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

This question tests your understanding of inheritance patterns, specifically how two heterozygous parents (carriers) can produce offspring with different genotypes and phenotypes. It focuses on autosomal recessive inheritance, such as cystic fibrosis.

Key Terms and Formulas

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

• Homozygote: An individual with two identical alleles (FF or ff).

• Punnett Square: A diagram used to predict the genotypes of offspring from a genetic cross.

For two heterozygotes (Ff x Ff):

Step-by-Step Guidance

1. Set up the Punnett square with one parent's alleles (F and f) on the top and the other parent's alleles (F and f) on the side.

2. Fill in the Punnett square by combining each possible allele from the parents to show all possible offspring genotypes.

3. List the resulting genotypes: FF, Ff, fF, and ff.

4. Identify which genotype corresponds to the affected phenotype (usually ff for a recessive disorder).

5. Count the number of affected genotypes and calculate the probability (fraction or percentage) of producing an affected child.

Try solving on your own before revealing the answer!

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

Background

Topic: Population Genetics, Allele Frequency

This question explores why certain alleles, especially those associated with recessive disorders, persist in populations even when they are disadvantageous. It relates to concepts like carrier advantage, mutation, and genetic drift.

Key Terms and Concepts

• Allele frequency: The proportion of a specific allele in a population.

• Recessive allele: An allele that only affects phenotype when two copies are present.

• Carrier: An individual with one copy of a recessive allele (heterozygote).

• Selection: The process by which certain traits become more or less common depending on their impact on survival and reproduction.

Step-by-Step Guidance

1. Consider that heterozygous carriers (Ff) do not show the affected phenotype and can pass the f allele to offspring.

2. Think about whether selection acts only on the homozygous recessive (ff) individuals, not on carriers.

3. Reflect on how new mutations can introduce the f allele into the population.

4. Consider the role of genetic drift, especially in small populations, in maintaining or changing allele frequencies.

5. Evaluate whether there could be any advantage to being a carrier (heterozygote advantage) in certain environments.

Try solving on your own before revealing the answer!