Mechanisms of Evolution: Step-by-Step Study Guidance

Q1. What is the chance that two heterozygotes will produce an affected child? Make a Punnett square to show your results.

Background

Topic: Mendelian Genetics – Punnett Squares and Probability

This question tests your understanding of how to use a Punnett square to predict the probability of offspring genotypes from two heterozygous parents, specifically for a recessive genetic disorder.

Key Terms and Formulas

• Heterozygote: An individual with two different alleles for a gene (e.g., Ff).

• Homozygous recessive: An individual with two copies of the recessive allele (ff), which is typically the affected genotype for a recessive disorder.

• Punnett Square: A diagram used to predict the genotypes of offspring from parental alleles.

Step-by-Step Guidance

1. Write out the genotypes of both parents. In this case, both are heterozygotes: Ff × Ff.

2. List the possible gametes each parent can produce. Each can contribute either an F or an f allele.

3. Set up a 2x2 Punnett square. Place the possible gametes from one parent across the top and from the other parent down the side.

4. Fill in the Punnett square by combining the alleles from each parent in each box to show all possible offspring genotypes.

5. Count the number of boxes that represent the affected genotype (ff) and determine the probability as a fraction or percentage.

Try solving on your own before revealing the answer!

Final Answer: 25% chance (1 in 4) that two heterozygotes will produce an affected child (ff).

The Punnett square for Ff × Ff yields the following genotypes: FF, Ff, Ff, and ff. Only one out of four boxes is ff, so the probability is 1/4 or 25%.

Q2. Why might the frequency of the f allele fail to drop over time?

Background

Topic: Population Genetics – Allele Frequency and Evolutionary Forces

This question explores why a recessive allele (such as the one causing cystic fibrosis) may persist in a population, even if it is associated with a harmful phenotype when homozygous.

Key Terms and Concepts

• Allele Frequency: The proportion of a specific allele among all alleles for a gene in a population.

• Heterozygote: Individuals who carry one copy of the recessive allele but do not express the affected phenotype.

• Selection: The process by which certain genotypes are favored or disfavored in a population.

Step-by-Step Guidance

1. Consider how the f allele is inherited and expressed. Remember that only individuals with two copies (ff) are affected.

2. Think about what happens to the f allele in heterozygous individuals (Ff). Are they affected by selection against the disease?

3. Reflect on whether selection can efficiently remove the f allele from the population if it is 'hidden' in carriers (Ff).

4. Consider other factors that might maintain the f allele in the population, such as mutation, genetic drift, or possible heterozygote advantage.

Try solving on your own before revealing the answer!

Final Answer:

The frequency of the f allele may fail to drop over time because it is carried by heterozygotes (Ff), who do not show the affected phenotype and are not selected against. As a result, the allele can persist in the population, especially if there is no strong selection against carriers or if new mutations introduce the allele. In some cases, heterozygote advantage can also maintain the allele.