FIU Mendelian Simple Dominance Inheritance Practice Problems
FIU Mendelian Simple Dominance Inheritance Practice Problems
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Inheritance Practice Problems vSp22 I. Problems Involving One Gene. Mendelian Simple Dominance. 1. In cats, long hair is recessive to short hair. A true-breeding (homozygous) short-haired male is mated to a long-haired female. What will their kittens look like? 2. Two cats are mated. One of the parent cats is long-haired (recessive allele). The litter which results contains two short-haired and three long-haired kittens. What does the second parent look like, and what is its genotype? 3. Mrs. And Mr. Smith both have widow’s peaks (dominant). Their first child also has a widow’s peak, but their second child doesn’t. Mr. Smith accuses Mrs. Smith of being unfaithful to him. Is he necessarily justified? Why or why not? Work the genetics problem predicting the frequencies of the versions of this trait among their prospective children. 4. Mr. and Mrs. Jones have six children. Three of them have attached earlobes (recessive) like their father, and the other three have free earlobes like their mother. What are the genotypes of Mr. and Mrs. Jones and of their numerous offspring? 5. In certain portions of the Jewish population, there is a genetic disease called Tay Sachs disease, which is fatal to infants within the first five years of life. This disease is caused by a recessive allele of a single gene. Why does this disease persist, even though it is invariably fatal long before the afflicted individual reaches reproductive age? (In other words, why doesn’t the allele for Tay Sachs disease simply disappear?) 6. About 80% of the human population can taste the chemical phenolthiocarbamide (PTC), while the other 20% can’t. This characteristic is governed by a single gene with two alleles, a tasting allele and a non-tasting allele. What does this statistic tell us about which allele (tasting or non-tasting) is dominant? 7. What is the probability of obtaining an individual with the genotype B_ from a cross between two individuals with the genotype Bb? 8. In peas, tall is dominant over dwarf. If a plant homozygous for tall is crossed with one homozygous for dwarf: a. What will be the appearance (phenotype) of the F1 plants? b. What will be the phenotypes of the F2, and what fraction of the offspring will have each phenotype? c. What will be the phenotypes and fractions if an F1 plant is crossed with its tall parent? d. What will be the phenotypes and fractions if an F1 plant is crossed with its short parent? 9. A tall plant crossed with a dwarf one produces offspring, of which about half are tall and half are dwarf. What are the genotypes of the two parents? 10. If the tall parent in problem #9 is self-fertilized, what is the probability that the first offspring will be tall? 11. If a pure-breeding purple-flowered pea plant is crossed with a pure-breeding whiteflowered pea plant, all the offspring have purple flowers. Suppose two F1 plants are crossed, and 2400 offspring are obtained. How many white-flowered plants will you expect? 12. When humans interlace their fingers, crossing left thumb over right thumb is a dominant trait. Two left-over-right parents have a right-over-left daughter. What are the genotypes of the parents and their daughter? II. Problems Involving Incomplete Dominance 1. Mr. and Mrs. Anderson both have tightly curled hair. (The hair form gene shows incomplete dominance. There are two alleles, curly and straight. The heterozygote has wavy hair.) The Andersons have a child with wavy hair. Mr. Anderson accuses Mrs. Anderson of being unfaithful to him. Is he necessarily justified? Why or why not? 2. Two wavy haired people (one male and one female) marry and have eight children. Of these eight, how many would you expect to be curly haired, how many wavy haired and how many straight haired, assuming that the family follows the expected statistically predicted pattern? 3. Basic body color for horses is influenced by several genes, one of which has several different alleles. Two of these alleles—the chestnut (dark brown) allele and a diluting (pale cream) allele (often incorrectly called ‘albino’)—display incomplete dominance. A horse heterozygous for these two alleles is a palomino (golden body color with flaxen mane and tail). Is it possible to produce a herd of pure- breeding palomino horses? Why or why not? Work the Punnett’s square for mating a palomino to a palomino and predict the phenotypic ratio among their offspring. III. Problems Involving Genes With Multiple Alleles 1. In a particular family, one parent has Type A blood, the other has Type B. They have four children. One has Type A, one has Type B, one has Type AB, and the last has Type O. What are the genotypes of all six people in this family? NOTE: The ABO blood type gene has three alleles. IA and IB are codominant; i (for Type O) is recessive to both. 2. Refer to problem I.3. Mrs. Smith has blood type A. Mr. Smith has blood type B. Their first child has blood type AB. Their second child has blood type O. Now is Mr. Smith justified? What are Mr. and Mrs. Smith’s genotypes for these two genes? 3. In a recent case in Spokane, Washington, a young woman accused a soldier of being the father of her child. The soldier, of course, denied it. The soldier’s lawyer demanded that blood types be taken to prove the innocence of his client. The following results were obtained: Alleged father, Type O. Mother, Type A. Child, Type AB. The court found the soldier guilty on the basis of the woman’s remarkable memory for dates and details that apparently eliminated all other possible fathers. a. What are the possible genotypes for these three people? b. Do you agree with the court’s decision? Why or why not? 4. It was suspected that two babies had been exchanged in a hospital. Mr. and Mrs. Jones received baby #1 and Mr. and Mrs. Simon received baby #2. Blood typing tests on the parents and the babies showed the following: Mr. Jones: Type A Mrs. Jones: Type O Mr. Simon: Type AB Mrs. Simons: Type O Baby #1: Type A Baby #2 Type O Were the babies switched? How do you know whether they were or they weren’t? 5. A man with type O blood marries a woman with Type AB blood. Among their children, what proportion would you expect to have blood types like one or the other of these parents? What proportion would have expected to have blood types different from both parents? Explain. 6. A woman has a daughter. There are three men whom she claims might have been the father of the child. The judge in the paternity court orders that all three men, the child, and the mother have blood tests. The results are: mother, Type A; Daughter, Type O; Man #1, Type AB; Man #2, Type B; Man #3, Type O. The mother claims that this proves that Man #3 must be the little girl’s father. a. Is the mother correct? Why or why not? b. The judge isn’t satisfied, so he asks for the medical records of the people involved. He discovers that the little girl is colorblind. Men #’s 1 and 2 are also colorblind; Man #3 has normal color vision, as does the mother. (NOTE: Color Blindness is X-linked and recessive.) Assuming that one of these three men must be the father, can you now determine which of the three it is? 7. Another woman has the same problem. Her blood type is A, her child’s is B. She again has three candidates for fatherhood. Their blood types are: Man #1, B; Man #2, AB; Man #3, O. Based on blood types, the mother says it must have been #1. a. Do you agree? Why or why not? b. This child, a son this time, is also colorblind. The only one of the men in question to share this characteristic is #2. The mother is not colorblind. Can you now determine who the father of the little boy is, assuming it must be one of these men? Explain your answer.
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