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BIOLOGY 30•GENETICS WORKSHEET 1 1. In sheep, the gene for white wool is dominant, the gene for black wool is recessive. What three crosses could result in a black sheep in the family? 2. In one experiment Mendel crossed a true-breeding pea plant having green pods with a truebreeding pea plant having yellow pods. All of the F1 plants had green pods. a) Which trait (green or yellow pods) is recessive? Explain how you arrived at your conclusion. b) Suppose the F1 plants are self-pollinated (F1 cross) and 135 F2 plants are produced. What phenotypes should be present in the F2 generation, and how many of the plants in that generation should show each of those phenotypes? 3. In a certain animal, one breed is known to always have a hairy tail another breed is known to always have a naked tail. How would you determine which trait is dominant? 4. In laboratory mice, the normal gray colour is dominant over the albino (all-white) colour. Starting with pure-bred albino and pure-bred gray as parents what is the ratio of phenotypes in the F2 generation? 5. In man, normal pigmentation is due to a dominant gene, A, albinism to its recessive allele, a. A normal man marries an albino woman and their first child is albino. What are the genotypes of these three persons? If they have several other children, what would these children probably be like? 6. Could two blue-eyed parents have a brown-eyed child? Could two brown-eyed parents have a blue-eyed child? 7. A blue-eyed man, both of whose parents were brown-eyed, marries a brown-eyed woman whose father was blue-eyed and whose mother is brown-eyed. This man and woman have a blue-eyed child. What are the genotypes of all of the individuals mentioned? 8. In the question above, what is the probability that a brother or sister of the blue-eyed child would have blue eyes, brown eyes? 9. What does it mean to say that a characteristic skips a generation? Could it skip 2 or 3 generations? Explain. 10. In guinea pigs short hair is dominant over long hair. A short-haired male and a short-haired female produce mostly short-haired offspring, but a few were long-haired. Show how you can determine the genotypes of the parents. 11. Mendel crossed a true-breeding, tall purple-flowered pea plant with a true-breeding dwarf (short), white-flowered plant. All of the F1 plants were tall and purple-flowered. If an F1 plant is now self-pollinated, what is the probability of obtaining an F2 plant heterozygous for the genes controlling height and flower colour? 12. One gene has alleles A and a, another gene has alleles B and b. For each of the following genotypes, what type(s) of gamates will be produced? a) AABB b) AaBB c) Aabb d) AaBb 13. Referring still to the preceding problem, what genotypes will be present in the offspring from the following matings? (Indicate the frequencies of each genotype among the offspring.) a) AABB X aaBB b) AaBB X AABb c) AaBb X aabb d) AaBb X AaBb 14. In addition to the genes mentioned in the problem above, assume you now study a third gene having alleles C and c. For each of the following genotypes, indicate what type(s) of gamates will be produced. a) AABBCC b) AaBBCc c) AaBBcc d) AaBbCc BIOLOGY 30•GENETICS WORKSHEET 2 Incomplete Dominance, Multiple Alleles and Multiple Genes 1. The shape of radishes may be long, round or oval. The following results were obtained in the different possible crosses: long X oval gave: 50% long and 50% oval. oval X round gave: 50% oval and 50% round. long X round gave: 100% oval. oval X oval gave: 25% long, 50% oval and 25% round. Explain these results. 2. In shorthorn cattle, the polled condition, H, is dominant over the horned condition, h. Also the heterozygous condition of red coat, W, and white coat, w, is roan. If a homozygous polled red animal is bred to a white horned one, what will the F1 and the F2 generations be like? 3. Explain the difference between the way in which the ABO blood groups are inherited and the way in which skin colour is inherited. 4. A woman having blood type A marries a man having blood type B. They have five children. Two sons have blood type 0. One daughter has blood type AB, another daughter type A, and a third type B. a) Draw a pedigree showing the blood types of the parents and offspring. Use letters in the pedigree symbols to represent the individuals' blood types. Figure out the genotypes of the parents and children. b) 5. The ABO blood system has often been employed to settle cases of disputed paternity. Suppose as an expert in genetics you are called to testify in a case where the mother has type A blood and the child has type 0 blood, and the alleged father has type B blood. How would you respond to the following statements of the attorneys: Attorney of the alleged father: "Since the mother has type A blood, the type 0 blood of the child must have come from the father, and since my client has type B blood, he obviously could not have fathered this child.'' Attorney of the mother: "Further tests revealed that this man is heterozygous and therefore he must be the father.'' 