Quiz 25: Genetics Basis of Cancer
Biology
Q 1Q 1
Give several examples of quantitative traits. How are these quantitative traits described within groups of individuals
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Height, weight, pigmentation in flowers and skin, milk production metabolic rate, speed and wavy hair as well as hidden traits such as musical talent, blood pressure, longevity and complex diseases like cancer, diabetes, hypertension and arthritis are some examples for quantitative traits. These are so called because their expression in individual depends on their mutual expression in various amounts. The quantitative traits can be described by statistical study of traits that is calculating Mean, Variance (V), Standard Deviation (S.D) and degree of variation for all measured values of individuals in a population for a trait. These statistical tools help us in analysing the variation of quantitative traits with in a group of individuals.
Q 2Q 2
Here are data for height and weight among 10 male college students.
A. Calculate the correlation coefficients for this group.
B. Is the correlation coefficient statistically significant Explain.
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(A)First, we have to calculate the means of height and weights of the students and difference between each value and mean.
Calculate variance and standard deviation ( S.D ) for both height and weight
Variance of height V X = 140.277 Standard deviation of height ( S.D X ) =11.84
Variance of weight V Y =121.43 Standard deviation of weight ( S.D Y ) =11.02
Then calculate covariance
Covariance CoV (X,Y) =123.15
By using above data let's calculate correlation coefficient( r ) 
(B)It is statistically significant with 8 degrees of freedom. That mean the association of these two variables is not because of random sampling errors. The association is true and frequent.
Q 3Q 3
Discuss why heritability is an important phenomenon in agriculture. Discuss how it is misunderstood.
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Heritability is important in agriculture because, in selective breeding heritability plays a major role. The breeder selects the plants as breeding stock for next generation, whose heritability value is high to improve the desired traits. People misunderstood the concept of heritability, by thinking that the variation of a trait is governs by genetics alone. But, Heritability is the amount of phenotypic variation is due to genetic variation in a particular population raised in a particular environment. Heritability depends on both genetical and environmental factors. Heritability does not describe the relative impact of the genetic variation and effect of environment on the outcome of a trait.
Q 4Q 4
At the molecular level, explain why quantitative traits often exhibit a continuum of phenotypes within a population. How does the environment help produce this continuum
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Q 5Q 5
The abdomen length (in millimeters) was measured in 15 male Drosophila, and the following data were obtained: 1.9, 2.4, 2.1, 2.0, 2.2, 2.4, 1.7, 1.8, 2.0, 2.0, 2.3, 2.1, 1.6, 2.3, and 2.2. Calculate the mean, standard deviation, and variance for this population of male fruit flies.
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Q 6Q 6
From a biological viewpoint, speculate as to why many traits seem to fit a normal distribution. Students with a strong background in math and statistics may want to explain how a normal distribution is generated, and what it means. Can you think of biological examples that do not fit a normal distribution
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Q 7Q 7
What is a normal distribution Discuss this curve with regard to quantitative traits within a population. What is the relationship between the standard deviation and the normal distribution
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Q 8Q 8
You need to understand solved problem S5 before answering this question. The average weights for two varieties of cattle were 514 kg and 621 kg. The genetic variance for weight calculated for both strains was 382 kg 2. What is the minimum number of genes that affect weight variation in these two varieties of cattle
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Q 9Q 9
What is heterosis Discuss whether it is caused by a single gene or several genes. Discuss the two major hypotheses proposed to explain heterosis. Which do you think is more likely to be correct
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Q 10Q 10
Explain the difference between a continuous trait and a discontinuous trait. Give two examples of each. Are quantitative traits likely to be continuous or discontinuous Explain why.
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Q 11Q 11
Using the same strategy as the experiment of Figure 25.6, the following data are the survival of F 2 offspring obtained from backcrosses to insecticide-resistant and control strains:
Interpret these results with regard to the locations of QTLs.
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Q 12Q 12
What is a frequency distribution Explain how the graph is made for a quantitative trait that is continuous.
