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Consider the following family history:

Bob has a genetic condition that affects his skin.
Bob’s wife, Eleanor, has normal skin. No one in Eleanor’s family has ever had the skin condition.
Bob and Eleanor have a large family. Of their eleven children, all six of their sons have normal skin, but all five of their daughters have the same skin condition as Bob.

Based on Bob and Eleanor’s family history, what inheritance pattern does the skin condition most likely follow?

View Available Hint(s)for Part A

X-linked dominant
autosomal recessive
autosomal dominant
Y-linked

1 answer

Based on the family history provided, the skin condition is likely to be X-linked recessive. Here's the reasoning:

  1. Bob has the condition, and since he is male, he has one X chromosome (from his mother) and one Y chromosome (from his father). Being affected means he has the recessive allele for the skin condition on his X chromosome.

  2. Eleanor has normal skin and no family history of the condition, suggesting she likely has two normal alleles (XX), with no carriers in her lineage.

  3. The children’s inheritance pattern shows that all six sons have normal skin while all five daughters have the condition. Sons inherit their X chromosome from their mother (Eleanor) and Y chromosome from their father (Bob). Since Eleanor is normal, her sons must have received her normal X chromosome. Consequently, they cannot inherit the skin condition from Bob, who only passes on his Y chromosome to his sons.

  4. On the other hand, the daughters inherit one X from Bob and one X from Eleanor. Since Bob has the condition, he contributes the affected X chromosome to all his daughters, leading to their having the same condition.

Thus, the inheritance pattern aligns with traits that are X-linked recessive, where males are affected but females can show the condition if they have one affected X chromosome (from their father) and do not have a normal one to mask the effect (from their mother).

In summary:

  • Sons are unaffected (inherited normal X from the mother).
  • Daughters are affected (inherited affected X from the father).

Therefore, the most likely inheritance pattern for Bob’s skin condition is X-linked recessive.