6. In Duroc jersey hogs, coat colour is dependent on two pairs of alleles, R and r and S and s. Any genotype containing at least one R gene and at least one S gene results in red coat colour. The double recessive genotype results in white coat colour. All other genotypes result in sandy coat colour. if two hogs, each heterozygous for both pairs of genes, are mated, what kinds of offspring will be produced in what ratio? BIOLOGY 30•GENETICS WORKSHEET 3 Sex-linked, Sex-influenced and Sex-limited Traits 1. What are the differences between a sex-linked, a sex-limited and a sex-influenced trait? Give an example of each. 2. Recall that human sex chromosomes are XX for females and XY for males. a) b) c) d) Does a male child inherit his X chromosome from his mother or father? With respect to an X-linked gene how many different types of gamates can a male produce? If a female is homozygous for an X-linked gene, how many different types of gamates can she produce with respect to this gene? If a female is heterozygous for an X-linked gene, how many different types of gamates can she produce with respect to this gene? 3. One human gene that may be Y-linked controls the length of hair on mens' ears. One allele at this gene locus produces non-hairy ears, another allele produces rather long hairs (hairy pinnae). a) Why would you not expect females to have hairy pinnae? b) If a man with hairy pinnae has sons, all of them will have hairy pinnae, if he has daughters, none of them will. Explain this statement. 4. In hemophilia A, the body's blood-clotting mechanism is defective. This condition has been traced to a recessive allele of an X-linked gene. Refer now to figure on page 660. a) b) Why are only the females shown as carriers of the recessive allele? If Victoria had married a hemophiliac, what phenotypes would have been expected among their children and with what probabilities? 5. A woman heterozygous for colour blindness XCXc marries someone with normal colour vision. What is the probability that their first child will be colour blind? The second child? If they only have two children, what is the probability that both will be colour blind? 6. A colour-blind man marries a woman with normal red-green vision. They have two daughters and one son. The son is colour-blind. One daughter is colour-blind and the other has normal vision. a) Draw a pedigree showing the phenotypes of the parents and children. Shade in those symbols representing colour-blind individuals. Symbols representing people with normal colour vision should be left blank. What are the genotypes of the mother and father? b) BIOLOGY 30•GENETICS WORKSHEET 4 Nondisjunction 1. A person afflicted with Turner's syndrome has only a single sex chromosome (X only), yet may survive. In contrast, a person having a single Y chromosome and no X chromosome cannot survive. What does this tell you about the genetic contents of the X and the Y chromosomes? 2. Fertilization of a normal egg by a sperm that has no sex chromosomes (male nondisjunction) can lead to Turner's syndrome. Also fertilization of an egg that has no sex chromosomes (female nondisjunction) by a sperm carrying one X chromosome can lead to the same disorder. Suppose a hemophilic male and a carrier (heterozygous) female have a child. The child is nonhemophillic and is afflicted with Turner's syndrome. In which parent did the nondisjunction occur? 3. The trisomic XXY condition is also called Klinefelter's syndrome. How could this syndrome arise if nondisjuction occured in the female parent of an afflicted individual? How could it arise if nondisjunction occurred in the male parent? 4. If nondisjunction occurs for X chromosomes during oogenesis, then some eggs having two X chromosomes and others having no X chromosomes are produced at about equal frequencies. If normal sperm fertilize these two types of eggs, what karyotypes are possible? 5. Expectant parents want to learn whether the fetus the woman is carrying has sickle-cell disease. Could a genetic counselor determine this by looking at the fetus's karyotype? Explain. 6. Individuals afflicted with Down's syndrome typically have an extra chromosome 21, so their cells have a total of 47 chromosomes. However, in a few cases of Down's syndrome 46 chromosomes are present. Included in this total are two normal-appearing chromosomes 21, and a longer than normal chromosome 14. Interpret this observation and indicate how these few individuals can have a normal chromosome number. 7. Since any pair of chromatids can fail to separate during meiosis, there are theoretically 23 possible kinds of monosomy and trisomy. However, monosomies and trisomies for most of the 23 chromosome pairs are extremely rare or unheard of in live babies. Why do you think this is so? BIOLOGY 30•GENETICS WORKSHEET 5 Population Genetics 1. In the gene pool for a certain population of rabbits, the gene frequency for brown fur (B) is 63%. What is the gene frequency of white fur? 2. Twenty-five homozygous brown rabbits are placed with 75 homozygous white rabbits. What are the gene frequencies for brown fur (B) and white fur (b)? 