Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels

Problema 21b

Capitolo 20, Problema 21b

The frequency of tasters and nontasters of PTC varies among populations. In population A, 64% of people are tasters (an autosomal dominant trait) and 36% are nontasters. In population B, tasters are 75% and nontasters 25%. In population C, tasters are 91% and nontasters are 9%.
Assuming that Hardy–Weinberg conditions apply, determine the genotype frequencies in each population.

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Start by recalling the Hardy-Weinberg principle, which states that the genotype frequencies in a population can be represented as p² (homozygous dominant), 2pq (heterozygous), and q² (homozygous recessive), where p is the frequency of the dominant allele and q is the frequency of the recessive allele. Also, p + q = 1.

For each population, identify the frequency of nontasters (q²), as nontasting is the recessive trait. For example, in population A, q² = 0.36, in population B, q² = 0.25, and in population C, q² = 0.09.

Calculate q (the frequency of the recessive allele) for each population by taking the square root of q². For example, q = √q².

Determine p (the frequency of the dominant allele) for each population using the equation p + q = 1. Solve for p by subtracting q from 1.

Finally, calculate the genotype frequencies for each population: p² (homozygous dominant), 2pq (heterozygous), and q² (homozygous recessive). Use the values of p and q obtained in the previous steps to compute these frequencies using the formulas p² = p × p, 2pq = 2 × p × q, and q² = q × q.

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Hardy-Weinberg Principle

The Hardy-Weinberg Principle is a fundamental concept in population genetics that describes how allele and genotype frequencies remain constant from generation to generation in a population under certain conditions. These conditions include a large breeding population, random mating, no mutation, no migration, and no natural selection. This principle provides a baseline to compare actual genetic variation in populations.

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Hardy Weinberg

Autosomal Dominance

Autosomal dominance refers to a pattern of inheritance where only one copy of a dominant allele is necessary for the expression of a trait. In the case of PTC tasting, the ability to taste is dominant, meaning individuals with at least one dominant allele (T) will be tasters, while those with two recessive alleles (tt) will be nontasters. Understanding this concept is crucial for calculating genotype frequencies based on phenotype ratios.

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Variations on Dominance

Genotype Frequencies

Genotype frequencies represent the proportion of different genotypes in a population. In the context of the Hardy-Weinberg equation, these frequencies can be calculated using the allele frequencies. For a trait with two alleles, the frequencies of homozygous dominant (TT), heterozygous (Tt), and homozygous recessive (tt) genotypes can be derived from the overall frequency of the dominant and recessive phenotypes, allowing for predictions about genetic variation in populations.

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Gamete Genotypes

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The frequency of tasters and nontasters of PTC varies among populations. In population A, 64% of people are tasters (an autosomal dominant trait) and 36% are nontasters. In population B, tasters are 75% and nontasters 25%. In population C, tasters are 91% and nontasters are 9%.

Calculate the frequency of the dominant (T) allele for PTC tasting and the recessive (t) allele for nontasting in each population.

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Tay–Sachs disease is an autosomal recessive neurological disorder that is fatal in infancy. Despite its invariably lethal effect, Tay–Sachs disease occurs at very high frequency in some Central and Eastern European (Ashkenazi) Jewish populations. In certain Ashkenazi populations, 1 in 750 infants has Tay–Sachs disease. Population biologists believe the high frequency is a consequence of genetic bottlenecks caused by pogroms (genocide) that have reduced the population multiple times in the past several hundred years. In the population described, what is the frequency of the recessive allele that produces Tay–Sachs disease?

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Tay–Sachs disease is an autosomal recessive neurological disorder that is fatal in infancy. Despite its invariably lethal effect, Tay–Sachs disease occurs at very high frequency in some Central and Eastern European (Ashkenazi) Jewish populations. In certain Ashkenazi populations, 1 in 750 infants has Tay–Sachs disease. Population biologists believe the high frequency is a consequence of genetic bottlenecks caused by pogroms (genocide) that have reduced the population multiple times in the past several hundred years. Explain how a genetic bottleneck and its aftermath could result in a population that carries a lethal allele in high frequency.

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Epidemiologic data on the population in the previous problem reveal that before the application of modern medical treatment, natural selection played a major role in shaping the frequencies of alleles. Heterozygous individuals have the highest relative fitness, and in comparison with heterozygotes, those who are βᴬβᴬ have a relative fitness of 82%, but only about 32% of those with SCD survived to reproduce. What are the estimated equilibrium frequencies of βᴬ and βˢ in this population?

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Sickle cell disease (SCD) is found in numerous populations whose ancestral homes are in the malaria belt of Africa and Asia. SCD is an autosomal recessive disorder that results from homozygosity for a mutant β-globin gene allele. Data on one affected population indicates that approximately 8 in 100 newborn infants have SCD.

What is the frequency of carriers of SCD in the population?

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