"More Genetics In Problem 29, we learned that for some diseases, such as sickle-cell anemia, an individual will get the disease only if he or she receives both recessive alleles. This is not always the case. For example, Huntington’s disease only requires one dominant gene for an individual to contract the disease. Suppose that a husband and wife, who both have a dominant Huntington’s disease allele (S) and a normal recessive allele (s), decide to have a child.
b. What is the probability that the offspring will not have Huntington’s disease? In other words, what is the probability that the offspring will have genotype ss? Interpret this probability.
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Verified step by step guidance

1. Identify the genotypes of the parents. Both the husband and wife have one dominant allele (S) and one recessive allele (s), so their genotypes are both Ss.
2. Set up a Punnett square to determine the possible genotypes of their offspring. The possible alleles from each parent are S or s, so the combinations are SS, Ss, sS, and ss.
3. Calculate the probability of each genotype by counting the number of times each appears in the Punnett square. Remember that Ss and sS are equivalent and both represent having the dominant allele.
4. Focus on the genotype ss, which means the child has two recessive alleles and will not have Huntington’s disease. Determine the probability of this genotype by dividing the number of ss outcomes by the total number of possible outcomes.
5. Interpret the probability of the offspring having genotype ss as the chance that the child will not have Huntington’s disease, since the disease requires only one dominant allele (S) to be present.

Key Concepts

Dominant and Recessive Alleles

Alleles are different forms of a gene, where dominant alleles express their trait even if only one copy is present, while recessive alleles require two copies to express the trait. In Huntington’s disease, the dominant allele (S) causes the disease, so only one copy is needed for the condition to manifest.

Genotype Probability Using Punnett Squares

A Punnett square is a tool used to predict the probability of offspring genotypes based on parental alleles. By combining the alleles from each parent, we can calculate the likelihood of different genotypes, such as SS, Ss, or ss, which helps determine the chance of inheriting a disease.

Interpreting Genetic Probability

Genetic probability quantifies the chance that an offspring will inherit a particular genotype. For Huntington’s disease, calculating the probability of the ss genotype means finding the chance the child will not have the disease, as ss represents two recessive alleles without the dominant disease-causing allele.