Problem 9 - Biology Concepts and Applications

Question

Alleles for Tay-Sachs disease are inherited in an autosomal recessive pattern. Why would two parents with a normal phenotype have a child with Tay-Sachs? a. Both parents are homozygous for a Tay-Sachs allele. b. Both parents are heterozygous for a Tay-Sachs allele. c. A new mutation gave rise to Tay-Sachs in the child. d. \(b\) or \(c\)

Step-by-Step Solution

Answer

Both parents are likely heterozygous carriers (option b), allowing for a child with Tay-Sachs.

Step 1: Understanding Genetic Terms

Tay-Sachs disease is inherited in an autosomal recessive pattern, meaning it occurs when a person inherits two recessive alleles (one from each parent) for the disease. Individuals with one recessive allele (heterozygous) are carriers and have a normal phenotype.

Step 2: Exploring Parental Genotype

To have a child with Tay-Sachs, both parents can be carriers of the Tay-Sachs allele. If both parents are heterozygous, they have a normal phenotype but can pass on the Tay-Sachs allele to their children with a probability of 25%.

Step 3: Analyzing Potential Scenarios

Since the parents have a normal phenotype, they cannot be homozygous for the Tay-Sachs allele; otherwise, they would both have the disease. Therefore, option (a) is incorrect. Having a new mutation (option c) is possible but less common statistically than two carrier parents passing the allele to the child.

Step 4: Concluding the Correct Answer

The most probable scenario is that both parents are heterozygous carriers, leading to option b. Option d suggests either scenario b or c, but given the higher likelihood of two carriers having a child with the disease, option b is sufficient.

Key Concepts

Autosomal Recessive Inheritance

In the realm of genetics, autosomal recessive inheritance is a way in which certain traits or diseases are passed down from parents to their children. This type of inheritance involves two specific alleles, which are forms of a gene located on one of the non-sex chromosomes, also known as autosomes.
To understand autosomal recessive inheritance, it's crucial to recognize that each individual carries two alleles for each gene—one from each parent. For a child to express an autosomal recessive trait or disease, they must receive two recessive alleles, one from each parent. Only when both alleles are recessive does the trait become visible or the disease manifest.
Here's how it typically works:

• If an individual has two recessive alleles, they will express the recessive trait or condition (e.g., Tay-Sachs disease).
• If an individual has one dominant allele and one recessive allele, they will not express the recessive trait but can be a carrier of it.
• If an individual has two dominant alleles, they will not carry or express the recessive trait.

In the case of autosomal recessive inheritance, carriers—individuals with one recessive allele—play a significant role as they can unknowingly pass the gene to their offspring.

Tay-Sachs Disease

Tay-Sachs disease is a genetic disorder that is inherited through an autosomal recessive pattern, which primarily affects nerve cells in the brain and spinal cord. Children born with Tay-Sachs disease have a deficiency in a critical enzyme called Hexosaminidase A (Hex-A), responsible for breaking down fatty substances in brain cells.
This enzymatic deficiency leads to the buildup of harmful substances, causing destruction, damage, and death of nerve cells, which deteriorates motor skills, vision, and basic bodily functions over time.
Tay-Sachs is most frequently observed in specific populations, including:

• Individuals of Ashkenazi Jewish descent
• Certain French Canadian communities
• Specific groups in Louisiana's Cajun population

Because of its autosomal recessive nature, Tay-Sachs can occur even if both parents appear perfectly healthy. It's vital to note that parents who are carriers do not show symptoms themselves because they have only one copy of the recessive allele. It is the inheritance of two copies from parents that results in the development of the disease in their child.

Heterozygous Carriers

Heterozygous carriers are individuals who possess two different alleles of a gene, one dominant and one recessive. In the context of diseases like Tay-Sachs, these carriers have inherited one normal allele and one mutated allele for the condition.
Despite having one diseased allele, heterozygous carriers do not exhibit the symptoms of Tay-Sachs disease because the normal allele compensates for the deficiency. However, they play a pivotal role in the transmission of the disease through generations.
As parent carriers of a recessive disorder, the probability for each of their children to inherit the disorder is as follows:

• 25% chance of inheriting both recessive alleles, resulting in the child having the disease.
• 50% chance of being a carrier like the parents, having one dominant and one recessive allele, but with no symptoms.
• 25% chance of inheriting both dominant alleles, resulting in the child neither having the disease nor being a carrier.

Genetic counseling is often recommended for carrier parents to understand their risks of passing on genetic conditions and to explore their reproductive options.