Chapter 27 & 28 Genetics

What is a "SNP"? SNPs represent _____% of all of variations in DNA sequences that occur among humans.

An SNP is a single-nucleotide polymorphism, which is the smallest type of genetic change that can occur within a given gene and are also the most common type of change. SNPs represent 90% of all variations in DNA sequences that occur among humans.

What is the range of correlation coefficients? Format your answer as: # to #

-1 to 1

If broad sense and narrow sense heritability are the same value, you can conclude that V(D) and V(I) equal what value?

0

If a population is at Hardy-Weinberg equilibrium, at what frequency (decimal form; 1 significant digit) of the dominant allele is the frequency of heterozygotes maximized?

0.5

The mean weight of cows in a population is 550 kg. Animals with a mean weight of 575 kg are used as parents and produce offspring with a mean weight of 565 kg. What is the narrow-sense heritability for body weight of this population of cows?

0.6

Assume that a population at Hardy-Weinberg equilibrium undergoes directional selection. The starting p value is 0.6 and the relative fitness values for the homozygous dominant, heterozygous, and homozygous recessive individuals are 1.0, 0.6, and 0.1. What is the mean fitness of the population (rounded to two significant digits)?

0.66

If a population of monomorphic mice are created by inbreeding, V(G) = _____ (number) and all phenotypic variance observed is due to _______ (one word) variance.

0; environmental

How many types of variance contribute to the genetic variance?

3

The standard deviation of a sample of plant heights is 1.77 cm. What is the variance of this sample (2 significant digits)?

3.1

Consider the following data collected for plant height at 3 weeks (cm) 8, 9, 4, 7, 7, and 11 What is the average value (2 significant digits) for this data set?

7.7

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.

A discontinuous trait is one that falls into discrete categories. Examples include brown eyes versus blue eyes in humans and purple versus white flowers in pea plants. A continuous trait is one that does not fall into discrete categories. Examples include height in humans and fruit weight in tomatoes. Most quantitative traits are continuous; the trait falls within a range of values. The reason why quantitative traits are continuous is because they are usually polygenic and greatly influenced by the environment.

C1. What is the gene pool?

A gene pool is all of the alleles of every gene in a population.

What does it mean when a correlation coefficient is negative? Can you think of examples?

A negative correlation means that two variables are related to each other in opposite ways. For example, a high amount of snowfall in the winter might be negatively correlated with the survival of wild species.

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?

A normal distribution varies in a symmetrical way around a mean value. When graphed, it exhibits a bellshaped curve. It is common for quantitative traits in a population to have a normal distribution around a mean value. If a distribution is normal, the standard deviation can predict the percentage of individuals that will fall within certain limits. For example, approximately 68% of all individuals in a population will be within one standard deviation unit (plus or minus) from the mean

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 are equal for both types of alleles (i.e., 50% heavy alleles and 50% light alleles), what percentage of individuals will be homozygous for the light alleles in all of the genes affecting this trait, if the trait was determined by the following number of genes? A. Two B. Three C. Four

A. 6.25% B. 1.56% C. 0.39%

A gene exists in two alleles and the heterozygote has the highest fitness. This scenario is likely to result in : A. Balancing selection B. Directional selection C. Stabilizing selection D. Disruptive selection

A. Balancing selection

The heritability for egg weight in a group of chickens on a farm in Maine is 0.95. Are the following statements regarding this 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.

A. False. The environment has very little impact on the amount of variation in the trait. However, environment always contributes greatly to the outcome of every trait. You could not have a living organism without an environment. B. True. C. Probably true, but we cannot say for sure. Quantitative traits are usually polygenic. D. False. You cannot say anything about the heritability of egg weight in Montana chickens. It depends on the amount of genetic variation in the population and on the type of environment in which they are raised.

C4. Identify each of the following as an example of allele, genotype, and/or phenotype frequency: A. Approximately 1 in 2500 people of Northern European descent is born with cystic fibrosis. B. The percentage of carriers of the sickle cell allele in West Africa is approximately 13%. C. The number of new mutations for achondroplasia, a genetic disorder, is approximately 5 × 10-5.

A. Genotype frequency and genotype frequency B. Genotype frequency C. Allele frequency

Two different varieties of potato plants produce potatoes with the same mean weight of 1.5 pounds. One variety has a very low variance for potato wieght, 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.

