Question 1

Which of the following best distinguishes a discrete (qualitative) trait from a continuous (quantitative) trait?

Accepted Answer

Discrete traits are typically determined by variation at a single gene and show distinct phenotypic categories, while continuous traits are influenced by multiple genes and the environment, resulting in a range of phenotypes.

Question 2

In a cross between two pure-breeding wheat plants, one with red kernels and one with white kernels, the F2 generation shows a 1:4:6:4:1 ratio of kernel colors. What does this ratio suggest about the genetic control of kernel color?

Accepted Answer

Kernel color is controlled by two additive genes, each with two alleles.

Question 3

If a polygenic trait is controlled by $$n$$ additive genes, what is the formula to calculate the number of distinct phenotypic categories possible in the F2 generation?

Accepted Answer

$$2n+1$$

Question 4

A researcher is studying corolla length in a plant species. The parental lines are pure-breeding and show no variation within each line. The F1 generation is also genetically identical. Which of the following statements is true regarding the variance in these populations?

Accepted Answer

The phenotypic variance in both parental and F1 generations is due entirely to environmental variance.

Question 5

Which of the following best describes broad-sense heritability ($$H^2$$)?

Accepted Answer

$$H^2 is $$the proportion of total phenotypic variance that is due to all genetic variance, including additive, dominance, and epistatic effects.

Question 6

Given the following variances for a trait: total phenotypic variance $$V_P = 20$$, genetic variance $$V_G = 12$$, and environmental variance $$V_E = 8$$, what is the broad-sense heritability ($$H^2$$)?

Accepted Answer

$$H^2$$ = \frac{12}{20} = 0.6$$

Question 7

Which of the following statements about heritability is correct?

Accepted Answer

Heritability measures the proportion of phenotypic variance in a population that is due to genetic variance.

Question 8

A plant breeder wants to select for increased yield in corn. The trait has a high narrow-sense heritability ($$h^2). $$What does this imply for the breeder's selection program?

Accepted Answer

Most of the phenotypic variance is due to additive genetic variance, so selection will be effective.

Question 9

Which of the following is true about Quantitative Trait Loci (QTLs)?

Accepted Answer

QTLs are loci on chromosomes associated with variation in a quantitative trait.

Question 10

In a genome-wide association study (GWAS) for Crohn's disease, what is the main purpose of comparing SNPs between cases and controls?

Accepted Answer

To identify SNPs that are associated with the presence or absence of Crohn's disease.

Question 11

A population of plants is grown in two different environments. In Environment 1, the phenotypic variance ($$V_P) $$for height is 10, and in Environment 2, $$V_P is 30. If $$the genetic variance ($$V_G) is $$the same in both environments, what can be concluded about heritability in these two environments?

Accepted Answer

Heritability will be higher in Environment 1.

Question 12

Which of the following best explains why Mendelian principles are less useful for understanding complex traits?

Accepted Answer

Complex traits are influenced by many genes and environmental factors, making it difficult to distinguish discrete phenotypic categories.

Question 13

If the additive genetic variance ($$V_A) $$for a trait is 6 and the total phenotypic variance ($$V_P) is 12$$, what is the narrow-sense heritability ($$h^2$$)?

Accepted Answer

$$h^2$$ = \frac{6}{12} = 0.5$$

Question 14

A tomato breeder is mapping QTLs for fruit size. They use SNP markers to track inheritance. Why are SNPs useful in this context?

Accepted Answer

SNPs serve as genomic markers that are physically linked to QTLs, allowing breeders to track regions associated with the trait.

Question 15

Which of the following scenarios would result in a heritability estimate close to zero for a trait?

Accepted Answer

All individuals are genetically identical and grown in a highly variable environment.

Question 16

A frequency histogram of human height in a population is bell-shaped and continuous. What does this suggest about the genetic architecture of height?

Accepted Answer

Height is a polygenic trait influenced by multiple genes and environmental factors.

Question 17

In a breeding experiment, the mean time to maturity for three pure-breeding parental lines is 20, 22, and 24 days. The F1 mean is 22 days, and the F2 variance is 14.26. The environmental variance ($$V_E) is $$estimated as the average variance of the parental lines: $$(1.92 + 2.05 + 2.88)/3 = 2.28. $$What is the estimated genetic variance ($$V_G) in $$the F2?

Accepted Answer

$$V_G = 14.26 - 2.28 = 11.98$$

Question 18

Which of the following is a correct interpretation of a heritability estimate of 0.8 for a trait in a specific population?

Accepted Answer

Eighty percent of the phenotypic variance in the population is due to genetic differences among individuals.

Question 19

Why is it important to distinguish between broad-sense and narrow-sense heritability in breeding programs?

Accepted Answer

Narrow-sense heritability reflects the proportion of variance due to additive genetic effects, which are most relevant for predicting response to selection.

Quantitative Genetics: Discrete and Continuous Traits, Polygenic Inheritance, and Heritability

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