3. Fifty heterozygous brown rabbits are placed with 100 homozygous white rabbits. What are the gene frequencies for brown (B) and white (b) fur? a) b) c) 4. If there were an equal distribution of males and females in both colours and random matings occured, what distribution of white and brown rabbits would your expect to get in the F1 generation? What distribution of white and brown rabbits would your expect to get with this gene pool? Is there a difference between your answers in a) and b). If yes explain. The gene frequency for the dominant allele for tongue rolling in a certain high school is 70%. Construct a cross-multiplication table for this gene. a) b) What percentage of the student population is heterozygous for this trait? What percentage of the population is homozygous recessive? 5. Population geneticists have studied a human population in which 9% of the people are homozygous for a recessive gene that causes red hair. What is the frequency of this gene in the total population. 6. Phenylketonuria (PKU) is an autosomal recessive condition. About 1 of every 50 individuals is heterozygous for the gene but displays no symptoms of the disorder. a) b) c) If you select a symptom-free male at random from the population, what is the probability that he will be heterozygous? If you select a symptom-free female at random from the population, what is the probability that she will be heterozygous? If you select a symptom-free male and a symptom-free female at random, what is the probability that both will be heterozygous? What is the probability that they could have a child afflicted with PKU? 7. Laws restricting marriage between close relatives (consanguineous matings) are widespread, the rationale being that such marriages generally lead to an increase in the incidence of genetic defects among offspring. Suppose you are a carrier (Heterozygous) for PKU. If you pick a potential mate at random from the population, what is the probability that he or she would also be a PKU carrier? If you marry your first cousin, do you think he or she would have the same probability of being a PKU carrier as your randomly selected mate? Explain. 8. A small population of deer becomes geographically isolated from others of the same species. The isolated deer mate among themselves. a) If this situation persists for many years, harmful traits due to mutations may increase. What genetic principle may explain this? b) If this isolation persists for a much longer time period, what is likely to happen to this particular group of deer. Explain. Now try these for a challenge: 1. A man is homozygous dominant for ten different genes, which assort independently. How many genotypically different types of sperm could he produce? A woman is homozygous recessive for eight of these ten genes, and she is heterozygous for the other two. How many genotypically different types of eggs could she produce? (HINT: Look at the first worksheet (#18-20) and see what can you conclude regarding the relationship between the number of different gamates possible and the number of heterozygous and homozygous genes that are present.) Try predicting the number of different types of gamates produced by a man heterozygos for ten different genes. For a species (or population), what might be the biological benefits, if any, of possessing, a large number of heterozygotes? 2. A pure breeding mink with black fur and a white tipped tail was crossed with a pure breeding brown furred mink without a white tip. The offspring were all black with a white tip on their tail. A test cross of numerous matings produced: 36 black, white tipped 29 black, no white tips 21 brown, no white tips 32 brown, white tipped Are the two genes linked? If so what is the percentage of crossover? 3. What conditions result in the following phenotypic ratios? (HINT: more than 1 locus may be involved.) a) 3: 1 e) b) 1: 2: 1 f) c) 9: 3: 3: 1 d) 1: 1 3: 6: 3: 1: 2: 1 47: 47: 3: 3 g) 1: 2: 1: 2: 4: 2: 1: 2: 1 4. In mice colour is controlled by two gene loci, each with two alleles. The B allele produces black and the b allele produces brown coat depending upon the action of the second locus. The C allele produces the colour controlled by the first locus and the c allele produces an absence of colour (white or albino) irregardless of the first locus. It two mice each heterozygous at both loci mate determine the genotypic and phenotypic ratios. 5. The manx cat is a tailless or short tailed cat produced by the presence of a dominant allele M. Normal tailed cats are homozygous mm. The homozygous MM situation is lethal. Use to explain why, on average, manx X manx breedings produce manx to tailed cats in a 2 to 1 ratio.