A. The differences in variance could be explained by variation in the way the potatoes are affected by environmental variation, or they could be explained by genetic differences. The strain with a higher variance may have more genetic variation. B. On the one hand, the strain with a higher variance may be better because the farmer may want to select for individuals that produce larger potatoes. On the other hand, if the farmer wants the size of his/her potatoes to be uniform, the population with a smaller variance would be better. C. You would use the strain with a higher variance, hoping that a significant proportion of the variance wasdue to genetic variation

C6. Cystic fibrosis (CF) is a recessive autosomal disorder. In certain populations of Northern European descent, the number of people born with this disorder is about 1 in 2500. Assuming Hardy-Weinberg equilibrium for this trait: A. What are the frequencies for the normal and CF alleles? B. What are the genotype frequencies of homozygous normal, heterozygous, and homozygous affected individuals? C. Assuming random mating, what is the probability that two phenotypically unaffected heterozygous carriers will choose each other as mates?

A. The genotype frequency for the CF homozygote is 1/2,500, or 0.004. This is equal to q^2. The allele frequency is the square root of this value, which equals 0.02. The frequency of the corresponding dominant allele is 1 - 0.02 = 0.98 B. The frequency for the CF homozygote is 0.004; for the unaffected homozygote, it is (098)2 = 0.96; and for the heterozygote, it is 2(0.98)(0.02) = 0.039. C. If a person is known to be a heterozygous carrier, the chances that this person will happen to choose another heterozygous carrier as a mate is equal to the frequency of heterozygous carriers in the population, which is 0.039, or 3.9%. The chances that two randomly chosen individuals will choose each other as mates equals 0.039 × 0.039 = 0.0015, or 0.15%.

For selective breeding to be successful, the starting population must: a. have genetic variation that affects the trait of interest. b. be very large. c. be amenable to phenotypic variation caused by environmental effects. d. have very little phenotypic variation

A. have genetic variation that affects the trait of interest

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?

At the molecular level, quantitative traits often exhibit a continuum of phenotypic variation because they are usually influenced by multiple genes that exist as multiple alleles. A large amount of environmental variation will also increase the overlap between genotypes and phenotypes for polygenic traits

Which one of the following is a factor that does not promote widespeard changes in allele or genotype frequencies? A. Natural selection B. New mutation C. Migration D. Nonrandom mating E. Genetic drift

B. New mutation

If two or more different genotypes do not respond to environmental variation in the same way, this outcome is due to a. a genotype-environment association. b. a genotype-environment interaction. c. the additive effects of alleles. d. both a and b.

B. a genotype-environment interaction

One way to estimate narrow-sense heritability for a given trait is to compare __________ for _________. a. variances, related pairs of individuals b. correlation coefficients, related pairs of individuals c. variances, unrelated pairs of individuals d. correlation coefficients, unrelated pairs of individuals

B. correlation coefficients; related pairs of individuals

C8. In a population, the frequencies of two alleles are B = 0.67 and b = 0.33. The genotype frequencies are BB = 0.50, Bb = 0.37, and bb = 0.13. Do these numbers suggest inbreeding? Explain why or why not

BB = (0.67)^2 = 0.45, or 45% Bb = 2(0.67)(0.33) = 0.44, or 44% bb = (0.33)^2 = 0.11, or 11% The actual data show a higher percentage of homozygotes (compare 45% with 50% and 11% with 13%) and a lower percentage of heterozygotes (compare 44% with 37%) than expected. Therefore, these datawould be consistent with inbreeding, which increases the percentage of homozygotes and decreases the percentage of heterozygotes

C9. The ability to roll your tongue is inherited as a recessive trait. The frequency of the rolling allele is approximately 0.6, and that of thedominant (nonrolling) allele is 0.4. What is the frequency of individuals who can roll their tongues?

Because this is a recessive trait, only the homozygotes for the rolling allele will be able to roll their tongues. If p equals the rolling allele and q equals the nonrolling allele, the Hardy-Weinberg equation predicts that the frequency of homozygotes who can roll their tongues would be p^ 2In this case, p 2 = (0.6) 2 = 0.36, or 36%.

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?

