Question

What does the age at diagnosis tell you about the mutation?

Answer 1

Probably not much, AFAIK. Do you have a specific disease and mutation in mind?

Answer 2

In general, the mutations causing early onset of disease (i.e., in childhood or early adulthood, when they affect the reproductive potential of the individual) tend to be spontaneously occurring ones. They are not the inherited kind, because the affected individual does not live long enough to pass them on. Also, they tend to be dominant to produce phenotypic results because (unless we are talking about mutations on the male X chromosomes, of which they have only one) each cell has a normal or wild type allele for the same gene on the other chromosome, which is usually sufficient to confer a normal phenotype.

By contrast, diseases caused by mutations in old age do not affect the reproductive success of the individual - they are as likely to pass the mutant gene on as a normal individual is to pass on the normal gene. So these tend to be recessive.

Answer 3

Sorry but I don't agree.
Counter-examples: Tay-Sachs, PKU.
What you are overlooking is reassortment: a newborn baby has 1 copy of each autosome from father and mother. For severe alleles, the parents are usually heterozygotes and unaffected. Also it is not true that they are usually dominant, for the same reason.

Answer 4

Of course there are counter examples and Tay Sachs is a splendid of one. The disease is completely dosage dependent.

But if, in general, you were to sort through all genetic diseases, I would argue that the vast majority of them follow this pattern.

And if you weighted for frequency of mutation - for example, if you compared the tens of millions of men and women worldwide with the cancer causing brac 1 and 2 mutations (chromosomals translocation conferring a strong predisposition to breast and other forms of cancer) to the what - possibly two dozen? - kids with the Tay Sachs mutation - then I suspect that the generality above would hold almost universally.

Answer 5

Nice answer. I'll think about it.

Answer 6

Nice to meet you too. :)