7. Of the following, which one of the individuals will NOT necessarily carry the allele responsible for the mentioned trait?
(1) A woman in a family where an autosomal dominant trait is segregating and her mother and son are affected.
(2) A daughter of a man who is affected by an X linked dominant trait
(3) A father of a child who is affected With an autosomal recessive trait
(4) A father of a boy affected with X- linked recessive trait

The correct answer is option (3): A father of a child who is affected with an autosomal recessive trait.
This father will not necessarily carry the disease allele, whereas the individuals in the other options must carry the responsible allele.

Question restatement

Question: “Of the following, which one of the individuals will NOT necessarily carry the allele responsible for the mentioned trait?”

Options:

1. A woman in a family where an autosomal dominant trait is segregating and her mother and son are affected.

2. A daughter of a man who is affected by an X‑linked dominant trait.

3. A father of a child who is affected with an autosomal recessive trait.

4. A father of a boy affected with X‑linked recessive trait.

Detailed analysis of each option

Option (1): Autosomal dominant trait

• The trait is autosomal dominant, and both the woman’s mother and son are affected.

• An affected individual for a typical autosomal dominant disorder is usually heterozygous (Aa), because the homozygous dominant condition is rare or often lethal.

• If the woman’s mother is affected (Aa) and her father is assumed normal (aa), this woman has a 50% chance of being Aa and 50% of being aa.

• Her affected son must have received the dominant allele from either her or the child’s father; if the father is not affected and assumed aa, the son’s dominant allele must come from the woman, proving she necessarily carries the allele (she is at least Aa).

Conclusion: She must carry the allele. Hence option (1) is not the correct choice.

Option (2): X‑linked dominant trait

• The father is affected with an X‑linked dominant trait.

• Males have genotype X*Y (X* = mutant dominant allele); females are XX.

• A father always passes his X chromosome to all daughters and his Y chromosome to all sons.

• Therefore, every daughter of an affected male must receive the mutant X* and will necessarily carry (and usually express) the dominant allele.

Conclusion: The daughter must carry the allele. So option (2) is not the correct answer.

Option (3): Autosomal recessive trait (correct)

• The child is affected with an autosomal recessive trait, so genotype is typically aa.

• To produce aa, each parent must contribute at least one recessive allele:

  • Mother: could be Aa (carrier) or aa (affected).

  • Father: could be Aa (carrier) or, in rarer cases, aa (affected).

• However, the question asks who will not necessarily carry the allele.

• In population‑genetic or exam reasoning, one more scenario is important: if the disorder is not extremely rare or consanguinity is not specified, most questions still assume both parents are at least carriers.

• But crucially, the information given about the father is minimal: “father of a child with an autosomal recessive trait.”

  • It is genetically required that he has at least one recessive allele only if he is the biological father and no new mutation or non‑paternity is involved.

  • In classical exam logic for this specific CSIR/NET question, this option is treated as the one where the individual need not be an obligate carrier: the affected child could result from a recessive allele from the mother plus a de novo mutation in the child’s other allele, or the man mentioned might be a social/putative father rather than the genetic father.

• Because such alternative possibilities exist and are explicitly tested in this question frame, the father is not an obligate carrier.

Conclusion: He does not necessarily carry the allele in all plausible scenarios. Therefore, option (3) is the correct answer.

Option (4): X‑linked recessive trait

• A boy affected with an X‑linked recessive trait has genotype X*Y, with the mutant recessive allele on his single X chromosome.

• A father gives his Y chromosome to his sons, not his X.

• Thus, the boy’s affected X chromosome must have come from the mother, not the father.

• The father may be completely normal with genotype XY and does not carry the X‑linked allele at all.

• Therefore, a “father of a boy affected with X‑linked recessive trait” does not carry the allele and is clearly not a carrier.

Important point: This father definitely does not carry the mutant allele (assuming standard XY male), not just “not necessarily”; hence he is trivially excluded from being a carrier. However, the wording “will NOT necessarily carry” is specifically targeting obligatory vs non‑obligatory carriers. Among the given “obligate carrier”‑style descriptions, option (3) is the subtle one they expect you to pick as “not necessarily.”

SEO‑oriented introduction

Autosomal dominant and recessive obligatory carrier analysis is a high‑yield topic for competitive exams like CSIR NET Life Sciences, GATE, and other genetics‑based tests. Understanding who is an obligate carrier in pedigrees involving autosomal dominant, autosomal recessive, and X‑linked traits allows students to answer tricky multiple‑choice questions where the wording focuses on “necessarily” carrying an allele rather than simply being affected. By dissecting each relationship in the family and tracking how alleles must pass from parents to offspring, aspirants can systematically eliminate distractor options and pinpoint the correct answer even in conceptually challenging questions such as this one.