I think that since every generation has males affected only, the trait must be Y-linked. Also, since every generation expresses the trait, I think it should be dominant. However, the answer key says that the answer is autosomal dominant. Could someone please clarify this and give a tip on how to recognise the difference?

  • $\begingroup$ Unfair question. With that short pedigree, it's hard to distinguish Y-linked from autosomal. And given that none of the females is affected I would have also opted for Y-linked dominant. $\endgroup$
    – alec_djinn
    Commented Nov 14, 2017 at 11:06

2 Answers 2


You're right that this could just as well be Y-linked dominant (althought that is pretty rare due to the small size of the chromosome).

The only reason pointing more towards autosomal dominant is the mention of "extra fingers and toes". Most cases of polydactyly are inherited in an autosomal dominant way.

Does seem like an unfair question, though, since (at least in theory) there's more than one possible answer.

  • $\begingroup$ (more likely to be X-linked recessive than Y-linked dominant, though) Why is that? What about the fact that none of the females are affected? $\endgroup$
    – S. Pai
    Commented Nov 14, 2017 at 17:04
  • $\begingroup$ Y-linked is very rare in general, just because there are so few genes on the Y chromosome. - X-linked recessive, on the other hand, is fairly common (hemophilia, anyone?) and also only affects males (because females have two X chromosomes, therefore one mutant copy doesn't affect them) $\endgroup$ Commented Nov 14, 2017 at 17:59
  • $\begingroup$ "more likely to be X-linked recessive than Y-linked dominant, though". This is not correct: the father doesn't pass the X chromosome to the son. For this to be the case, all mothers should be heterozigous, and some daughters would be affected. $\endgroup$
    – user24284
    Commented Nov 15, 2017 at 6:03
  • $\begingroup$ ooops, yes, you're right - thanks for pointing it out; I'll edit my answer. $\endgroup$ Commented Nov 15, 2017 at 6:13

Ok so this is how I solve a pedigree:

enter image description here

This clearly shows that the disease is autosomal dominant. But there is one problem.

An autosomal dominant pedigree has the following features: 1. Appears in both sexes with equal frequency. 2. Both sexes transmit the trait to the offspring. 3. Doesn’t skip generations. 4. When one parent (heterozygous) is affected, and the other parent is unaffected, there is a 50% chance of the offspring getting affected. (You can refer to the cross I’ve drawn below your picture)

The pedigree in the question satisfies all 3 points except the last one. If there are 6 offsprings, at least 2 of them should be affected (if not 3), but there is only one, and that too a male.

Since only the male is affected in all generations, you’re right in thinking it’s Y linked. In my opinion, it can be Y linked dominant, since that’s very rare. And your question specifies that the trait is rare. It could DEFINITELY be autosomal dominant, had there been more affected offsprings in the third generation.


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