as a geneticist, you must testify in a case in which the mother has type A blood, the child has type O blood, and the alleged father has type B blood. How would you respond to the following statements?
Attorney of the alleged father: “The mother has type A blood so the child’s type O blood must have come from the father. My client has type B blood; he could not have fathered this child.”
Mother’s attorney: “Further tests prove this man is heterozygous, so he must be the father.”
Use a Punnett Square to defend your response.
9. As in Labrador retrievers, fur color in mice is governed by genes concerned with producing and distributing melanin. At one-gene location, a dominant allele (B), specifies dark brown and a recessive allele (b) specifies light brown, or tan. At another gene location, a dominant allele (C) permits melanin production and a recessive allele (c) shuts it down and results in albinism.
à A homozygous bb cc albino mouse mates with a homozygous BB CC brown mouse. State the probable genotypic and phenotypic ratios for the F1 and F2offspring.
à If an F1 mouse from the previous problem is backcrossed with its albino parent, which phenotypic and genotypic ratios would you expect?
10. A dominant allele W confers black fur on guinea pigs. If the guinea pig is homozygous recessive (ww), it has white fur. Molly would like to know if her pet black-furred guinea pig is homozygous WW or heterozygous (Ww). How might she determine her pet’s genotype? Explain using a Punnett Square to illustrate how the approach you advise would work.
11. An allele codes for a mutant form of hemoglobin (HbS instead of HbA). Homozygotes (HbSHbS) that are affected have sickle cell anemia. Heterozygotes (HbAHbS) show few outward symptoms. Suppose a female whose father was homozygous for the HbS allele marries a heterozygous male, and they consider having children. For each pregnancy,
- What is the probability of having a child homozygous for the HbS allele?
- What is the probability of having a child homozygous for the HbA allele?
- What is the probability of having a child heterozygous HbAHbS?
12. An inability to produce melanin results in albinism. A recessive allele, a, is responsible for this phenotype. State the possible genotypes of both parents and the offspring of the following crosses.
a. Both parents have normal phenotypes but have both albino and normal children.
b. Both parents are albino and have only albino children.
c. A normal woman and an albino man who have two albino and two normal children.
13. The so-called “blue” (really gray) Andalusian variety of chicken is produced by a cross between the black and white varieties. Only a single pair of alleles is involved. What color of chickens (and in what proportions) would you expect if you crossed two blues? If you crossed a blue and a black? Explain.
14. In snapdragons, the allele that produces tall stems is completely dominant to the allele for dwarf stems, while the allele that produces red flowers is only partially dominant to that for white flowers. Describe the phenotype (height and flower color) to the F1 plants resulting from a cross between a homozygous tall, red-flowered plant and a homozygous dwarf, white-flowered plant. If one of these F1 plants self-pollinates, what will be the appearance and proportions of the phenotypes in the resulting F2 generation? Which two of these phenotypes will breed true?
15. In Duroc-Jersey pigs, coat color is determined by two genes, R and S. The homozygous recessive condition, rrss, produces a white coat. The presence of at least one copy each of R and S produces red. The presence of one or the other allele (either R or S) produces a new phenotype, sandy. Give the phenotypes of the following genotype:
RRSS rrss
RrSs rrSs
RRSs rrSS
RrSS RRss
16. Mating a red Duroc-Jersey boar to sow A (white) gave pigs in the ratio of 1 red: 2 sandy: 1 white. Mating this same boar to sow B (sandy) gave 3 red: 4 sandy: 1 white. When this boar was mated to sow C (sandy), the litter had equal numbers of red and sandy piglets. Using the information presented in Question 15, give the possible genotypes of the boar and the three sows.
17. In Jimson weed, the allele that produces violet petals is dominant over that for white petals, and the allele that produces prickly capsules is dominant over that for smooth capsules. A plant with white petals and prickly capsules was crossed with one that had violet petals and smooth capsules. The F1 generation was composed of 47 plants with white petals and prickly capsules, 45 plants with white petals and smooth capsules, 50 plants with violet petals and prickly capsules, and 46 plants with violet petals and smooth capsules. What were the genotypes of the parents?
18. Among Hereford cattle there is a dominant allele called polled; the individuals that have this allele lack horns. After college, you acquire a herd consisting entirely of polled cattle, and carefully determine that no cow in the herd has horns. Among the calves born that year, however, some grow horns. You remove them from the herd and make certain that no horned adult has gotten into your pasture. Despite your efforts, more horned calves are born next year. What is the reason for the appearance of the horned calves? If your goal is to maintain a herd consisting entirely of polled cattle, what should you do? Use a Punnett Square to defend you answer.
19. An inherited trait among humans in Norway caused affected individuals to have very wavy hair, not unlike that of sheep. The trait, which is called woolly, is very evident when it occurs in families; no child possesses woolly hair unless at least one parent does. Imagine you are a Norwegian judge, and that you have before you a woolly-haired man suing his normal-haired wife for divorce because their first child has normal hair. The husband claims this constitutes evidence of infidelity on the part of his wife. Do you accept his claim? Justify your decision.
Punnett Square – Monohybrid Cross – You can copy and insert this table to answer questions.
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Punnett Square – Dihybrid Cross – You can copy and insert this table to answer questions.
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