Problem 7 - Biology Concepts and Applications

Question

Color blindness is a case of __________ inheritance. a. autosomal dominant b. autosomal recessive c. X-linked dominant d. X-linked recessive

Step-by-Step Solution

Answer

Color blindness is a case of X-linked recessive inheritance.

Step 1: Understanding Color Blindness

To solve this problem, we need to recognize the genetic pattern color blindness follows. Color blindness is often a genetic condition where the most common form is red-green color blindness.

Step 2: Considering Genetic Inheritance Patterns

There are two main categories of genetic inheritance: autosomal (pertaining to non-sex chromosomes) and sex-linked (pertaining to the sex chromosomes, X and Y). Each can be dominant or recessive.

Step 3: Identifying the Inheritance Type for Color Blindness

Color blindness is most frequently linked to the X chromosome, making it X-linked. Since males have only one X chromosome, the presence of the color blindness allele on that X chromosome will lead to the condition. Females, with two X chromosomes, will only exhibit the condition if they have the allele on both chromosomes. This pattern indicates recessive inheritance.

Step 4: Choosing the Correct Answer

Given that color blindness is passed down through the X chromosome and is recessive, the correct option is d. X-linked recessive.

Key Concepts

X-linked recessive

Genetic disorders can be inherited in a variety of ways, but one important mode is through X-linked recessive inheritance. This means the gene that carries the disorder is located on the X chromosome, one of the two sex chromosomes. Everyone has a pair of sex chromosomes: females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). Since the gene is recessive, a single copy of the gene (from one X chromosome) in females may not lead to the disorder because typically the normal gene on the second X chromosome compensates. However, a male has only one X chromosome, and thus if it carries the recessive gene, there is no other X chromosome to compensate, leading the male to express the trait immediately.

• Females need two copies of the recessive gene to express the trait.
• Males only need one copy of the gene on their X chromosome to show the trait.

Because of this, X-linked recessive disorders tend to appear more frequently in males than in females.

Color Blindness

Color blindness is an example of an X-linked recessive disorder. It often impacts one's ability to see and distinguish colors correctly, with red-green color blindness being the most common form. The gene responsible for color vision is located on the X chromosome. Because men have only one X chromosome, if it carries the gene mutation causing color blindness, they are highly likely to exhibit this trait.

• Women, possessing two X chromosomes, can carry one mutated gene without being colorblind -- acting as carriers instead.
• They would need both X chromosomes to carry the mutation to experience color blindness themselves, which occurs less frequently.

Understanding the genetic pattern helps explain why males are more often affected by color blindness.

Sex-linked Inheritance

Sex-linked inheritance refers to heritable characteristics that are determined by genes located on the sex chromosomes — X and Y. Most often, this type of inheritance involves X-linked genes, as the X chromosome carries a significant number of genes, while the Y chromosome carries fewer.

• X-linked traits are much more common than Y-linked traits for this reason.
• Men are more likely to express X-linked recessive traits due to possessing only one X chromosome.

In sex-linked inheritance, the pattern of inheritance and expression differs between males and females, largely due to the differences in their sex chromosomes. This explains why conditions such as color blindness occur more frequently in males and how females can be carriers. Understanding these patterns is crucial for predicting the spread and occurrence of sex-linked genetic traits across generations.