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Chapter 20: Problem 9

What kind of heritability estimates (broad sense or narrow sense) are obtained from human twin studies?

Short Answer

Expert verified
Answer: Human twin studies provide estimates of narrow sense heritability (h^2) because they focus on the portion of phenotypic variation that is attributable to additive genetic effects. This is achieved by comparing the phenotypic similarities between monozygotic and dizygotic twins, while non-additive effects, such as dominance and epistasis, are not precisely captured.

Step by step solution

01

Understanding Broad Sense and Narrow Sense Heritability

Broad sense heritability (H^2) is the proportion of phenotypic variation in a population that is attributable to genetic variation. It includes all genetic effects (additive, dominance, and interaction). Narrow sense heritability (h^2) refers to the proportion of phenotypic variation that is due to additive genetic effects only. Additive genetic effects are those that can be passed from parents to offspring and can be used to predict the response to selection.
02

Understanding Twin Studies

Twin studies are used to estimate heritability by comparing the phenotypic similarities between monozygotic (identical) twins and dizygotic (fraternal) twins. Monozygotic twins share 100% of their genes, while dizygotic twins share, on average, 50% of their genes. By comparing the phenotypic similarities of these two types of twins, researchers can estimate the heritability of a particular trait.
03

Heritability Estimates from Twin Studies

Twin studies provide estimates of narrow sense heritability (h^2). This is because the additive genetic effects are directly measured by comparing the phenotypic similarities between monozygotic and dizygotic twins, while non-additive effects (such as dominance and epistasis) are not precisely captured. By comparing the phenotypic correlations between monozygotic and dizygotic twins, researchers can estimate the proportion of phenotypic variation due to additive genetic effects, which is the narrow sense heritability. To sum up, human twin studies are suited for obtaining narrow sense heritability estimates since they focus on the portion of phenotypic variation that is attributable to additive genetic effects.

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Most popular questions from this chapter

Define the following: (a) polygenic, (b) additive alleles, (c) monozygotic and dizygotic twins, (d) heritability, and (e) QTL.

In a herd of dairy cows the narrow-sense heritability for milk protein content is \(0.76,\) and for milk butterfat it is \(0.82 .\) The cor- relation coefficient between milk protein content and butterfat is \(0.91 .\) If the farmer selects for cows producing more butterfat in their milk, what will be the most likely effect on milk protein content in the next generation?

List as many human traits as you can that are likely to be under the control of a polygenic mode of inheritance.

A strain of plants has a mean height of \(24 \mathrm{cm} .\) A second strain of the same species from a different geographical region also has a mean height of \(24 \mathrm{cm}\). When plants from the two strains are crossed together, the \(F_{1}\) plants are the same height as the parent plants. However, the \(\mathrm{F}_{2}\) generation shows a wide range of heights; the majority are like the \(P_{1}\) and \(F_{1}\) plants, but approximately 4 of 1000 are only $12 \mathrm{cm}\( high, and about 4 of 1000 are \)36 \mathrm{cm}$ high. (a) What mode of inheritance is occurring here? (b) How many gene pairs are involved? (c) How much does each gene contribute to plant height? (d) Indicate one possible set of genotypes for the original \(\mathrm{P}_{1}\) parents and the \(\mathrm{F}_{1}\) plants that could account for these results. (e) Indicate three possible genotypes that could account for \(\mathrm{F}_{2}\) plants that are \(18 \mathrm{cm}\) high and three that account for \(\mathrm{F}_{2}\) plants that are \(33 \mathrm{cm}\) high.

A dark-red strain and a white strain of wheat are crossed and produce an intermediate, medium-red \(\mathrm{F}_{1}\). When the \(\mathrm{F}_{1}\) plants are interbred, an \(\mathrm{F}_{2}\) generation is produced in a ratio of 1 dark-red: 4 medium-dark-red: 6 medium-red: 4 light-red: 1 white, Further crosses reveal that the dark-red and white \(\mathrm{F}_{2}\) plants are true breeding. (a) Based on the ratios in the \(\mathrm{F}_{2}\) population, how many genes are involved in the production of color? (b) How many additive alleles are needed to produce each possible phenotype? (c) Assign symhols to these alleles and list pnssible genotypes that give rise to the medium-red and light-red phenotypes. (d) Predict the outcome of the \(F_{1}\) and \(F_{2}\) generations in a cross between a true-breeding medium-red plant and a white plant.
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