# Situation: A nurse assists in the genetic counseling and fertility clinic of a medical center, where couples come for preconception advice. A 27-year-old woman's father has hemophilia A, an X-linked recessive disorder. Her mother is not a carrier, and her husband does not have hemophilia. She asks about the risk to her future children. Which statement is accurate?

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> subject: Nursing Practice II — Maternal and Child Health Nursing

## 문제

Situation: A nurse assists in the genetic counseling and fertility clinic of a medical center, where couples come for preconception advice.

A 27-year-old woman's father has hemophilia A, an X-linked recessive disorder. Her mother is not a carrier, and her husband does not have hemophilia. She asks about the risk to her future children. Which statement is accurate?

## 보기

1. Each daughter has a 50% chance of hemophilia, and each son a 50% chance of carrying it
2. Each son has a 50% chance of hemophilia; each daughter, 50% of being a carrier **✔ 정답**
3. Each son has a 25% chance of hemophilia; each daughter, 25% of being a carrier
4. Every son will have hemophilia, and every daughter will be a carrier like her

**정답: 2**

## 해설

A father passes his X chromosome to every daughter, so this woman is an obligate carrier. Each son receives one of her two X chromosomes and has a 50% chance of inheriting the affected one and having hemophilia. Each daughter also has a 50% chance of receiving the affected X, but she receives a normal X from her father and becomes a carrier rather than affected.

## 심화 해설

Inheritance pattern in this family

Hemophilia A follows an X-linked recessive pattern. The affected gene is F8, located on the X chromosome. In this scenario, the woman’s father has hemophilia A, which means his single X chromosome carries the pathogenic F8 variant. Because a father always transmits his X chromosome to every daughter, this woman necessarily received the affected X from her father. Her mother is not a carrier and therefore contributed a normal X. As a result, the woman is an obligate carrier: she has one normal X and one affected X, and she is phenotypically unaffected because the normal allele compensates for the recessive defect.

The key point is that a father cannot pass his X chromosome to a son, so hemophilia A is never transmitted directly from father to son. The affected X must pass through a carrier daughter before it can appear in a grandson.

Watch out! The woman’s carrier status is certain, not probabilistic. She does not have a 50% chance of being a carrier; she already is one because her father’s only X is affected.

| Family member | X chromosomes | Phenotype |
| --- | --- | --- |
| Woman’s father | XhY | Hemophilia A |
| Woman’s mother | XX | Non-carrier, unaffected |
| Woman | XhX | Obligate carrier, unaffected |
| Woman’s husband | XY | Unaffected |

Risk to each future pregnancy

The woman’s husband is unaffected, so he contributes a normal Y chromosome to each son and a normal X chromosome to each daughter. The woman’s gametes are the variable factor: each oocyte carries either the affected X or the normal X with equal probability.

For each son, the sex chromosome combination is determined by the father’s Y plus one of the mother’s X chromosomes. If the son receives the affected X, he has no second X to compensate, so he will have hemophilia A. The probability is 50%.

For each daughter, the father always contributes a normal X, and the mother contributes either the affected X or the normal X. A daughter who receives the affected X becomes a carrier like her mother, but she is not affected because the paternal normal X provides functional factor VIII. The probability is also 50%.

| Child | Maternal X received | Paternal sex chromosome | Genotype | Phenotype | Probability |
| --- | --- | --- | --- | --- | --- |
| Son | Affected X | Y | XhY | Hemophilia A | 50% |
| Son | Normal X | Y | XY | Unaffected | 50% |
| Daughter | Affected X | Normal X | XhX | Carrier, unaffected | 50% |
| Daughter | Normal X | Normal X | XX | Non-carrier, unaffected | 50% |

Why the other options are incorrect

Option 1 reverses the expected phenotypes by stating that daughters would have hemophilia and sons would be carriers. In X-linked recessive inheritance, carrier status applies only to individuals with two X chromosomes, so males cannot be carriers in the usual sense; they are either affected or unaffected.

Option 3 incorrectly reduces the risk to 25%. That figure would apply only if the mother’s carrier status were uncertain, such as when there is no known family history and carrier testing has not been performed. Here, the mother’s carrier status is established by pedigree logic, so the conditional probability for each child is 50%.

Option 4 states that every son will be affected and every daughter will be a carrier. This would be true only if the mother were homozygous for the affected allele, which is not the case. A carrier has one normal X, so half of her offspring receive the normal allele.

Key point! In X-linked recessive conditions, the risk to offspring of a carrier mother is calculated per pregnancy, not per family. Each son independently has a 50% chance of being affected, and each daughter independently has a 50% chance of being a carrier.

Clinical and counseling considerations

Preconception carrier screening identifies couples at risk for transmitting X-linked recessive conditions such as hemophilia A, allowing discussion of reproductive options including preimplantation genetic diagnosis, prenatal diagnosis, and targeted newborn testing [1]. In this family, the woman’s carrier status is already established by the pedigree, so the counseling focus shifts to explaining the 50% per-child risk and reviewing available reproductive choices.

Molecular confirmation of the specific F8 variant can refine counseling, particularly because some structural variants such as intron 1 inversions or duplications may be missed by standard sequencing and require long-read sequencing for accurate characterization . Carrier females are typically phenotypically normal, but genetic analysis can identify the causative variant and support prenatal or preimplantation testing in subsequent pregnancies .

Watch out! Although carrier females are generally asymptomatic, factor VIII levels can occasionally be low enough to cause mild bleeding tendencies. This does not change the inheritance risk calculation, but it is relevant for obstetric and perioperative management.References (research sources)

- [1]Carrier screening in the reproductive setting-Are there medical implications for the heterozygote?-A guide for clinicians.Research articleRosenfeld EB, Kasatkin N, Yu BL, Velinov M, Brandt JS, Ashkinadze E. (2026) · DOI: 10.1002/pmf2.70280

## 임상 시나리오

Hemophilia A Carrier Risk CounselingX-linked recessive inheritance from an affected father
A father with hemophilia A transmits his affected X chromosome to every daughter, making her an obligate carrier. The affected X is never passed directly from father to son.

For each pregnancy of a carrier woman with an unaffected partner, each son has a 50% chance of inheriting the affected X and having hemophilia. Each daughter has a 50% chance of being a carrier, but she is not affected because she receives a normal X from her father.

CautionDo not describe the woman's carrier status as a 50% probability; it is certain because her father's only X carries the pathogenic F8 variant. Each pregnancy outcome is independent, so avoid stating that all sons or all daughters will be affected or carriers.

## 핵심 개념

- **Obligate carrier** — A female who must carry an X-linked recessive allele because her father is affected and she receives his only X chromosome
- **X-linked recessive** — Inheritance pattern where the gene is on the X chromosome and two copies are needed for expression in females, while one copy causes disease in males
- **Hemophilia A** — Bleeding disorder caused by deficiency of clotting factor VIII due to pathogenic variants in the F8 gene on the X chromosome
- **Carrier** — A female with one normal and one affected X chromosome who is phenotypically unaffected but can transmit the affected allele
- **Punnett square** — A grid used to predict the probability of offspring genotypes from parental gametes

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