Broad-sense heritability takes into account all genetic factors that affect the phenotypic variation in a trait. Narrow-sense heritability considers only alleles that behave in an additive fashion. In many cases, the alleles affecting quantitative traits appear to behave additively. More importantly, if a breeder assumes that the heritability of a trait is due to the additive effects of alleles, it is possible to predict the outcome of selective breeding.

The mean weight of cows in a population is 520 kg. Animals with a mean weight of 540 kg are used as parents and produce offspring that have a mean weight of 535 kg. What is the narrow-sense heritability (hN 2) for body weight in this population of cows? a. 0.25 b. 0.5 c. 0.75 d. 1.0

C. 0.75

In a population of squirrels in North Carolina, the heritability for body weight is high. This means that a. body weight is primarily controlled by genes. b. the environment has little influence on body weight. c. the variance in body weight is mostly due to genetic variation. d. both a and b are correct

C. the variance in body weight is mostly due to genetic variation

Distinguish between continuous and discontinuous traits. Which one is associated with quantitative traits?

Continuous traits, such as height and weight, do not fall into discrete categories. Continuous traits are quantitative and discontinuous traits are qualitative.

Saying that a quantitative traits follows a continuum means that : A. environmental effects are additive. B. the trait continuously changes during the life of an individual. C. the numerical value of the trait increasese with the age of the individual. D. Phenotypes are continuous and don't fall into discrete categories.

D. Phenotypes are continuous and don't fall into discrete categories.

________ (one word) fitness is defined as the relative likelihood that one genotype will contribute to the gene pool of the next generation compared with other genotypes.

Darwinian

Which of the following can affect the overlaps between phenotypes and different genotypes? A. Gene number B. Heritability C. Environmental variation D. A and B E. A and C

E. A and C

Which of the following factors can contribute to a selection limit being observed? A. Monomorphic alleles B. Low environmental variance C. Negative effects on fitness D. A and B E. A and C F. B and C G. A, B, and C

E. A and C

Which one of the following is NOT a condition/assumption of the Hardy-Weinberg equation? A. No natural selection B. No migration C. No new mutations D. Random mating E. Unchanging allele frequencies in successive generations F. All of the above are correct conditions/assumptions

F. All of the above are correct conditions/assumptions

True or False: Genetic drift is more likely to affect dominant alleles in a population than recessive alleles.

False

C7. For a gene existing in two alleles, what are the allele frequencies when the heterozygote frequency is at its maximum value, assuming Hardy-Weinberg equilibrium? What if there are three alleles?

For two alleles, the heterozygote is at a maximum when they are 0.5 each. For three alleles, the two heterozygotes are at a maximum when each allele is 0.33.

C3. What is genetic polymorphism? What is the source of genetic variation?

Genetic polymorphism refers to the observation that many traits display variation within a population. Mutation is the ultimate source of genetic variation.

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?

If the broad-sense heritability equals 1.0, it means that all of the variation in the population is due to genetic variation rather than environmental variation. It does not mean that the environment is unimportant in the outcome of the trait.

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 28.11. Explain why it occurs

In some cases, the alleles that are being selected will become monomorphic in the population, and further selection will have no effect. Alternatively, a second possible reason for a selection limit is natural selection. Some genes that respond to artificial selection have a negative impact on the overall fitness of a population. A selection limit is reached in which the desired effects of artificial selection are balanced by the negative effects on fitness.

C10. What evolutionary factors can cause allele frequencies to change and possibly lead to a genetic polymorphism? Discuss the relative importance of each type of process.

Migration, genetic drift, and natural selection are the main factors that alter allele frequencies within a population.

What are the four mechanisms that can alter existing genetic variation? Which one of these mechanisms doesn't result in changes to allele frequency?

Natural selection, genetic drift, migration and nonrandom mating. Nonrandom mating does not change allele frequency.

What two-word phrase is equivalent to a "bell-shaped curve"?

Normal distribution

What is polygenic inheritance? Discuss the issues that make polygenic inheritance difficult to study.

Polygenic inheritance refers to the transmission of traits that are governed by two or more different genes. Such traits are difficult to study because there may be multiple genes with multiple alleles, and environmental effects may be significant.

Give several examples of quantitative traits. How are these quantitative traits described within groups of individuals?

