Problem 11 - Biology Today and Tomorrow Without Physiology

Question

Is this statement true or false? A son can inherit an X-linked recessive allele from his father.

Step-by-Step Solution

Answer

False, a son inherits the X chromosome from his mother, not his father.

Step 1: Understand X-Linked Inheritance

In X-linked recessive inheritance, the gene causing the trait or disorder is located on the X chromosome. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). This means that males inherit their X chromosome from their mother and their Y chromosome from their father.

Step 2: Determine Inheritance for Males

A son inherits an X chromosome from his mother and a Y chromosome from his father, as males have one X and one Y chromosome. This setup means that an X-linked recessive allele cannot be passed from father to son, as fathers provide a Y chromosome to their male offspring.

Step 3: Assess the Statement

Since a son receives his X chromosome from his mother, and the assertion describes the father passing an X-linked recessive allele to his son, the statement is logically false. Fathers contribute the Y chromosome to sons, which excludes the transfer of any X-linked genes from them.

Key Concepts

X-linked recessive inheritance

X-linked recessive inheritance refers to genetic traits or disorders that are determined by genes located on the X chromosome. This concept is key in understanding certain types of genetic conditions and how they are passed down through generations. In X-linked recessive inheritance:

• The gene responsible for a disorder resides on the X chromosome.
• Males (XY) have only one X chromosome and therefore one chance of inheriting an X-linked recessive allele.
• Females (XX) have two X chromosomes, providing two chances to inherit such alleles, which affects probability and phenotype expression.

Males express the trait if they inherit the recessive allele because they lack a second X chromosome that could carry a dominating allele. As a result, males typically show symptoms of X-linked recessive disorders more frequently than females do. In females, a second, normal allele on the other X chromosome can mask the effect of the recessive allele, making them carriers without expressing the disorder. Understanding this inheritance pattern is crucial for predicting genetic disorders in families.

Chromosome inheritance

Chromosome inheritance is the process where chromosomes, and thus genes, are passed down from parents to offspring during reproduction. This ensures the transfer of genetic information from one generation to the next. In humans:

• Females have two X chromosomes ( ext{XX}).
• Males have one X and one Y chromosome ( ext{XY}).

During reproduction, each parent contributes one sex chromosome to the offspring. From the mother, an offspring can inherit an X chromosome. From the father, the offspring can inherit either an X or a Y chromosome, determining the offspring's sex. Sons receive an X from their mother and a Y from their father, while daughters receive an X from each parent. Understanding chromosome inheritance is key to explaining not only sex determination but also how sex-linked traits are passed down.

Sex-linked traits

Sex-linked traits are characteristics determined by genes located on sex chromosomes, most commonly on the X chromosome. These traits display unique patterns of inheritance due to the differences in sex chromosome makeup between males and females.

• An example of a sex-linked trait is red-green color blindness, a common X-linked recessive condition.
• Males with a single X-linked recessive allele will express the trait since they lack a second X chromosome.
• Females must inherit two copies of the recessive allele to express the trait.

Sex-linked traits often affect males more severely or frequently due to their singular X chromosome. However, females can be carriers if they carry one recessive allele without showing any trait symptoms. This concept is important in genetic counseling and understanding the transmission of certain hereditary conditions.