It took me a while to figure this out ...

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Re: It took me a while to figure this out ...

Postby Wanderer » Mon Jan 16, 2017 9:53 am

A clarification for the readers:

There are 46 chromosomes in the standard human cell. Of these, the first 22 chromosomal pairs not involved in determining sex are called autosomes.

Polydactyly can be produced by many different mutations, regardless of type. Postaxial polydactyly -- the kind exhibited by Count Rugen in The Princess Bride -- is hereditary 14% of the time, and follows what is called an autosomal dominant pattern of expression.

Autosomal: The mutation occurs in one of the first 22 nonsex chromosomal pairs.

Dominant: Even if the mutation occurs on only one of the chromosomes in the pair, and even though the other parent may not have the mutation, there is still a 50% chance for the mutation to be inherited.

Expression: Whether the mutation shows up in the body's construction. This consists of what happens when it shows up (expression) and how often it shows up when inherited (penetrance). Polydactyly of this sort has variable expression (if it does show up, it can be anything from a nub to a fully-functioning finger) and incomplete penetrance (not everyone who inherits shows an extra finger).

Of note is that, unlike the traits of Mendelian genetics (in which a dominant trait masks a recessive one, as in Mendel's pea experiment), this concerns expression and inheritance. This type of mutation has a flat 50% rate of being inherited, and may or may not result in an extra finger of some description.
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Re: It took me a while to figure this out ...

Postby Skydancer » Tue Jan 17, 2017 1:28 am

This probably means it is triggered by other genes; you need it to get polydactyly, but only if something else is present - that something would appear to be fairly common given the apparent slightly-better-than 1 in 4 odds of getting it.
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