Quantitative traits are described numerically. Examples include height, weight, speed, and metabolic rate. In a population, a trait may be given a mean value, and the degree of variation may be described by thevariance and standard deviation.

Examine figure 28.4. Explain how gene number and environmental variation affects the overlaps between phenotypes and different genotypes.

The ability of geneticists to correlate genotype and phenotype depends on how many genes are involved and how much the environment causes the phenotype to vary.

What is the possible range of values for the correlation coefficient? Explain the interpretation of +, 0, and - correlation coefficients.

The range of values can be +, 0 or -. A positive value means that the two variables are relative to each other in the same way. A value of zero means that the two factors do not vary, and a negative value means that the two factors are opposite of each other.

Are polymorphisms common or rare in natural populations?

They are extremely common in natural populations. In humans, a polymorphic gene occurs about 10 times every 2000-3000 genes.

The variance for weight in a particular herd of cattle is 484 pounds . The mean weight is 562 pounds. How heavy would an animal have to be if it was in the top 2.5% of the herd? The bottom 0.13%?

To be in the top 2.5% is about two standard deviation above the mean. If we take the square root of the variance, the standard deviation would be 22 pounds. To be in the top 2.5% an animal would have to weigh at least 44 pounds more than the mean weight, or 562 + 44 = 606 pounds. To be in the bottom 0.13%, an animal would have to be three standard deviations lighter than the mean weight, or at least 66 pounds lighter, which gives a weight of 496 pounds.

True or False: An allele that is "fixed" in a population is regarded as "monomorphic".

True

True or False: Directional selection results in a normal, bell-shaped curve distribution of phenotypes.

True

True or False: Evolution results in changes to allele frequencies.

True

True or False: Quantitative traits are typically polygenic.

True

True or False: The maximum value of "w" is 1.0.

True

What is the expected correlation coefficient for identical twins? For full-sibs (i.e. children with the same father and mother)?

Twins: 1.0 Siblings: 0.50

C5. The term polymorphism can refer to both genes and traits. Explain what is meant by a polymorphic gene and a polymorphic trait. If a gene is polymorphic, does the trait that the gene affects also have to be polymorphic? Explain why or why not.

When a trait is polymorphic, this means that different individuals show phenotypic variation with regard to the trait. Gene polymorphism does not always result in phenotypic polymorphism.

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?

When you examine a trait in a population, it often varies among different members. Sometimes the variation is large, and sometimes it is small. The variation can be caused by genetic differences among the members of the population, and it can be caused by the fact that each member of the population is exposed to a slightly or greatly different environment.

What is a frequency distribution? Explain how such a graph is made for a quantitative trait that is continuous.

With regard to genetics, a frequency distribution is a graph that compares the number of individuals and their phenotypes for a given trait. This is a common way to depict the distribution of a quantitative trait in a population. To make a frequency distribution for a continuous quantitative trait, it is necessary to divide individuals into arbitrary groups.

Which axis (X or Y) of a frequency distribution presents the number of individuals displaying a specific phenotype?

Y

A gene exists in two alleles designated D and d. If 48 copies of this gene are the D allele and 152 are the d allele, what is the allele frequency of D? a. 0.24 c. 0.38 b. 0.32 d. 0.76

a. 0.24 Take 48/200 since 48+152 = 200

Consider a population of pea plants that consists of 325 homozygous tall, 500 heterozygous tall, and 400 dwarf plants. On the basis of this information, what is: a. The frequency (decimal form) of the heterozygous plants? Round to 2 significant digits. b. The sum of the frequencies of all three genotype frequencies (1 significant digit). c. The frequency (decimal form) of the dominant allele? Round to 2 significant digits.

a. 0.41 b. 1 c. 0.47

Assume a population of purple and white flower pea plants (2000 total plants) is in Hardy-Weinberg equilibrium. The frequency of white flower plants is 0.16. On the basis of this information, determine the following: a. The frequency (decimal form) of heterozygous purple plants. Round to 2 significant digits. b. The frequency (decimal form) of the dominant allele (1 significant digit). c. The number of homozygous purple flower plants.

a. 0.48 b. 0.6 c. 720

Which of the following is a factor that, by itself, does not promote widespread changes in allele or genotype frequencies? a. New mutation d. Migration b. Natural selection e. Nonrandom mating c. Genetic drift

a. New mutation

What is the expected correlation coefficient between: a. A parent and child? b. Two full siblings (i.e. same mother and father)? c. Identical twins?

