Question

When can a daughter be affected by an X-linked recessive trait, such as color blindness?

• A. When mother is affected and father is normal
• B. When father is affected and mother is normal
• C. When the father is affected and mother can be either affected or is a carrier
• D. The father is normal and the mother is a carrier

Hint:

Rules for X-linked recessive traits:
- An affected female must have an affected father ($X^c Y$).
- Her mother must be at least a carrier ($X^C X^c$).
- A normal father ($X^C Y$) can never produce an affected daughter.

Answer 1

The Correct Option is C

Concept:
X-linked recessive inheritance refers to the transmission of mutant alleles located on the non-homologous region of the X chromosome.
Females possess two X chromosomes ($XX$), whereas males possess one X and one Y chromosome ($XY$).
This question analyzes the parental genotypic requirements necessary for a female offspring (daughter) to express an X-linked recessive phenotype.

Step 1: Understanding Female Recessive Expression:
For a female to be phenotypically affected by an X-linked recessive trait (e.g., red-green color blindness, $X^c X^c$), she must be homozygous recessive.
She must receive one mutant allele ($X^c$) from her biological father and one mutant allele ($X^c$) from her biological mother.

Step 2: Deducing Paternal and Maternal Genotypes:
1. Paternal Genotype Requirement: A father donates his only X chromosome to all his daughters. If the father is genetically normal ($X^C Y$), every daughter receives a dominant normal allele ($X^C$) and will never be affected. Therefore, the father must be affected ($X^c Y$).
2. Maternal Genotype Requirement: The mother must provide the second mutant allele ($X^c$). Thus, the mother must either be a heterozygous carrier ($X^C X^c$) or a homozygous affected female ($X^c X^c$).

Step 3: Final Answer:
A daughter will only be affected if her father is affected and her mother is either affected or a carrier.
Thus, Option (C) is the correct choice.