Cystic fibrosis, which is usually lethal before the age of reproduction, is a homozygous recessive trait. Why do cases continue to arise, even though people with the disease rarely live to reproduce?

• A. New mutations continually introduce this harmful condition into the population.
• B. Mosquitoes can transfer the disease from person to person.
• C. People continue to make inappropriate lifestyle choices.
• D. None of the listed responses is correct.
• E. The harmful allele "hides" within heterozygous individuals, and one-fourth of the offspring of two heterozygotes would be afflicted.

Answer: (E)

Verified step by step guidance

1. Understand that cystic fibrosis is a genetic disorder caused by a recessive allele. This means that an individual must inherit two copies of the recessive allele (homozygous recessive) to express the disease.
2. Recognize that individuals who are heterozygous for the cystic fibrosis allele (carriers) do not exhibit symptoms of the disease. They have one dominant allele and one recessive allele, which means the dominant allele masks the effect of the recessive allele.
3. Consider the genetic inheritance pattern: when two carriers (heterozygous individuals) reproduce, there is a chance for their offspring to inherit two recessive alleles, resulting in cystic fibrosis.
4. Use a Punnett square to visualize the genetic cross between two heterozygous individuals. Each parent can pass on either the dominant allele or the recessive allele, leading to four possible combinations in their offspring.
5. Calculate the probability of each genotype from the Punnett square: 25% chance of homozygous dominant (healthy), 50% chance of heterozygous (carrier), and 25% chance of homozygous recessive (affected by cystic fibrosis). This explains why cases continue to arise despite the disease being lethal before reproductive age.