Problem 6 - Biology Concepts and Applications

Question

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

Step-by-Step Solution

Answer

The trait is characteristic of X-linked recessive inheritance.

Step 1: Understanding the Dominant and Recessive Traits

When a trait is autosomal dominant, it means that only one copy of the dominant allele is needed for the trait to be expressed. Therefore, if a child has an autosomal dominant trait, at least one of the parents must also have the trait, which contradicts the scenario where neither parent has the trait.

Step 2: Considering Autosomal Recessive Inheritance

For a trait to be autosomal recessive, both parents must at least be carriers (heterozygous) of the recessive allele for their child to express the trait. While the parents themselves do not express the trait, they can pass on the recessive allele to their offspring. This scenario fits with the male child displaying a trait not found in either parent.

Step 3: Examining X-Linked Recessive Inheritance

X-linked recessive traits are usually passed from mother to son. In this case, the mother may be a carrier of the trait (heterozygous) even though she does not express it. The father does not transmit his X chromosome to the son. This could explain why the trait appears in a male child and is absent in the parents.

Step 4: Eliminating Insufficient Information

Option D suggests that it is not possible to answer without more information. However, between option B (autosomal recessive) and option C (X-linked recessive), both are plausible explanations for how a trait could appear in a male child that neither parent visibly exhibits, unless further specific familial context is given, such as lineage information affecting autosomal recessive trait dominance.

Key Concepts

Autosomal Dominant

In genetic inheritance, an autosomal dominant trait is expressed when an individual possesses at least one copy of the dominant allele. This means if a person carries the dominant allele on one of their autosomal (non-sex) chromosomes, the trait will be visible. Since autosomal chromosomes are inherited from both parents, if a child shows a trait that follows autosomal dominance, one of the parents must also express the trait.

For example, if a parent is heterozygous for a dominant trait (carrying one dominant allele and one recessive allele), there is a 50% chance that they will pass this trait to any offspring.

Autosomal dominant conditions are often easier to identify within families because the inheritance pattern is straightforward. Everyone who has the dominant allele will show the trait unless the inheritance has very low penetrance or there's a new mutation involved.

• Suitable for traits where only one affected parent can produce affected offspring.
• Typical examples include Huntington's Disease and Marfan Syndrome.
• Offspring of a parent with the trait have a 50% chance of inheriting it.

Autosomal Recessive

Autosomal recessive inheritance requires two copies of the recessive allele for a trait to be expressed. This means a child must inherit one recessive allele from each parent. Parents who carry only one recessive allele (and one dominant) do not show the trait themselves; these parents are called carriers.

If a trait like this appears in a child, and neither parent visibly exhibits the trait, it’s common to deduce that they are carriers. Thus, a child inheriting one recessive allele from each carrier parent expresses the trait.

• Both parents must be either carriers or affected to pass on the trait.
• There is a 25% chance for two carriers to have an affected child.
• Examples include cystic fibrosis and sickle cell anemia.

This type of inheritance often stays hidden in family trees until two carriers have a child together, potentially unveiling surprising new family traits.

X-Linked Recessive Inheritance

X-linked recessive inheritance patterns often involve traits more commonly expressed in males. This is due to the male's XY chromosomal arrangement, as they have only one X chromosome. A single recessive allele on that X chromosome can lead to trait expression because there is no second X chromosome present to potentially mask the recessive allele with a dominant one.

Mothers, who have two X chromosomes, might carry one normal and one recessive allele without expressing the trait themselves. Since fathers pass on their Y chromosome to male children (which does not contain X-linked traits), a mother's carrier status directly affects the likelihood of passing on recessive X-linked conditions to her sons.

• Son of a carrier mother faces 50% chance of inheriting the trait.
• Famous examples include hemophilia and color blindness.
• Females can be carriers, but only express the trait when both X chromosomes have the recessive allele (a rare occurrence).

Thus, X-linked recessive traits can appear unexpectedly if maternal ancestry isn't thoroughly known for such recessive conditions.