Schafer
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Hey,
I am preparing for the BMAT test, and found a question I can't answer in a BMAT preparation book.

This picture shows the genetic inheritance of a certain disease:
Name:  Capture.JPG
Views: 222
Size:  9.7 KB
Legends:
Circle = woman
Square = man
Red = affected

The question was what is the pattern of inheritance for the disease.
While the correct answer in the book is "Autosomal dominant", I don't understand why it can't also be "Autosomal recessive".
If the mother in the first generation is a carrier of the gene, and it is recessive, then this answer is also valid.
Am I wrong, is there something i missed?
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Revenged
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the pedigree for an autosomal recessive would be like this:

http://www.geneticseducation.nhs.uk/...ive-conditions
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Hype en Ecosse
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(Original post by Schafer)
Hey,
I am preparing for the BMAT test, and found a question I can't answer in a BMAT preparation book.

This picture shows the genetic inheritance of a certain disease:
Name:  Capture.JPG
Views: 222
Size:  9.7 KB
Legends:
Circle = woman
Square = man
Red = affected

The question was what is the pattern of inheritance for the disease.
While the correct answer in the book is "Autosomal dominant", I don't understand why it can't also be "Autosomal recessive".
If the mother in the first generation is a carrier of the gene, and it is recessive, then this answer is also valid.
Am I wrong, is there something i missed?
There are a few characteristics for the pedigrees of autosomal dominant and autosomal recessive conditions:

Autosomal dominant conditions tend to show multi-generational affected; male-to-male transfer; 1 parent is normally affected and males and female are equally affected. It's easy to see why this is.

Autosomal recessive conditions, however, tend to only affect one generation; show affected children with unaffected parents; M:F is equal and there's a higher incidence of consanguinity (more like if there's been incestuous mating). As far as you're concerned, this is all you need to know about pedigree patterns. I'm going to go into why this is true:

So why is that pedigree not autosomal recessive. Well. It could be, but it's incredibly unlikely: let's see why. I'll take an autosomal recessive condition like cystic fibrosis as an example, for which the risk of a random unaffected member of the population with no family history of the disease carrying a CF allele is 1/22.

So let's suppose our granddad in the first generation is affected (has 2 mutant alleles) and our grandma is a carrier, with population risk as there's no indication of a history on her side. What's the risk of them having a child with CF? It's 1 in 44 (dad is definitely going to pass down a gene, but mum only has half the population risk of passing it down), and the risk of children being carriers, at least, for the mutation is 100% - because dad's affected. So already, the male in the second generation is unlucky to get both bad genes.

Now, this guy with cystic fibrosis goes on to meet another lovely lady. Her risk of being a carrier is also 1 in 22 - so the chance of any of their kids having cystic fibrosis is 1 in 44. So them having even one kid with CF is pretty unlikely - especially since CF has already been "passed down" once already. But what's the risk of them having two children with CF? It's 1 in 1936 (1 in 44 risk, twice, which - coincidentally - is the same chance of 2 randomers without a history of cystic fibrosis having a kid with cystic fibrosis). That's a <0.1% chance. So on top of the unlikely chance of it being passed down to one kid in one generation, we've got the further unlikelihood of it being passed down to 2 kids in the subsequent generation. So, as we can see, the chance of this being an autosomal recessive condition is incredibly unlikely. It's possible, but so unlikely that I'd be willing to bet that the average clinical geneticist will never see an autosomal recessive condition present in this fashion in their entire career.

So with the huge unlikelihood of children of an (affected) + (pop. risk of carrier) relationship contracting the disease, it's easy to see why only 1 person in 1 generation tends to be affected. It's also easy to see why it's more likely to happen with consanguinity (genetic defects "pile-up"). The fact that there're multiple generations affected here and it shows male-to-male transfer makes it quite clear that this is an autosomal dominant pattern of inheritance.
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