When a haemophilic carrier female marries a normal male, the chance of their daughter becoming a carrier is?

SCTIMST Staff Nurse - 2015 (Set-A)

Pathology & Genetics

Medium

Appeared in: SCTIMST Staff Nurse - 2015 (Set-A)

Explanation

Hemophilia is an X-linked recessive disorder, meaning the gene responsible is located on the X chromosome.
A carrier female has one normal X chromosome and one X chromosome with the hemophilia allele (genotype XᴴXʰ).
A normal male has one normal X chromosome and a Y chromosome (genotype XᴴY).
Daughters inherit one X chromosome from each parent. They will always receive the normal Xᴴ from the father.
From the carrier mother, a daughter has a 50% chance of inheriting the normal Xᴴ and a 50% chance of inheriting the affected Xʰ allele.
Therefore, the probability of a daughter having the genotype XᴴXʰ and being a carrier is 50%.

Option A: A 100% chance of a daughter being a carrier would require the father to have hemophilia (XʰY), as he would pass his only X chromosome (Xʰ) to all his daughters.
Option B: A 75% probability is not a standard outcome in Mendelian genetics for a single-gene, two-allele cross like this one.
Option D: This represents the probability of having a carrier daughter out of all possible children (1 out of 4 total possibilities). The question specifically asks for the probability among daughters only.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain X-linked recessive inheritance as background academic context rather than a clinical decision trigger.
Nurses have a vital role in educating families about the inheritance patterns of genetic disorders like hemophilia and referring them for genetic counseling.
Anticipatory guidance is crucial for at-risk families. This includes teaching parents to recognize signs of bleeding and implementing safety measures for male children, such as avoiding intramuscular injections and using protective gear.
Carriers of hemophilia should have their clotting factor levels checked, as some may have low levels (due to lyonization) and be at risk for bleeding, especially during surgery or after childbirth.

How to Approach the Question

First, identify the inheritance pattern of the condition. The question states the female is a 'haemophilic carrier,' which points to a genetic disorder, and hemophilia is a classic example of X-linked recessive inheritance.
Determine the genotypes of both parents based on the description. A 'carrier female' is heterozygous (XᴴXʰ), and a 'normal male' is (XᴴY).
Construct a Punnett square to map out all possible genetic combinations for their offspring.
Carefully read what the question is asking for. It specifically asks for the 'chance of their daughter becoming a carrier.' This means you should only consider the female offspring in your final calculation.
Analyze the female offspring results from the Punnett square. You will see two possibilities: a normal daughter (XᴴXᴴ) and a carrier daughter (XᴴXʰ).
Calculate the probability. Since there is one carrier outcome out of two possible daughter outcomes, the chance is 1/2 or 50%.

Concept Tested & Keywords

Concept Tested: X-linked recessive inheritance
Stem keywords: haemophilic carrier female, normal male, daughter, carrier
Lead-in keywords: chance
Clinical cues: Age/sex group narrows the expected diagnosis, intervention, or normal reference range.

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