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Q 13Q 13
In one strain of cabbage, you conduct an RFLP analysis of head weight; you determine that seven QTLs affect this trait. In another strain of cabbage, you find that only four QTLs affect this trait. Note that both strains of cabbage are from the same species, although they may have been subjected to different degrees of inbreeding. Explain how one strain can have seven QTLs and another strain four QTLs for exactly the same trait. Is the second strain missing three genes
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Q 14Q 14
The variance for weight in a particular herd of cattle is 484 lb 2. The mean weight is 562 lb. How heavy would an animal have to be if it was in the top 2.5% of the herd The bottom 0.13%
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Q 15Q 15
From an experimental viewpoint, what does it mean to say that an RFLP is associated with a trait Let's suppose that two strains of pea plants differ in two RFLPs that are linked to two genes governing pea size. RFLP-1 is found in 2000-bp and 2700-bp bands, and RFLP-2 is found in 3000-bp and 4000-bp bands. The plants producing large peas have RFLP-1 (2000 bp) and RFLP-2 (3000 bp); those producing small peas have RFLP-1 (2700 bp) and RFLP-2 (4000 bp). A cross is made between these two strains, and the F 1 offspring are allowed to self-fertilize. Five phenotypic classes are observed: small peas, small-medium peas, medium peas, medium-large peas, and large peas. We assume that each of the two genes makes an equal contribution to pea size and that the genetic variance is additive. Draw a gel and explain what RFLP banding patterns you would expect to observe for these five phenotypic categories. Note: Certain phenotypic categories may have more than one possible banding pattern.
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Q 16Q 16
Two different varieties of potatoes both have the same mean weight of 1.5 pounds. One group has a very low variance, and the other has a much higher variance.
A. Discuss the possible reasons for the differences in variance.
B. If you were a potato farmer, would you rather raise a variety with a low or high variance Explain your answer from a practical point of view.
C. If you were a potato breeder and you wanted to develop potatoes with a heavier weight, would you choose the variety with a low or high variance Explain your answer.
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Q 17Q 17
Let's suppose that two strains of pigs differ in 500 RFLPs. One strain is much larger than the other. The pigs are crossed to each other, and the members of the F 1 generation are also crossed among themselves to produce an F 2 generation. Three distinct RFLPs are associated with F 2 pigs that are larger. How would you interpret these results
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Q 18Q 18
If an r value equals 0.5 and N = 4, would you conclude that a positive correlation is found between the two variables Explain your answer. What if N = 500
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Q 19Q 19
Outline the steps you would follow to determine the number of genes that influence the yield of rice. Describe the results you might get if rice yield is governed by variation in six different genes.
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Q 21Q 21
A researcher has two highly inbred strains of mice. One strain is susceptible to infection by a mouse leukemia virus, whereas the other strain is resistant. Susceptibility/resistance is a polygenic trait. The two strains were crossed together, and all of the F 1 mice were resistant. The F 1 mice were then allowed to interbreed, and 120 F 2 mice were obtained. Among these 120 mice, 118 were resistant to the viral pathogen, and 2 were sensitive. Discuss how many different genes may be involved in this trait. How would your answer differ if none of the F 2 mice had been susceptible to the leukemia virus Hint: You should assume that the inheritance of one viral-resistance allele is sufficient to confer resistance.
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Q 22Q 22
When a correlation coefficient is statistically significant, what do you conclude about the two variables What do the results mean with regard to cause and effect
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Q 23Q 23
In a wild strain of tomato plants, the phenotypic variance for tomato weight is 3.2 g 2. In another strain of highly inbred tomatoes raised under the same environmental conditions, the phenotypic variance is 2.2 g 2. With regard to the wild strain,
A. Estimate V G.
B. What is h B 2 .
C. Assuming that all of the genetic variance is additive, what is h N 2 .
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Q 24Q 24
What is polygenic inheritance Discuss the issues that make polygenic inheritance difficult to study.
