A man with red-green color blindness (X-linked recessive) has children with a woman who has normal vision and no family history of color blindness. Which of the following best describes the expected outcomes for their children?
AAll sons will be color blind; all daughters will have normal vision
BAll daughters will be carriers; all sons will have normal vision
CHalf of sons will be color blind; half of daughters will be carriers
DNo children will be color blind; daughters will be obligate carriers
The affected father's genotype is X^a Y. The unaffected, non-carrier mother's genotype is X^A X^A. All daughters receive X^a from their father and X^A from their mother — making every daughter a carrier (X^A X^a). All sons receive Y from their father and X^A from their mother — so no son is affected. The common misconception (option A) is that an affected father passes the trait to sons, but sons inherit the Y chromosome from their father, not the X. The X-linked allele from the father goes only to daughters.
Question 2 Multiple Choice
A carrier mother (X^A X^a) and an unaffected father (X^A Y) have children. What is the probability that a son will be color blind?
A0% — the father is unaffected, so sons cannot inherit the allele
B25% — one in four children overall will be affected
C50% — each son has a 50% chance of receiving X^a from his mother
D100% — any son of a carrier mother will be affected
Sons inherit their X chromosome from their mother (the Y comes from their father). The carrier mother has genotype X^A X^a, so each son has an equal chance of receiving X^A (unaffected) or X^a (affected). Males are hemizygous — receiving X^a with no second X to mask it, they will be color blind. So the probability for each son is 50%. Option A is the classic criss-cross confusion: affected sons do NOT get the allele from their father. Option B confuses the probability for 'any child' with the probability for 'a son specifically.'
Question 3 True / False
A female cannot be affected by an X-linked recessive condition — she can mainly be a carrier.
TTrue
FFalse
Answer: False
Females CAN be affected by X-linked recessive conditions if they are homozygous (X^a X^a) — inheriting the recessive allele from both parents. This requires an affected father (who contributes X^a) and a carrier or affected mother (who also contributes X^a). While this is far rarer than male expression, it does occur. The statement confuses 'less likely' with 'impossible.' Women are affected less frequently because they need two copies of the allele; men are affected with a single copy because they are hemizygous.
Question 4 True / False
An affected father with an X-linked recessive condition cannot pass the trait directly to any of his sons.
TTrue
FFalse
Answer: True
This is the defining pedigree rule for X-linked recessive inheritance. Fathers pass their Y chromosome to sons and their X chromosome to daughters. An affected father (X^a Y) therefore gives all daughters X^a (making them obligate carriers) and gives all sons Y (passing no X-linked allele whatsoever). Affected sons inherit the allele from their carrier mothers, not their fathers. This 'no father-to-son transmission' pattern is a diagnostic signature in pedigree analysis for X-linked traits.
Question 5 Short Answer
Why do X-linked recessive traits appear far more frequently in males than in females, and what condition would be required for a female to be affected?
Think about your answer, then reveal below.
Model answer: Males are hemizygous — they carry only one X chromosome. A single recessive allele on that X is sufficient to produce the phenotype because there is no second allele to mask it. Females have two X chromosomes and can be heterozygous carriers, with one dominant allele suppressing expression of the recessive allele. For a female to be affected, she must be homozygous (X^a X^a), which requires inheriting the recessive allele from both parents — an affected father and a carrier or affected mother.
The hemizygosity of males is the central concept. It means that the usual dominance/recessiveness logic — requiring two copies of a recessive allele for expression — does not apply to males for X-linked genes. One copy is all they have, so one copy is expressed. This asymmetry between sexes is the key feature distinguishing X-linked inheritance from autosomal inheritance.