Problem 6 - Biology: The Unity and Diversity of Life

Question

A trait that is present in a male child but not in either of his parents is characteristic of __________ inheritance. a. autosomal dominant d. It is impossible to answer b. autosomal recessive \(\quad\) this question without c. X-linked recessive \(\quad\) more information.

Step-by-Step Solution

Answer

c. X-linked recessive

Step 1: Identify Inheritance Patterns

Begin by understanding the different inheritance patterns: - **Autosomal dominant**: If a child has the trait, at least one parent must also display the trait. - **Autosomal recessive**: Both parents can be carriers without displaying the trait, allowing the child to display the trait. - **X-linked recessive**: Typically, this trait appears in males. The mother can be a carrier, and the father does not display the allele on his single X chromosome.

Step 2: Analyze Given Information

The problem states the trait appears in a male child but not in either of his parents. This means it cannot be autosomal dominant, as one parent must display the trait. It can be autosomal recessive if both parents are carriers. It can also be X-linked recessive if the mother is a carrier.

Step 3: Decide on Most Likely Answer

Given the characteristic that the trait is present only in the son and not in either parent, this most closely aligns with X-linked recessive inheritance, where the issue manifests in males and can be carried silently by females.

Key Concepts

Autosomal Dominant

In autosomal dominant inheritance, a single copy of a mutant gene from an affected parent can cause the expression of the trait in the offspring. This means that if the trait is present in the child, at least one of the parents must also exhibit the trait.
This pattern of inheritance follows a straightforward path:

• Only one defective allele is needed for the trait to manifest.
• An affected parent typically has a 50% chance of passing the trait to each child.
• The trait appears in every generation without skipping any.

Since the exercise scenario describes a situation where neither parent has the trait, autosomal dominant inheritance can be ruled out. This pattern requires at least one parent to be visibly affected, making it impossible under the conditions provided in the problem.

Autosomal Recessive

Autosomal recessive inheritance requires two copies of the mutant gene for the trait to be expressed. This means both parents must either be carriers or at least one must be affected, though they may not show signs of the trait themselves. This leads to certain characteristics:

• Both parents often carry one copy of the mutant gene, without showing any symptoms.
• There’s a 25% chance for each child to be affected if both parents are carriers.
• Traits can skip generations, hidden by carriers who do not display the trait.

In the original exercise, since neither parent shows symptoms, it's possible for both to be carriers, and so autosomal recessive remains a potential explanation for the trait in the child. However, without further information, this explanation is not definitive.

X-linked Recessive

X-linked recessive inheritance is a type of genetic inheritance pattern specifically related to genes found on the X chromosome. These patterns often result in traits that manifest predominantly in males due to the following reasons:

• Males have one X and one Y chromosome, so a single recessive gene on the X chromosome can express the trait.
• Females have two X chromosomes, so they would need two copies of the recessive gene to express the trait, making them more likely to be carriers rather than affected.
• The mother can pass the affected X chromosome to her sons, who then express the trait.

In the scenario provided in the exercise, the description that the trait appears in a male child but is absent in both parents aligns well with X-linked recessive inheritance. Here, the trait can be passed on by a mother who is a carrier and does not exhibit the trait herself, making this the most likely explanation.