a. Parent-child: 0.48 +/- 0.04 b. Sibling-sibling: 0.50 +/- 0.04 c. Indentical twins: 0.95 +/- 0.01

The variance is a. a measure of the variation around the mean. b. computed as a squared deviation. c. higher when there is less phenotypic variation. d. Both a and b are correct

a. a measure of the variation around the mean

In natural populations, most genes are a. polymorphic. c. recessive. b. monomorphic. d. both a and c

a. polmorphic

What term (one word) is used to describe alleles contributing to the outcome of a trait in an incremental way?

additive

What is an equivalent term for selective breeding (two words)?

artificial selection

Which of the following statistics is used to compare two variables? a. Mean b. Correlation coefficient c. Variance d. Standard deviation

b. correlation coefficient

A QTL is a __________ where one or more genes affecting a quantitative trait are ________ . a. site in a cell, located b. site in a chromosome, located c. site in a cell, expressed d. site in a chromosome, expressed

b. site in a chromosome; located

Which genetic effect (one word) can influence genetic drift via significant changes in the gene pool composition due to the loss of population members by particular natural events (e.g. disease)?

bottleneck

What term is defined as ratio of genetic variance to phenotypic variance?

broad-sense heritability

To map QTLs, strains are crossed that differ with regard to a. a quantitative trait. b. molecular markers. c. a quantitative trait and molecular markers. d. a quantitative trait and a discontinuous trait.

c. a quantitative trait and molecular markers

A gene pool is: a. all of the genes in a single individual. b. all of the genes in the gametes from a single individual. c. all of the genes in a population of individuals. d. the random mixing of genes during sexual reproduction.

c. all of the genes in a population of individuals

Saying that a quantitative trait follows a continuum means that a. the numerical value for the trait increases with the age of the individual. b. environmental effects are additive. c. the phenotypes for the trait are continuous and do not fall into discrete categories. d. the trait continuously changes during the life of an individual

c. the phenotypes for the trait are continous and do not fall into discrete categories

What term (one word) is used to describe the population that consists of immigrants and the original members?

conglomerate

Narrow-sense heritability values can also be estimated using a ratio of _________ (one word) coefficients.

correlation

Which of the following is an example of a quantitative trait? a. Height b. Rate of glucose metabolism c. Ability to learn a maze d. All of the above are quantitative traits.

d. All of the above are quantitative traits

Gene flow is depends on: a. Migration b. The ability of migrant alleles to be passed on to subsequent generations c. Genetic drift d. a and b e. a and c f. a, b, and c

d. a and b

For many quantitative traits, genotypes and phenotypes tend to overlap because a. the trait changes over time. b. the trait is polygenic. c. environmental variation affects the trait. d. both b and c are true.

d. both b and c are true

If two traits have a negative correlation coefficient, then as the values for one trait increase, values for the second trait tend to _________

decrease

Examine figure 28.4, part A. What factor/effect (one word) explains the difference between the two graphs?

environmental

What are two primary sources of variance (one word each) that contribute to observed phenotype variance in a population?

environmental; genetic

V(I) is the variance due to the effects of alleles that interact in an ______ (one word) manner.

epistatic

What are the two possible extreme outcomes for allele frequencies with regards to genetic drift (one word each)?

fixation; loss

Which genetic effect (one word) can influence genetic drift via migration of individuals from one population to another?

founder

What phrase (2 words) is used to describe changes in a populations allele frequencies due to random fluctuations?

genetic drift

The mean fitness of a population is calculated based on relative fitness and ______ (one word) frequencies.

genotype

Realized heritability is also defined as _______ (two words) heritability.

narrow; sense

Darwin and Wallace proposed the theory of evolution by ___________ (2 words).

natural selection

Natural selection acts on _______ that are derived from the individuals' _________.

phenotypes; genotypes

Realized heritability is the proportion of __________ variance that can be used to predict changes in the population ______ for a trait when selection is practiced.

phenotypic; mean

The equation to calculate the selection coefficient is:

s = 1-w

The standard deviation is calculated as the ___________ ___________ of the variance.

square root

Which type of selection reduces the variation of the phenotype range of a trait?

stabilizing