# Possible answers

> A. X-Linked

No, it can't be as the son express the rare condition

> B. Autosomal recessive

It is indeed possible. However as the condition is rare, it is unlikely that both the parents of the paternal grandmother were both carrying the mutant allele and also the mother of the two kids is carrying the mutant allele. I would tend to think it is therefore unlikely. If we had more information (such as the frequency of the disease and/or how many siblings bother the paternal grandmother has), then we could have a better opinion about the likelihood of "B." being the good answer.

> C. Autosomal dominant

No, it can't be as none of the parents don't express the condition

> D. Multifactorial

The term "multifactorial" is a little unclear to me. It might mean that there is environmental variance and/or it might mean that there are several loci that affect the inheritance of this condition (with eventually some epistasis). All of these are definitely possible and likely (see below).

> E. Mitochondria

It is unlikely as 1) the disease is rare and 2) the father's mother expressed the disease. The father did not transmit the mtDNA (as mtDNA is inherited from the mother only).

# What I would answer

For the vast majority of phenotypic conditions we consider, the inheritance is controlled by several SNPs and there is some non-zero environmental variance (that is, heritability is different from 1; see [this post](https://biology.stackexchange.com/questions/42273/why-is-a-heritability-coefficient-not-an-index-of-how-genetic-something-is)).

I would hence consider than "multifactorial" is most likely a good descriptor for this condition.