A Trait That Skips a Generation
In a family, neither parent has a particular condition, but their daughter has it and their son does not. The mother's father also had the condition. A student concludes: 'The trait must be dominant, because it shows up in the daughter even though her parents don't have it.' Another student says: 'It must skip generations, so it's recessive.' The teacher asks the group to decide whether the pedigree is consistent with a recessive allele, and to explain why 'skipping a generation' is not proof by itself.
1. Explain why the pedigree is consistent with a recessive trait, using the parents' genotypes.
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If the condition is recessive, a person shows it only with two copies (aa). The affected daughter is aa, so she must inherit one 'a' from each parent. Since neither parent is affected, both parents must be carriers (Aa) - they each have one recessive allele but also a dominant normal allele, so they appear unaffected.
This fits: Aa × Aa gives a 1-in-4 chance of an aa child. The affected mother's father being affected is also consistent - he could be aa, passing an 'a' to the mother. So the pedigree is fully consistent with a recessive allele. It is also possible for a dominant trait to appear in a child of unaffected parents only if a new mutation occurred, which is less likely but not impossible - so the pedigree alone narrows but does not absolutely prove recessiveness.
2. Explain why 'it skipped a generation' is weak evidence on its own, and name one additional observation that would strengthen the recessive conclusion.
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'Skipping a generation' is a description, not a mechanism. Recessive traits often appear to skip because carriers are unaffected, but a dominant trait can also appear to skip if it has reduced penetrance (not everyone with the allele shows it) or if a new mutation arises. So the phrase alone does not distinguish recessive from dominant.
Stronger evidence:
- Consanguinity or carrier testing: if both parents are confirmed carriers (Aa) by a genetic test, recessiveness is strongly supported.
- Ratio in many offspring: if carrier parents have many children, roughly 1 in 4 affected and unaffected carriers among the rest fits recessive inheritance.
- Affected relatives on both sides without an affected parent in between is typical of recessive.
Any one of these - especially direct carrier testing - moves the conclusion from 'consistent with' to 'strongly supported.'