Please observe the following pedigree and come up with a conclusion as to which type of inheritance is most likely

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Now it should be obvious that it's either autosomal dominant or autosomal recessive. BUT it could also be X-recessive if the women I2 and II5 are carriers.

Anyways, if it's autosomal dominant than about 50% of children should be sick according to a punnet square.

If autosomal recessive about 25% should be sick according to the same logic.

33% of children are sick so what do you answer? My book states a very clear and only answer without explaining how it could be other forms too, autosomal dominant. Why not autosomal recessive just as likely?

  • 2
    $\begingroup$ 1. Please read our homework policy. 2. Demanding answers generally does not go over too well. 3. Look up "incomplete penetrance". Even if a trait is autosomal dominant doesn't mean it will occur in exactly 50% of the carriers all the time. Unfortunately, Gregor Mendel fudged a lot of things, and Punnett Squares ≠ reality. $\endgroup$ – MattDMo Jun 6 '15 at 15:32
  • $\begingroup$ It's not really homework, I'm preparing for exams and I already have the answer as stated in the question. $\endgroup$ – Paze Jun 6 '15 at 15:41
  • $\begingroup$ That is included in the policy. Please read it. $\endgroup$ – MattDMo Jun 6 '15 at 15:50
  • $\begingroup$ You should use better titles... "Very easy pedigree problems annoying me heavily": what does this mean? Are you asking why some easy problems are annoying you? Keep your title to the point. $\endgroup$ – WYSIWYG Jun 6 '15 at 16:08
  • $\begingroup$ Some edits have been made. I'd prefer not to have it closed as I did receive an answer that I am satisfied with, $\endgroup$ – Paze Jun 6 '15 at 17:38

Let's consider all the options.

X-linked recessive: I:2 is a carrier and II:5 is also a carrier, and I:1 is affected. Everyone who marries into this family(except for II:2, lucky guy. He better not have any sons though.) is a carrier for the same genetic disorder. While it does work, it's vastly less likely than the other options. Observe: II:4 is affected, but only because I:2 is a carrier. I:1 is entirely irrelevant.

Autosomal recessive: I:2 and II:5 are still required to carry the same genetic disorder, but at least I:1 contributes.

Autosomal dominant: The only individual required to have the disease genotype is the affected I:1 individual, and everyone else inherits from him.

The problem asks which inheritance pattern is most likely and not which inheritance pattern is possible because of cases like this. Most genetic disorders are rare, and it's far more likely that random chance had 25% instead of 50% of four children affected than this small family inherited two sources of this genetic illness.


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