Problem 9 - Biological Science

Question

In humans, albinism is caused by loss-of-function mutations in genes involved in the synthesis of melanin, the dark pigment in skin. Only people homozygous for a loss-of-function allele (genotype \(a a\) ) have the albino phenotype. In Americans of northern European ancestry, albino individuals are present at a frequency of about 1 in 10,000 (or 0.0001 ). Assuming that genotypes are in Hardy-Weinberg equilibrium, what is the predicted frequency of Caucasians in the United States who carry a single allele for albinism?

Step-by-Step Solution

Answer

The predicted frequency of Caucasians in the United States who carry a single allele for albinism is approximately 1.98%.

Step 1: Identify the given information

According to the exercise, the frequency of individuals with the albino phenotype (homozygous recessive genotype, aa) is 1 in 10,000, or 0.0001.

Step 2: Calculate the frequency of the recessive allele (q)

The frequency of the homozygous recessive genotype (aa) is given as 0.0001, which is equal to \(q^2\). To find the frequency of the recessive allele (q), we need to take the square root of 0.0001. \(q = \sqrt{0.0001} = 0.01\)

Step 3: Calculate the frequency of the dominant allele (p)

Now that we know the frequency of the recessive allele (q), we can calculate the frequency of the dominant allele (p), since p and q must sum to 1. \(p = 1 - q = 1 - 0.01 = 0.99\)

Step 4: Calculate the frequency of individuals carrying a single allele for albinism

We want to find the frequency of individuals who carry a single allele for albinism, which means they have the heterozygous genotype (Aa). According to the Hardy-Weinberg equation, this is represented by \(2pq\). The frequency of individuals carrying a single allele for albinism is: \(2pq = 2 * 0.99 * 0.01 = 0.0198\)

Step 5: Express the answer as a percentage

To express the result as a percentage, multiply the frequency by 100: \(0.0198 * 100 = 1.98\%\) So, the predicted frequency of Caucasians in the United States who carry a single allele for albinism is approximately 1.98%.

Key Concepts

Genetic Mutation

Genetic mutations are alterations in the DNA sequence that can lead to changes in an organism's traits. These changes can be as small as a single base pair (point mutations) or as large as rearrangements of large sections of chromosomes. Mutations can occur during DNA replication, by exposure to mutagens such as radiation or chemicals, or can be inherited from a parent.

Though often associated with negative effects, such as diseases or disorders, mutations are also a source of genetic variation within a population. This variation is essential for the process of natural selection and the evolution of species. In the context of the Hardy-Weinberg equilibrium, mutations can introduce new alleles into a gene pool or change the frequencies of existing alleles over time, potentially disrupting the equilibrium.

Albinism

Albinism is a group of genetic conditions characterized by a lack of melanin, the pigment that colors skin, hair, and eyes. The most common type, oculocutaneous albinism (OCA), involves the eyes, hair, and skin. People with albinism often have lighter-than-average skin and hair coloration and can experience vision problems due to lack of pigmentation in the eyes.

The condition is inherited in an autosomal recessive pattern, which means a person must receive two copies of the mutated allele (one from each parent) to exhibit the traits of albinism. If an individual has only one copy of the mutated allele, they do not show the characteristics of albinism but are carriers and can pass the allele to their offspring.

Allele Frequency

Allele frequency, also known as gene frequency, refers to how often a particular allele appears within a given population's gene pool. It is a measure of genetic variation and is used to predict how traits can spread or evolve. In a stable population, where the Hardy-Weinberg equilibrium applies, allele frequencies remain constant from one generation to the next, barring any disturbances such as mutations, genetic drift, migration, or selection.

The allele frequency is essential for calculating expected genotype frequencies for a population under the Hardy-Weinberg equilibrium. By analyzing changes in allele frequency, scientists can infer patterns of genetic diversity, the impact of evolutionary forces, and predict the likelihood of genetic diseases.

Homozygous Recessive

An individual with a homozygous recessive genotype has two copies of the same recessive allele for a particular gene. This genetic configuration means that the recessive trait will be expressed, as there is no dominant allele present to mask it. In classical Mendelian genetics, a person with a homozygous recessive genotype regarding a trait like albinism will display that trait.

Recessive traits often require that an individual receives the recessive allele from both parents. In the context of Hardy-Weinberg equilibrium, the frequency of homozygous recessive individuals (\(q^2\) in the formula) can be used to calculate the allele frequency (\(q\) and \(p\) values) and predict genetic trait distribution in the population.