Question
In a pedigree chart, two unaffected parents have both affected and unaffected children, with the affected trait appearing in roughly 1/4 of offspring. One of the grandparents (maternal grandfather) was also affected. How do you determine whether this is autosomal dominant or autosomal recessive? Analyse the evidence systematically.
Solution — Step by Step
The problem states that two unaffected parents have affected children. This is the single most diagnostic observation for determining dominance:
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If the trait were dominant, affected individuals have at least one dominant allele. Two unaffected parents would have two recessive alleles each — and cannot produce a dominant-allele-carrying offspring. So an unaffected couple cannot have an affected child if the trait is dominant.
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If the trait is recessive, unaffected parents can be carriers (Aa × Aa). Then 1/4 of their offspring will be affected (aa).
Conclusion from this step: Since unaffected parents have affected children, the trait is almost certainly autosomal recessive.
The problem states the affected trait appears in roughly 1/4 of offspring. This is exactly the expected Mendelian ratio from a carrier × carrier cross (Aa × Aa):
| Genotype | Phenotype | Ratio |
|---|---|---|
| AA | Unaffected | 1 |
| Aa | Unaffected (carrier) | 2 |
| aa | Affected | 1 |
1 affected out of 4 offspring = 1/4 proportion. This confirms autosomal recessive.
If it were autosomal dominant, affected parent × unaffected parent would give ~1/2 affected offspring (not 1/4).
The maternal grandfather was affected. This means he had the recessive allele (in fact, genotype aa if recessive). He passed one allele to the mother. Since the mother is unaffected but has the allele from her father, she is a carrier (Aa).
The father (unaffected, having affected children) must also be a carrier (Aa). But where did the father get his allele? Check his parents — either one or both may be carriers or affected.
This pattern — a skip-generation where an affected grandparent has unaffected children but affected grandchildren — is classic autosomal recessive inheritance.
X-linked recessive would produce:
- Affected males > affected females (males have one X)
- No father-to-son transmission
Without information about sex-specific rates in this pedigree, we can note that the problem didn’t mention sex-specific predominance, and the affected grandfather passing through an unaffected mother (carrier female) to affected grandchildren is equally consistent with both X-linked and autosomal recessive.
However, if the problem mentions both male and female children are equally affected (or close to equal), then X-linked recessive is ruled out. X-linked would affect mostly males.
Final conclusion: Based on (1) unaffected parents having affected children, (2) 1/4 ratio, and (3) the skip-generation pattern via a carrier mother — the trait is autosomal recessive.
Why This Works
Pedigree analysis is a deductive process — you rule out possibilities until one pattern remains consistent with all observations. The four key rules:
- Two unaffected parents → affected child = recessive trait (dominant is excluded)
- Affected parent × unaffected parent → all unaffected children = rare for dominant, rules nothing out
- Roughly 1/4 affected offspring = both parents are carriers (Aa × Aa) → recessive
- Every affected individual has at least one affected parent = dominant (unless incomplete penetrance)
- Father-to-son transmission = autosomal (not X-linked)
- Mostly males affected = X-linked (especially X-linked recessive)
Alternative Method
Create a logic table and test each possibility:
| Observation | Autosomal Dominant | Autosomal Recessive |
|---|---|---|
| Unaffected parents → affected child | Impossible ✗ | Possible ✓ (carriers) |
| 1/4 ratio in offspring | Inconsistent ✗ | Expected ✓ |
| Affected grandfather → carrier mother → affected grandchild | No ✗ | Yes ✓ |
Every observation is consistent only with autosomal recessive. The dominant hypothesis is excluded by the first observation alone.
NEET Biology and CBSE Class 12 frequently give pedigrees and ask students to determine the inheritance pattern. The fastest analysis: first check if two normal parents have an affected child. If yes → recessive. Then check if males are predominantly affected → X-linked. Otherwise → autosomal recessive. This two-step filter handles 80% of pedigree problems.
Common Mistake
Students sometimes conclude “autosomal dominant” when an affected grandparent is present, reasoning that the trait is “being passed down.” This is wrong. Autosomal recessive traits also pass through generations — just invisibly in carriers. The presence of an affected ancestor does NOT imply dominant inheritance. Always base your conclusion on the most diagnostic observation: whether unaffected parents have affected children.