Questions: Quantitative Genetics and Polygenic Traits

Question 1

A study reports that the heritability of height in a large European population is h² = 0.80. Which interpretation is correct?

• A. 80% of any individual's height is determined by their genes
• B. In this population, 80% of the variation in height between individuals is attributable to genetic differences
• C. 80% of the population has their height primarily controlled by genetics
• D. Environmental factors can account for at most 20% of height in any single person

Answer: (B)

Heritability is a population-level statistic about variance, not an individual-level claim. h² = 0.80 means that 80% of the phenotypic variance in height among people in that population is due to genetic variance. It says nothing about how much of any one person's height is 'caused' by their genes — that question doesn't have a meaningful answer in the same framework.

Question 2

A wheat breeder selects plants whose average height is 10 cm above the population mean (selection differential S = 10 cm). If heritability for height is h² = 0.40, what is the predicted mean height gain in the next generation?

• A. 40 cm, because the response scales with 1/h²
• B. 10 cm, because the selection differential is fully transmitted
• C. 4 cm, applying the breeder's equation R = h² × S
• D. 25 cm, because h² represents the proportion of variation not lost to environment

Answer: (C)

The breeder's equation R = h² × S gives R = 0.40 × 10 = 4 cm. Only the heritable fraction of the selection differential is transmitted to offspring. The selection differential of 10 cm includes both genetic and environmental components; heritability filters out the non-heritable part.

Question 3

A heritability estimate of 0.85 for IQ obtained in one high-income Western population can be directly generalized to predict that IQ heritability will also be near 0.85 in low-income populations with greater environmental variation.

• T. True
• F. False

Answer: False

Heritability is a population-specific statistic that depends on the amount of genetic and environmental variance present in that population. When environmental variation is greater (as in resource-poor settings), a larger fraction of phenotypic variance may be attributable to environment, and h² will be lower. The same trait can have very different heritabilities in different populations or environments.

Question 4

Polygenic traits show approximately normal phenotypic distributions in populations partly because the combined effect of many small, independent allele contributions tends toward a normal distribution as the number of loci increases.

• T. True
• F. False

Answer: True

This follows directly from the central limit theorem: the sum of many small, independent random variables — here, the additive contributions of alleles at many loci — converges on a normal distribution as the number of terms grows. With just a few loci, the distribution is stepped; with many loci plus environmental variation, it smooths into a bell curve.

Question 5

Explain why heritability is described as a property of a population in a specific environment, rather than a fixed property of a trait. What would happen to the heritability of a trait if all individuals in the population were raised in identical environments?

Model answer: Heritability = V_genetic / V_phenotypic. If all individuals experience identical environments, V_environmental approaches zero, so V_phenotypic ≈ V_genetic, and h² approaches 1.0 — regardless of how 'genetic' the trait really is. Conversely, if a population is genetically homogeneous but environments vary widely, h² approaches zero. This shows that heritability reflects the balance of genetic vs. environmental variance in a particular population and environment, not an intrinsic property of the trait itself.