Heather was surprised to discover she suffered from red-green colorblindness. She told her biology professor, who said, 'Your father is colorblind, too, right?' How did her professor know this? Why did her professor not say the same thing to the colorblind males in the class?

Verified step by step guidance

1. Understand the genetic basis of red-green colorblindness: Red-green colorblindness is primarily caused by mutations in the X chromosome. Since females have two X chromosomes (XX) and males have one X and one Y chromosome (XY), the inheritance patterns differ between genders.
2. Recognize the inheritance pattern in females: For a female to express red-green colorblindness, she must inherit the defective gene on the X chromosome from both parents. This is because having one normal X chromosome can usually compensate for the defective one, preventing the expression of colorblindness.
3. Consider the inheritance from the father: Since Heather's professor mentioned her father, it implies that her father must have the colorblind gene on his X chromosome. Males only have one X chromosome, and if it carries the colorblind gene, they will express the condition. Heather must have inherited this X chromosome from her father.
4. Explain why the professor did not mention fathers for colorblind males: Males only need one defective gene on their X chromosome to be colorblind, which they inherit from their mothers, as fathers pass on their Y chromosome to male offspring. Therefore, the status of the father's colorblindness is irrelevant to the male's condition.
5. Summarize the genetic transmission: Heather's professor inferred her father's condition based on the understanding that Heather, being female, would need two copies of the defective gene to be colorblind—one from her mother and one from her father. This is not the case for males, who only need one defective gene from their mother.

Key Concepts

X-Linked Inheritance

Red-green colorblindness is primarily inherited in an X-linked recessive pattern. This means that the gene responsible for this condition is located on the X chromosome. Males, having only one X chromosome, are more likely to express the trait if they inherit the affected X. Females, with two X chromosomes, must inherit two copies of the affected gene to express colorblindness.

Genetic Carrier Status

In X-linked inheritance, females can be carriers of the colorblindness gene without expressing the trait themselves. A carrier has one normal X chromosome and one affected X chromosome. If Heather's father is colorblind, it indicates that he has the affected X chromosome, which he passed to Heather, making it likely that she is a carrier if her mother has a normal X chromosome.

Sex-Linked Traits

Sex-linked traits, such as colorblindness, are associated with genes located on sex chromosomes. Since males have one X and one Y chromosome, they are more susceptible to expressing recessive traits linked to the X chromosome. In contrast, females have two X chromosomes, which provides a higher chance of having a normal allele that can mask the expression of the recessive trait, explaining why the professor did not make the same assumption about colorblind males in the class.