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Q 25Q 25
The average thorax length in a Drosophila population is 1.01 mm. You want to practice selective breeding to make larger Drosophila. To do so, you choose 10 parents (5 males and 5 females) of the following sizes: 0.97, 0.99, 1.05, 1.06, 1.03, 1.21, 1.22, 1.17, 1.19, and 1.20. You mate them and then analyze the thorax sizes of 30 offspring (half male and half female): 0.99, 1.15, 1.20, 1.33, 1.07, 1.11, 1.21, 0.94, 1.07, 1.11, 1.20, 1.01, 1.02, 1.05, 1.21, 1.22, 1.03, 0.99, 1.20, 1.10, 0.91, 0.94, 1.13, 1.14, 1.20, 0.89, 1.10, 1.04, 1.01, 1.26. Calculate the realized heritability in this group of flies.
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Q 26Q 26
What is a quantitative trait locus (QTL) Does a QTL contain one gene or multiple genes What technique is commonly used to identify QTLs
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Q 27Q 27
In a strain of mice, the average 6-week body weight is 25 g and the narrow-sense heritability for this trait is 0.21.
A. What would be the average weight of the offspring if parents with a mean weight of 27 g were chosen
B. What weight of parents would you have to choose to obtain offspring with an average weight of 26.5 g
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Q 28Q 28
Let's suppose that weight in a species of mammal is polygenic, and each gene exists as a heavy and light allele. If the allele frequencies in the population were equal for both types of allele (i.e., 50% heavy alleles and 50% light alleles), what percentage of individuals would be homozygous for the light alleles at all of the genes affecting this trait, if the trait was determined by the following number of genes
A. Two
B. Three
C. Four
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Q 29Q 29
Two tomato strains, A and B, both produce fruit that weighs, on average, 1 lb each. All of the variance is due to V G. When these two strains are crossed to each other, the F1 offspring display heterosis with regard to fruit weight, with an average weight of 2 lb. You take these F1 offspring and backcross them to strain A. You then grow several plants from this cross and measure the weights of their fruit. What would be the expected results for each of the following scenarios
A. Heterosis is due to a single overdominant gene.
B. Heterosis is due to two dominant genes, one in each strain.
C. Heterosis is due to two overdominant genes.
D. Heterosis is due to dominance of several genes each from strains A and B.
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Q 30Q 30
The broad-sense heritability for a trait equals 1.0. In your own words, explain what this value means. Would you conclude that the environment is unimportant in the outcome of this trait Explain your answer.
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Q 31Q 31
You need to understand solved problem S5 before answering this question. The variance in height for fathers (in square inches) was 112, the variance for sons was 122, and the covariance was 144. The mean height for fathers was 68 inches, and the mean height for sons was 69 inches. If a father had a height of 70 inches, what is the most probable height of his son
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Q 32Q 32
Compare and contrast the dominance and overdominance hypotheses. Based on your knowledge of mutations and genetics, which do you think tends to be the more common explanation for heterosis
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Q 33Q 33
A danger in computing heritability values from studies involving genetically related individuals is the possibility that these individuals share more similar environments than do unrelated individuals. In the experiment of Figure 27.8, which data are the most compelling evidence that ridge count is not caused by genetically related individuals sharing common environments Explain.
FIGURE 27.8 Heritability of human fingerprint patterns.
Starting material: A group of human subjects from Great Britain.
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Q 34Q 34
What is hybrid vigor (also known as heterosis) Give examples that you might find in a vegetable garden.
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Q 35Q 35
A large, genetically heterogeneous group of tomato plants was used as the original breeding stock by two different breeders, named Mary and Hector. Each breeder was given 50 seeds and began an artificial selection strategy, much like the one described in Figure 25.13. The seeds were planted, and the breeders selected the 10 plants with the highest mean tomato weights as the breeding stock for the next generation. This process was repeated over the course of 12 growing seasons, and the following data were obtained:
