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This image is from the NCERT textbook. In diagram A, which represents an autosomal dominant trait, I found out that autosomal recessive inheritance also satisfies the pedigree. Yet, the NCERT text book states that it is a dominant trait. How can that be? Is it because the dominant trait is more probable here? If so, how is that it is more probable in this case?

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Sure, both are possible here!

Your book probably makes an implicit assumption of low frequency for the "red allele".

Assuming, "red" means "disease", it is quite standard to assume that the "red allele" is at low frequency in the population. If this is the case, then it is unlikely that partners of the members of the main family being represented in the tree carry this allele. It is therefore quite standard to assume that partners does not carry the "red allele". If this assumption is made, then recessive autosomal cannot explain the observed tree.

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Both are possible here, as Remi.b stated.

It could be autosomal dominant, assuming that the first male in the pedigree is heterozygous for that trait (assuming that "red" means the trait). Since he is heterozygous, and the female is homozygous recessive, there is a 2:2 chance of their children getting that trait or not, which is actually shown!

So if the male is heterozygous recessive for autosomal dominant, and passed it on to 2 of his children, then those children that received that trait are also heterozgous, resulting in the person with that phenotype from the F2 generation.

Also the trait could be seen as autosomal recessive, as you had said, if the females in the parent generation and F1 are carriers.

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  • $\begingroup$ how about X-linked dominant trait? It is also possible for this pedigree. So, how can is test which is more probable, autosomal dominant or X linked dominant? $\endgroup$ – Rakhal Chandran.B Mar 5 '18 at 14:47
  • $\begingroup$ If it was sex-linked dominant, the other female offspring would also be seen as having it. $\endgroup$ – Beariclymenus Mar 23 '18 at 22:28

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