six-fingered dominance

whether or not J was serious about me commenting on six-fingered dominance, it's certainly too interesting a subject to overlook. (i think people are beginning to question my definition of 'interesting'...)

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i took only one course in genetics and that was more than enough. for u UofT science students out there, i'm talking about the infamous HMB265. no one walks away from the course unscathed. in the end, u either love it or u hate it. i think i did both. genetics is one of the COOLEST things on earth to study but it takes a LOT to grasp the full extent of its coolness. so the theory part i managed just fine, but i had very little patience with the problems. (of course the biggest chunk of marks always comes from solving the problems... aiy.) anyway, i don't remember the six-fingered trait being brought up at all. maybe i missed that lecture? or i probably skipped over that problem or something... whatever. i'm catching up now, aren't i?

i found this problem on a berkeley site:

A six-fingered man and a five-fingered woman have 20 children (10 males and 10 females) with six fingers.
1. Do you know if six-fingered is dominant or recessive?
2. Do you know if six-fingered is sex-linked?
3. Do you know with 100% percent certainty that the male is homozygous for six-fingered?
4. What is the probability that if the male is heterozygous for six-fingered that all 20 children are six-fingered?

here are the official answers:

1. Autosomal: Six-fingered is most likely dominant. Note that if it is recessive, the five-fingered woman must be heterozygous and five-fingered is dominant. If 20 out of 20 children are six-fingered, then the chance of mom always donating the six-fingered allele randomly is (1/2)^20 -- not very likely (1 in a million since 2^10 is about 1000). These odds are actually much better than when you play the lottery.

2. The male must have the six-fingered allele and a Y chromosome. Thus the daughters will always get a six-fingered allele from dad and the sons will get the Y chromosome from dad. Mom is either homozygous for the five-fingered allele (if it is recessive) or she is heterozygous with the five-fingered allele being dominant. If she is heterozygous with the five-fingered allele being dominant, then 1/2 the boys should be five-fingered and 1/2 should be six-fingered. Likewise 1/2 the girls should be five-fingered and the other half should be six-fingered. If five-fingered is recessive, then all the boys must be five-fingered and all the girls six-fingered. This is not the case for the boys, so if the trait is sex-linked, then the five-fingered allele must be dominant.

3. This would imply autosomal. If six-fingered is autosomal dominant and the male is homozygous, then all of the offspring should be six-fingered. This is most likely the case.

4. As calculated earlier, the chance of getting the above results is one in a million if the male is heterozygous. Possible, but not likely.

makes plenty of sense to me. basically, the pattern of inheritance for polydactyly (ie. having extra digits) is autosomal dominant, and i read somewhere that the trait also exhibits complete penetrance and variable expressivity.

WHO THE HECK CARES?!
if this is what u're voicing (as i presume most of u are), u can stop right here :)

WHAT DOES IT ALL MEAN?!
curiosity killed the cat, they say... so by all means, read on!

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*the following review is far from comprehensive but will do for the purposes of this post*

• humans carry 23 pairs of chromosomes. the sex chromosomes make up one pair. there are 2 kinds of sex chromosomes: X and Y. (females are XX and males are XY.) the other 22 pairs are called autosomes. so, if a trait is sex-linked, that means the corresponding gene is found on a sex chromosome (ie. whether or not u carry or express that trait depends on whether u're a guy or a girl). autosomal means that the trait expressed by the gene is found on an autosome (ie. the trait has nothing to do with whether u're a guy or a girl).

• genes usually exist in more than one form, and each form is called an allele. we get 2 alleles per gene that are present in an autosome pair--one from mom and one from dad. (sex chromosomes are a bit different but i won't get into that here.) an allele is either dominant or recessive.

• let's use eye-colour as an example. having brown eyes is a dominant trait, so let's represent it with a capital B. having blue eyes is a recessive trait, so let's represent it with a small b. since we have two alleles for our eye-colour gene (one from each parent), we have one of the following combinations: BB, Bb, bB, or bb. of course, if u carry only brown-eyed alleles (ie. BB) then ur eyes would be brown, and if u carry only blue-eyed alleles (ie. bb), then ur eyes would be blue. but what if u carry one of each?

• if an allele is dominant, that means it will override the recessive allele. therefore, if u have one brown-eyed allele and one blue-eyed allele (ie. Bb or bB--same thing), u will end up having brown eyes. a homozygous combination is made up of two of the same alleles (eg. BB or bb) and a heterozygous combination is made up two different alleles (eg. Bb).

• soooo... let's bring it back to the six-fingered trait. it's autosomal dominant, which means that the polydactyl gene (let's represent polydactyly with a capital D and the five-fingered gene with a small d) is on one of the non-sex chromosomes, and it is always expressed in the homozygous combo of DD or even if only one copy of the gene is present, as in the heterozygous combo of Dd.

• now what does it mean to have complete penetrance? in this case, the mutation of six fingers has complete penetrance because its symptoms are present in all individuals who carry this mutation. that is, if u carry a D, it's gonna show without question. other genes may have what's called incomplete penetrance which means that even if u carry it, u don't show it.

• lastly, what does variable expressivity mean? in the case of the six-fingered gene, variable expressivity means that the type and severity of the six-fingered mutation can vary. for example, if u have a D, u can have an extra thumb, or only a part of a thumb, while another person also carrying a D can have an extra index finger.

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hmm... looking at my explanation, i don't think i made things any less complicated. oh well, that's genetics (and grace) for ya ;)