A. Explain these results.
B. Another tomato breeder, named Martin, got some seeds from Mary's and Hector's tomato strains (after 12 generations), grew the plants, and then crossed them to each other. The mean weight of the tomatoes in these hybrids was about 1.7 lb. For a period of 5 years, Martin subjected these hybrids to the same experimental strategy that Mary and Hector had followed, and he obtained the following results:
Explain Martin's data. Is heterosis occurring Why was Martin able to obtain tomatoes heavier than 1.5 lb, whereas Mary's and Hector's strains appeared to plateau at this weight
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Q 36Q 36
From an agricultural point of view, discuss the advantages and disadvantages of selective breeding. It is common for plant breeders to take two different, highly inbred strains, which are the product of many generations of selective breeding, and cross them to make hybrids. How does this approach overcome some of the disadvantages of selective breeding
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Q 37Q 37
The correlation coefficients for height were determined for 15 pairs of individuals with the following genetic relationships:
Mother-daughter: 0.36
Mother- granddaughter: 0.17
Sister-sister: 0.39
Sister-sister (fraternal twins): 0.40
Sister-sister (identical twins): 0.77
What is the average heritability for height in this group of females
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Q 38Q 38
Many beautiful varieties of roses have been produced, particularly in the last few decades. These newer varieties often have very striking and showy flowers, making them desirable as horticultural specimens. However, breeders and novices alike have noticed that some of these newer varieties are not very fragrant compared with the older, more traditional varieties. From a genetic point of view, suggest an explanation why some of these newer varieties with superb flowers are not as fragrant.
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Q 39Q 39
An animal breeder had a herd of sheep with a mean weight of 254 lb at 3 years of age. He chose animals with mean weights of 281 lb as parents for the next generation. When these offspring reached 3 years of age, their mean weights were 269 lb.
A. Calculate the narrow-sense heritability for weight in this herd.
B. Using the heritability value that you calculated in part A, what weight of animals would you have to choose to get offspring that weigh 275 lb (at 3 years of age)
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Q 40Q 40
In your own words, explain the meaning of the term heritability. Why is a heritability value valid only for a particular population of individuals raised in a particular environment
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Q 41Q 41
The trait of blood pressure in humans has a frequency distribution that is similar to a normal distribution. The following graph shows the ranges of blood pressures for a selected population of people. The red line depicts the frequency distribution of the systolic pressures for the entire population. Several individuals with high blood pressure were identified, and the blood pressures of their relatives were determined. This frequency distribution is depicted with a blue line. (Note: The blue line does not include the people who were identified with high blood pressure; it includes only their relatives.)
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Q 42Q 42
What is the difference between broad-sense heritability and narrow-sense heritability Why is narrow-sense heritability such a useful concept in the field of agricultural genetics
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Q 43Q 43
The heritability for egg weight in a group of chickens on a farm in Maine is 0.95. Are the following statements regarding heritability true or false If a statement is false, explain why.
A. The environment in Maine has very little effect on the outcome of this trait.
B. Nearly all of the phenotypic variance for this trait in this group of chickens is due to genetic variance.
C. The trait is polygenic and likely to involve a large number of genes.
D. Based on the observation of the heritability in the Maine chickens, it is reasonable to conclude that the heritability for egg weight in a group of chickens on a farm in Montana is also very high.
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Q 44Q 44
In a fairly large population of people living in a commune in the southern United States, everyone cares about good nutrition. All of the members of this population eat very nutritious foods, and their diets are very similar. How do you think the height of individuals in this commune population would compare with that of the general population in the following categories
A. Mean height
B. Heritability for height
C. Genetic variation for alleles that affect height
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Q 45Q 45
When artificial selection is practiced over many generations, it is common for the trait to reach a plateau in which further selection has little effect on the outcome of the trait. This phenomenon is illustrated in Figure 27.11. Explain why.
FIGURE 27.11 Common results of selective breeding for a quantitative trait.
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Q 46Q 46
Discuss whether a natural population of wolves or a domesticated population of German shepherds is more likely to have a higher heritability for the trait of size.
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Q 47Q 47
With regard to heterosis, would the following statements be consistent with the dominance hypothesis, the overdominance hypothesis, or both
A. Strains that have been highly inbred have become monomorphic for one or more recessive alleles that are somewhat detrimental to the organism.
B. Hybrid vigor occurs because highly inbred strains are monomorphic for many genes, whereas hybrids are more likely to be heterozygous for those same genes.
C. If a gene exists in two alleles, hybrids are more vigorous because heterozygosity for the gene is more beneficial than homozygosity of either allele.
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