Q: If a quantitative trait is controlled by three independently assorting loci, each with two alleles contributing additively to the phenotype, how many distinct phenotypic categories are expected in the F2 generation?

$$2n + 1 = 2 \times 3 + 1 = 7$$

Q: A scientist crosses two pure-breeding lines of insects with wing length variances of $$40\ \mathrm{mm}^2$$ and $$80\ \mathrm{mm}^2. $$The F1 progeny have a variance of $$54\ \mathrm{mm}^2$$, and the F2 have a variance of $$92\ \mathrm{mm}^2. $$What is the estimated genotypic variance ($$V_G$$)?

$$V_G = V_{F2} - \text{mean}(V_{P1}, V_{P2}, V_{F1}) = 92 - ((40 + 80 + 54)/3) = 92 - 58 = 34\ \mathrm{mm}^2$$

Question 1

Which of the following best describes a quantitative trait?

Accepted Answer

A trait that shows a continuous distribution in a population due to the additive effects of multiple genes and environmental factors

Question 2

If a quantitative trait is controlled by three independently assorting loci, each with two alleles contributing additively to the phenotype, how many distinct phenotypic categories are expected in the F2 generation?

Accepted Answer

$$2n + 1 = 2 	imes 3 + 1 = 7$$

Question 3

Which equation correctly partitions phenotypic variance in a population?

Accepted Answer

$$V_P = V_G + V_E$$

Question 4

A scientist crosses two pure-breeding lines of insects with wing length variances of $$40\ \mathrm{mm}^2$$ and $$80\ \mathrm{mm}^2. $$The F1 progeny have a variance of $$54\ \mathrm{mm}^2$$, and the F2 have a variance of $$92\ \mathrm{mm}^2. $$What is the estimated genotypic variance ($$V_G$$)?

Accepted Answer

$$V_G = V_{F2} - 	ext{mean}(V_{P1}, V_{P2}, V_{F1}) = 92 - ((40 + 80 + 54)/3) = 92 - 58 = 34\ \mathrm{mm}^2$$

Question 5

Which of the following statements about broad-sense heritability ($$H^2) is $$correct?

Accepted Answer

$$H^2 is $$calculated as $$H^2$$ = \frac{V_G}{V_P}$$

Question 6

In twin studies, which formula is used to estimate broad-sense heritability ($$H^2) $$using monozygotic (MZ) and dizygotic (DZ) twins?

Accepted Answer

$$H^2 = 2(r$$_{MZ} - r_{DZ})$$

Question 7

Which of the following best explains why a high broad-sense heritability does NOT guarantee a strong response to selection?

Accepted Answer

Because broad-sense heritability includes all genetic variance, not just additive variance

Question 8

The breeder’s equation is $$R = h^2$ S. If $$the selection differential ($$S) is 4 $$and the response to selection ($$R) is 2$$, what is the narrow-sense heritability ($$h^2$$)?

Accepted Answer

$$h^2$$ = \frac{R}{S} = \frac{2}{4} = 0.5$$

Question 9

Which of the following is a key requirement for successful QTL mapping in a controlled cross?

Accepted Answer

A large number of individuals and markers to increase mapping precision

Question 10

In a QTL mapping experiment, which marker is most likely to be closely linked to a QTL influencing a trait?

Accepted Answer

A marker that shows very few recombinants with the trait

Question 11

If the phenotypic variance in a population is $$120$$, and the additive genetic variance is $$60$$, what is the narrow-sense heritability ($$h^2$$)?

Accepted Answer

$$h^2$$ = \frac{V_A}{V_P} = \frac{60}{120} = 0.5$$

Question 12

A population of plants has a mean height of 100 cm. After selecting and breeding the tallest plants (mean height 110 cm), the next generation has a mean height of 104 cm. What is the selection differential ($$S) $$and response to selection ($$R$$)?

Accepted Answer

$$S = 10\ \mathrm{cm},\ R = 4\ \mathrm{cm}$$

Question 13

Which of the following is an example of a threshold trait?

Accepted Answer

Blood pressure that results in hypertension only above a certain value

Question 14

Which of the following best describes the effect of environmental variance ($$V_E) on $$phenotypic variance ($$V_P$$)?

Accepted Answer

Increasing $$V_E$$ increases $$V_P$$

Question 15

In a diploid trihybrid cross ($$a^0a^1$; b^0$b^1$$; $$c^0c^1$ 	imes a^0$a^1$$; $$b^0b^1$; c^0$c^1$$), what is the frequency of the phenotypic class with a value of 4, assuming independent assortment and additive effects?

Accepted Answer

$$0.16$$

Question 16

Which of the following is true about additive gene effects in quantitative traits?

Accepted Answer

Each locus contributes equally and independently to the phenotype

Question 17

A QTL mapping experiment in dogs finds that marker B is strongly associated with canine hip dysplasia (CHD). What does this imply?

Accepted Answer

Marker B is located near a gene/locus that causes CHD

Question 18

Which of the following best describes the relationship between mean and variance in a normal distribution of a quantitative trait?

Accepted Answer

The mean determines the center of the distribution, while the variance determines the spread

Quantitative Traits, Variance, Heritability, and QTL Mapping

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