Problem 4 - Biology of Humans: Concepts, Applications, and Issues

Question

Huntington's disease is caused by a dominant allele. If a mother has Huntington's disease but the father does not carry the dominant allele, what is the probability that the first child will be normal? a. \(0 \%\) b. \(25 \%\) c. \(50 \%\) d. \(100 \%\)

Step-by-Step Solution

Answer

c. 50%

Step 1: Understand Dominance

Huntington's disease is caused by a dominant allele. This means that only one copy of the allele is needed for an individual to have the disease.

Step 2: Identify Parental Genotypes

The mother has Huntington's disease, indicating she has at least one dominant allele (Hh or HH). The father does not carry the dominant allele, so his genotype is hh.

Step 3: Set Up Punnett Square

We need to determine the possible genotypes of the children. For this, create a Punnett square with the mother's alleles on one axis (Hh if she is heterozygous) and the father's alleles (hh) on the other axis.

Step 4: Fill in Punnett Square

The Punnett square will have the following combinations: Hh (carrier, has disease), hh (normal, no disease), Hh (carrier, has disease), hh (normal, no disease).

Step 5: Calculate Probabilities

There are four possible outcomes and 2 of them are hh (normal). Therefore, the probability that their first child will be normal is 2 out of 4, or 50%.

Step 6: Conclusion

Based on the calculated probabilities, the correct answer is c. 50%.

Key Concepts

dominant allele

In genetics, an allele is a variant form of a gene. When an allele is dominant, it means that only one copy of that allele is needed to express a trait.
For instance, Huntington's disease is caused by a dominant allele. If someone possesses just one copy of the allele (H), they will exhibit the disease, regardless of the second allele.
This contrasts with recessive alleles which need two copies (one from each parent) for the trait to be expressed. Understanding dominant alleles is critical in predicting genetic disorders and inheritance patterns.

Punnett square

A Punnett square is a tool used in genetics to determine the probabilities of offspring inheriting particular genotypes.
It's like a genetic calculator, helping us visualize the possible combinations of parental alleles.

To create a Punnett square:

• Draw a grid with the parental alleles on the top and side.
• Fill in the squares by combining the alleles from each parent.

For the Huntington's disease exercise, we arrange the mother's alleles (Hh) on one axis and the father's alleles (hh) on the other.
By filling out the grid, we observe potential allele combinations for their children.

genotype probability

Genotype probability explores the likelihood of different genotypes appearing in the offspring.
When examining the Punnett square for our specific exercise:

• Hh (has disease)
• hh (normal)

Since the square reveals 2 Hh (disease) and 2 hh (normal) outcomes,
each genotype has an equal chance of appearing - meaning a 50% probability for each.
This concept helps in predicting the chances that offspring will exhibit or carry genetic traits or disorders.

genetic disorders

Genetic disorders are diseases caused by abnormalities in an individual's DNA.
These can be inherited from parents or result from mutations. Huntington's disease is one example and is caused by a dominant allele.

Understanding genetic disorders involves:

• Identifying if the condition is dominant or recessive.
• Using tools like Punnett squares to determine the probability of inheritance.

It's crucial to study these conditions to provide better medical advice and genetic counseling.
Accurate knowledge helps in making informed health and family planning decisions.