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Autosomal recessive genes

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ann_h | 22:15 Fri 08th Apr 2005 | Science
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Can anyone tell me how you could work out the whole population incidence of carriers based on the annual birth incidence (#) of a disorder in its full form?

eg  for every child born with 2 defective genes, one will be clear and two will be carriers. There are, say, 800,000 people born each year ( x 70 years lifespan = 56 million population). If # is 1:10,000 then carriers are 2:10,000 or 160 per year. If # is 1:100,000 then carriers are 16 per year.

Or is there a geometric element of  probability to complicate it?

This isn't my homework, by the way!

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Hi Ann

Your q is not very clear.

Incidence is, as you say, rate per year.

Prevalence is the flat how-many-there-are in a population at a certain time.

Arent you looking at the expansion of the Hardy Weinberg equation, p2 + 2pq + q2

if the prevalence of a disease is 1;400, (p2)

then the gene frequency is the square root of this, 1:20

Reading your question, I think this is what youa re getting at: I think youare asking if a disease has a frequency x what is the gene frequency, and the answer is the-square-root of x

For the reason given above.

Question Author

I think I mis-used terminology.

I know that the likelihood of a two-carrier couple producing an affected child is 1:4, with 2:4 carriers and 1:4 unaffected. This is standard. In this particular case, an affected child will not be able to have children, as they will die or be too disabled, so only the 2-in-4 that are carriers will count in my calculation of prevalence.

I have only been able to establish a rate of birth per year of affected children, so I am assuming that carriers will be double that, but maybe I am wrong.

Basically, if the birthrate is (x) per year, what are the odds of one carrier meeting another?

Hi Ann, there is nothing wrong with what you wondering, it is just that you havent got the words straight. You have used birthrate in a sense no-one else does.

Take a rarish disease like CF, prevalence 1: 400. Gene frequency 1 in 20.

Chance of two carriers meeeting 1/20 times 1/20.....and we are back to 1 in 400

Your point about lethality is not a factor so long as the disease is rare. If a gene hangs around for along time, even thugh the disease is lethal then it is called a founder effect - but that is not really what you're talking about, either.

If X knows he is a carrier, then the chance of meeting Y who is a carrier is 1/20 - conditional probability

Take something really rare like Ellis van Crefeld, 1 in 160 000, then the gene frequency is 1.400, and the upshot is that we never see it.....although oddly enough EVC disease shows a founder effect in the desert dwelling aborigines of Western Oz - which it shouldnt and the reasons for which are not really known

Does any of this help?

Question Author

Right then! Let's see if this is the right use of the words; then I can ask the question correctly!

Incidence = number/ratio of affected people born in any one year

Prevalence = number/ratio of affected people in the population as a whole

Gene frequency = number/ratio of people carrying the gene in the population as a whole

So, a gene frequency of 1:20 gives a prevalence of 1:400 by simple maths.

But, I only know the incidence (the annual number/ratio of births)

So what is the accepted principle by which gene frequency can be estimated from this?

(I didn't think it was the root as only a proportion of the population is of child-producing age, and they don't all have one baby every year)

Thanks for trying so hard, Peter, and sorry if I'm confusing you further!

Interesting about EVC, by the way.

The nice thing about AB is that one gets interested enough to ask someone who knows. One of my colleagues' wives is a geneticist, and I will ask.

The q is: If you know the incidence of a aut rec disease how can one estimate the gene frequency?

My suspicion is that you cant unless you know the number in the population and the average life span.

I tried this on a selection of my colleagues and they all came up with the first stab- square root of the incidence. They sort of agreed that you should nbeed the population, the average life span of the condition and the proportion of child bearing age -

gasp - the Lady who Knows, has been asked and we are breathlessly awaiting her answer!

Question Author

This is what I came up with, wait to see if they agree!

Incidence 1:90,000 of affected, who all die before procreating

Assume carriers therefore 2:90,000 or 22 per million

In a city of 1,000,000 people there are 22 carriers, of whom about 1 in 10 are in your age group.

So there's just the two of you.

The Lady who Knows hasnt answered. perhaps she thinks it is all too easy.

Incidence 1:90,000 - that is the number born per year remember. All dead by 15 say, so 16:90000 is the prevalence.

gene frequency 4:300

so in a city of however mmnay it is - quite a few more.

NOW - The nice thing about AB is finding outthe answer and so I wandered into a book shop. Try halliburton - theory of population genetic or something. p 77.

Estimation of gene frquency using the der- daaaah - hardy Weinberg equilibrium.

Anyway why are you doing this?

Question Author

It's such a rare disorder, I suspect somebody's skeleton just fell out of the cupboard

:(

A week later - the lady who knows er hasnt answered. I think I will have to give her a prod or something.

I havent got much further than reading Halliburton....

If you get to a sensible answer then please post it.

Question Author

I did find a website which seemed to agree with you, using the Hardy-Weinberg, except it came to 1:300 not 4 ( a square too far?!) It said there were some instances where you couldn't estimate. I'll have to find it again if you like.

As I said, it was a suspicion I had .

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