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Question: This pedigree can't be for ...

This pedigree can't be for

A

Dominant sex linked

B

Recessive sex linked

C

Sex-limited autosomal dominant

D

Autosomal recessive

Answer

Dominant sex linked

Explanation

Solution

To determine which mode of inheritance is not possible for the given pedigree, we will analyze each option against the observed patterns in the pedigree.

Pedigree Analysis:

  • Squares represent males, circles represent females.
  • Shaded shapes represent affected individuals, unshaded shapes represent unaffected individuals.

Key observations from the pedigree:

  1. I-1 (affected male) and I-2 (unaffected female) have an unaffected daughter II-3.
  2. II-2 (unaffected male) and II-3 (unaffected female) have an affected son III-1.

Let's evaluate each option:

A: Dominant sex linked (X-linked dominant)

  • Rule: An affected father transmits the trait to all his daughters.
  • In the pedigree, I-1 is an affected father. His daughter, II-3, is unaffected.
  • This directly violates the rule for X-linked dominant inheritance. If I-1 (X^DY) were affected by an X-linked dominant trait, he would pass his X^D chromosome to all his daughters, making them all affected. Since II-3 is unaffected, X-linked dominant inheritance is not possible.

B: Recessive sex linked (X-linked recessive)

  • Rule: Affected individuals are typically male. Affected sons can have unaffected carrier mothers. Affected fathers do not pass the trait to their sons. Affected fathers pass the trait (as a carrier) to all their daughters.
  • Let 'a' be the recessive allele and 'A' be the dominant allele.
  • III-1 (affected male) has genotype X^aY. He inherited X^a from his mother II-3. Since II-3 is unaffected, she must be a carrier (X^AX^a). His father II-2 is unaffected (X^AY). This is consistent.
  • I-1 (affected male) has genotype X^aY. He has an affected son II-1 (X^aY). For this to happen, I-2 (unaffected female) must be a carrier (X^AX^a).
  • If I-1 (X^aY) and I-2 (X^AX^a), their children can be:
    • Sons: X^AY (unaffected), X^aY (affected). Both II-1 (affected) and II-2 (unaffected) are possible.
    • Daughters: X^AX^a (unaffected carrier), X^aX^a (affected). II-3 is unaffected, so she is X^AX^a (unaffected carrier).
  • This mode of inheritance is consistent with the pedigree.

C: Sex-limited autosomal dominant

  • Rule: The gene is on an autosome, but its expression is limited to one sex. Let's assume the trait is expressed only in males, as III-1 (affected) is male.
  • Let 'A' be the dominant allele, 'a' be the recessive allele. Affected males are A_, unaffected males are aa. Females can be A_ or aa but are always phenotypically unaffected.
  • II-2 (unaffected male) has genotype 'aa'.
  • II-3 (unaffected female) and II-2 (aa) have an affected son III-1 (A_). For III-1 to be A_, he must have inherited 'A' from II-3 (since he got 'a' from II-2). So, II-3 must be A_ (e.g., Aa). Since she is female, she is unaffected. This is consistent.
  • I-1 (affected male) has genotype A_ (e.g., Aa). I-2 (unaffected female) could be Aa or aa. They have an unaffected son II-2 (aa). This is possible if I-1 is Aa and I-2 is Aa or aa. They also have an affected son II-1 (A_) and an unaffected daughter II-3 (A_ or aa). This is consistent.
  • This mode of inheritance is consistent with the pedigree.

D: Autosomal recessive

  • Rule: Affected individuals can have unaffected parents (carriers). The trait can skip generations. Males and females are affected equally.
  • III-1 (affected male) has genotype 'aa'. His parents, II-2 (unaffected male) and II-3 (unaffected female), must both be carriers (Aa). This is consistent with them being unaffected.
  • I-1 (affected male) has genotype 'aa'. His partner I-2 is unaffected. They have an unaffected child II-2 (Aa) and II-3 (Aa). This is possible if I-2 is a carrier (Aa). (If I-2 were AA, all children would be Aa and unaffected, which contradicts II-1 being affected). So I-1 (aa) and I-2 (Aa) can have II-1 (aa, affected), II-2 (Aa, unaffected), and II-3 (Aa, unaffected).
  • This mode of inheritance is consistent with the pedigree.

Conclusion:

The pedigree cannot be for Dominant sex linked inheritance because an affected father (I-1) has an unaffected daughter (II-3), which is impossible for X-linked dominant traits.

The final answer is A\boxed{A}

Explanation of the solution:

The pedigree cannot represent dominant sex-linked (X-linked dominant) inheritance because affected father I-1 has an unaffected daughter II-3. In X-linked dominant inheritance, an affected father transmits the trait to all his daughters.