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More Genetics In Problem \(33,\) 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. (a) List the possible genotypes of their offspring. (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. (c) What is the probability that the offspring will have Huntington's disease?

Step by step solution

01

Determine Parental Genotypes

Given that both parents have a dominant Huntington's disease allele (S) and a normal recessive allele (s), their genotypes are Ss each.

02

Create a Punnett Square

Draw a Punnett Square to predict the genotypes of the offspring. Place one parent's alleles (S and s) on the top and the other parent's alleles (S and s) on the side.

03

Fill in Punnett Square

Fill in the Punnett Square by combining the alleles from each parent. The four possible combinations (genotypes) are: SS, Ss, Ss, and ss.

04

List Possible Genotypes

The possible genotypes for the offspring are: SS, Ss, Ss, and ss.

05

Calculate Probability of Genotype ss

Out of the four possible combinations in the Punnett Square, only one combination is ss. Therefore, the probability that the offspring will have the genotype ss, and thus will not have Huntington's disease, is \( \frac{1}{4} \) or 25%.

06

Calculate Probability of Huntington's Disease

The offspring will have Huntington's disease if they inherit at least one dominant allele (S). The possible genotypes for this are SS or Ss. There are 3 out of 4 combinations (SS, Ss, Ss) that result in Huntington's disease. Thus, the probability that the offspring will have Huntington's disease is \( \frac{3}{4} \) or 75%.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Genotype Probability

In genetics, genotype probability helps us predict the likelihood of an offspring inheriting a specific set of genes. By using the Punnett Square, we can model and visualize these probabilities. In this example, we know both parents have the genotype Ss, where 'S' represents the dominant allele for Huntington's disease and 's' represents the normal recessive allele.
The Punnett Square allows us to combine these alleles to see all possible genotypes for their child. The possible combinations (genotypes) are:

• SS
• Ss
• Ss
• ss

This shows us the probability of each genotype appearing in the offspring. For instance, there's a 1 in 4, or 25%, chance for the child to be ss. Therefore, there’s a 25% chance the offspring won't get Huntington's disease.

Recessive and Dominant Alleles

Understanding recessive and dominant alleles is foundational in genetics. Alleles are different forms of the same gene. Dominant alleles (such as 'S' for Huntington's disease) overshadow recessive alleles (such as 's' for the normal allele) in expression. This means that if an individual inherits at least one 'S' allele, they will exhibit the trait for Huntington's disease.
In this exercise, both parents have a dominant Huntington's disease allele (S) and a normal recessive allele (s). Thus, their genotypes are Ss each. The dominant allele (S) determines whether the disease appears. If the genotype is SS or Ss, the individual will have Huntington's disease. Only the genotype ss results in no disease.
It's essential to remember the rules of dominance:

• Dominant alleles (S) hide the effects of recessive alleles (s).
• Only two recessive alleles (ss) lead to recessive trait expression, like normal health in this context.

Genetic Disorders

Genetic disorders are diseases caused by anomalies in an individual's DNA. These can be due to mutations or inherited alleles. In this problem, we look at Huntington's disease, a genetic disorder caused by a dominant allele (S).
Huntington’s disease only needs one copy of the dominant allele (S) for the disease to manifest. Contrast that with other disorders like sickle-cell anemia, which requires two copies of the recessive allele to express the disease.
The main conclusions are:

• Huntington’s disease appears with one dominant allele (genotypes SS or Ss).
• The probability of an offspring not having Huntington's disease (genotype ss) is 25%.
• There's a 75% chance of having Huntington's disease (genotypes SS or Ss).

Understanding how these genetic disorders pass from parents to offspring helps in predicting and managing potential health risks, emphasizing the importance of genetic counseling and testing.