Question

Assume two normal parents have a color-blind son. Which parent is responsible for color blindness in the son? a. the mother b. the father c. either parent d. Neither parent-two normal parents cannot have a color- blind son.

Short answer

The correct answer is (a) the mother. A color-blind son would inherit the X chromosome carrying the color-blind gene from his mother, and the Y chromosome from his father. Since both parents are normal, the mother must be a carrier for the color-blind gene with a genotype of X^(N)X^(b).

Step-by-step solution

Understand the inheritance of color blindness

Color blindness is inherited through the X chromosome. Males have one X chromosome (from the mother) and one Y chromosome (from the father), while females have two X chromosomes (one from each parent). Therefore, a male child inherits the responsible gene for his color blindness from his mother's X chromosome.

Analyze the parents' genotypes

Since both parents are normal (not color-blind), the mother must be a carrier for the color-blind gene in order for her to pass it to her son. The mother's genotype would be X^(N)X^(b), where X^(N) is a normal X chromosome and X^(b) is an X chromosome carrying the color-blind gene. The father, being normal and not a carrier, would have the genotype X^(N)Y.

Determine the offspring's genotypes and the responsible parent

Since the child is a color-blind male, his genotype must be X^(b)Y. The child inherits the X^(b) chromosome (with the color-blind gene) from his mother's X chromosome, and the Y chromosome from his father. Therefore, the responsible parent for color blindness in the son is the mother, who passes on the X chromosome carrying the color-blind gene. The correct answer is (a) the mother.

Key concepts

X-linked recessive inheritance

Understanding how color blindness passes from parents to children requires knowledge about a genetic mechanism called X-linked recessive inheritance. This pattern of inheritance is tied to genes located on the X chromosome, one of the two sex chromosomes in humans. Males possess one X and one Y chromosome (XY), while females have two X chromosomes (XX).
In the case of X-linked recessive conditions like color blindness, males are more frequently affected because they have only one X chromosome. A single recessive gene (like the one causing color blindness) on a male's X chromosome can lead to expressing the condition, as there is no corresponding gene on the Y chromosome to counteract it. Conversely, females, having two X chromosomes, typically require two copies of the recessive gene—one on each X chromosome—to show symptoms of an X-linked recessive condition. Hence, a female with one normal and one affected X chromosome will not exhibit the trait but can be a carrier, potentially passing it to her offspring.
In our exercise, the color-blind son's single X chromosome inherited from his mother must carry the gene for color blindness, offering a clear demonstration of X-linked recessive inheritance.

Genotype

To further fathom the hereditary puzzle of color blindness, it is vital to discuss genotype. A genotype refers to the set of genes in our DNA responsible for a particular trait. In humans, each gene has two alleles since they are located on paired chromosomes—one from the mother and one from the father. These alleles can either be dominant (typically symbolized as 'A') or recessive (symbolized as 'a').
In terms of color vision, the allele for normal color vision (let's call it 'N') is dominant, and the allele for color blindness ('b') is recessive. When considering X-linked traits like color blindness, the genotype is written using superscripts on the X chromosome, like this: X^N for a normal vision allele and X^b for a color blindness allele.
Applying this to the exercise, the mother's genotype is X^NX^b, indicating she is a carrier of the color blindness gene but does not express the condition because the 'N' allele is dominant. The father's genotype, X^NY, leads to normal color vision since Y chromosomes do not carry color vision genes at all.

Carrier status

Carrier status is a key concept within the realm of genetics, and understanding it can provide insights into how traits like color blindness are transferred hidden through generations. A carrier is an individual who has one copy of a recessive gene that does not produce symptoms when a dominant gene is also present. Carriers can, however, pass the recessive gene to their offspring, who might express the condition if they inherit recessive genes from both parents.
In X-linked recessive traits, females are typically carriers because they have two X chromosomes. If a woman has one affected X chromosome (X^b), her chance of passing on this gene is 50% for each child. With respect to our exercise, since the son inherits his X chromosome from his mother and is color blind, the mother is unequivocally a carrier, having one allele for normal vision and one for color blindness.

Chromosomal inheritance

A key aspect of genetics is chromosomal inheritance. Our genes are arranged on chromosomes, which are string-like structures within our cells. Humans have 46 chromosomes, arranged into 23 pairs, with one chromosome of each pair inherited from our mother and the other from our father. Of these, two chromosomes determine our sex—the X and Y chromosomes.
In the specific case of the exercise involving color blindness, the pattern of inheritance is tied directly to the X chromosome. Since males receive their only X chromosome from their mothers, any X-linked recessive condition such as color blindness will also be inherited from their mothers. It is essential to note that the Y chromosome, which males inherit from their fathers, contains very few genes and does not contain a gene for color vision. Therefore, father-to-son transmission of X-linked traits does not occur, as sons cannot inherit an X chromosome from their fathers.