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Step-by-Step Explanation
Step 1: Identify the Key Observations
• A man and a woman, who both appear healthy, have seven children (2 daughters and 5 sons).
• Three of the five sons are affected by a certain disease, but neither daughter is affected.
Step 2: Recall Modes of Inheritance
1. Autosomal Dominant: If the disease were autosomal dominant, one affected parent would typically pass the trait to about half the children, including daughters and sons alike.
2. Autosomal Recessive: Both parents would be carriers, and approximately 25% of their children would be expected to show the disease, but it should affect both sons and daughters equally.
3. Sex-Linked Recessive (X-linked Recessive): In this pattern, typically unaffected carrier mothers pass the defective gene on the X chromosome to about half of their sons (who become affected) and half of their daughters (who become carriers if the father is healthy). An unaffected father cannot pass a recessive X-linked disease to his sons, but his daughters can become carriers.
4. Sex-Linked Dominant (X-linked Dominant): An affected father would pass the trait to all his daughters and none of his sons. An affected mother would pass the trait to about half her sons and half her daughters.
Step 3: Match the Observations to X-linked Recessive Inheritance
• None of the daughters are affected: This suggests that the father does not carry a dominant disease gene on his X chromosome, because then all daughters would have been affected if the trait were X-linked dominant.
• Three out of five sons are affected: Sons receive their X chromosome from their mother. If the mother is a carrier (XcX), there is a 50% chance each son inherits the affected X (Xc), causing him to be affected (XcY).
• Parents appear healthy: The father is healthy (not carrying the defective X), and the mother must be a carrier (has one normal X and one mutant X). This matches a typical scenario for an X-linked recessive trait.
Step 4: Conclude the Mode of Inheritance
Given the pattern that only sons are affected (no daughters), and the parents appear unaffected (suggesting the mother is a carrier), the most likely mode of inheritance for this disease is Sex-linked Recessive (X-linked Recessive).
