If a boy's father has haemophilia and mother is heterozygous what is the chance that the boy will inherit haemophilia

Options

A. 0.5
B. 0.25
C. 0.75
D. 1

Answer

To solve the problem of determining the chance that a boy will inherit haemophilia from a father who has the condition and a mother who is heterozygous, we can follow these steps:

### Step 1: Understand the Genetics of Haemophilia
Haemophilia is a sex-linked recessive disorder, which means it is carried on the X chromosome. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).

### Step 2: Determine the Genotype of the Parents
- The father has haemophilia, so his genotype is X^hY (where X^h represents the X chromosome with the haemophilia allele).
- The mother is heterozygous, meaning she has one normal allele and one allele for haemophilia. Her genotype is X^HX^h (where X^H represents the normal allele).

### Step 3: Set Up a Punnett Square
To find the probability of the offspring's genotypes, we can set up a Punnett square with the mother's alleles on one side and the father's alleles on the other.

| | X^H | X^h |
|----------|-----------|-----------|
| **X^h** | X^HX^h | X^hX^h |
| **Y** | X^HY | X^hY |

### Step 4: Analyze the Offspring Genotypes
From the Punnett square, we can see the possible genotypes for the offspring:
1. X^HX^h (female, carrier but not affected)
2. X^hX^h (female, affected)
3. X^HY (male, normal)
4. X^hY (male, affected)

### Step 5: Determine the Probability for Males
Now, we focus on the male offspring:
- There are two male offspring possibilities: X^HY (normal) and X^hY (affected).
- Out of these two possibilities, one results in haemophilia (X^hY).

### Step 6: Calculate the Probability
The chance that a boy will inherit haemophilia is:
- Probability = Number of affected males / Total number of males = 1 (affected) / 2 (total males) = 0.5 or 50%.

### Conclusion
Thus, the chance that the boy will inherit haemophilia is **50%**.

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