Seriously, what are the odds?

[bldrnrpdx]

It was confirmed today - my dad has myasthenia gravis, which is the same thing Michael has. They're both Class I - located in the eyes only. It's possible that if they go off their meds that it'll spread across the body, but properly treated, they should only show signs in their eyes. My dad's been months from initial onset to diagnosis and

Comments

[whipartist]

Well, let's take the easy problem. Assume that you already have one, Michael. The odds of one exact person, your father, having it are 20/100000. I don't know if it's genetic or not, but let's assume that it isn't, since that makes it easier.

How many relatives do you have? The calculation is:

1 - (99980 / 100000) ^ N, where N = number of relatives. (The caret is raised to the Nth power.)

In decimal, 20/100000 = .0002. If you have ten relatives, then:

1 - (99980 / 100000) ^ 10 = .0019982

Another problem. What are the odds that if you pick any two people at random, they both have it?

(20 / 100000) ^ 2 = .00000004, aka four out of a hundred million. But that's for picking two very specific people.

I'll do the math for any two out of N relatives later.

[bldrnrpdx]

Let's assume direct relatives, as in one generation out or fewer. And because we're including Michael, we'll include the spouses/spouse-like equivalents of each relation as well. So dad and his wife, me and Michael, my brother and sister plus their partners, plus my brother's son (blood relation) and 3 stepkids. We should probably also include my mom, as she lived plenty long enough to have gotten the disease if she was destined to get it at all. That makes 13 direct relations for me. I'm related to two people who have the disease.

To the best of my understanding, while it is possible for a child to have congenital MG, it can be from a healthy parent. Some infants of mothers who have MG may have "transient neonatal myasthenia", but it typically resolves fairly quickly. MG in general does not have any direct inheritance.

[bldrnrpdx]

I'm glad I have friends who can do the math as well. I can sometimes do the arithmetic, but I don't always know the math to get started.

As for the disease - my dad's at an age where people acquire all sorts of diseases. As diseases go, this one's not too bad, all things considered (you know, other than the "it can kill you" part), certainly a better deal than *everything else* we could come up with, and we've got the plus of having a family member who is super-familiar with the ins and outs of it, including the "Oh, I'm a special snowflake, I don't actually need to pay attention to that precaution/doctor order/really good idea" followed shortly by "Hm, I guess I should pay attention to all that stuff after all" routine. So hopefully my dad can learn more quickly from both the good examples and the bad examples :)

[whipartist]

Here we go. 13 relatives. If you start with one person having it, then the odds of at least one other relative also having it are:

1 - (99980 / 100000) ^ 12 = .002397361760

Basically, we're calculating the probability that none of the other relatives will also have the disease, and then subtracting it from 1. So .24% chance, or just a little better than 1 in 400.

I punted on the other calculation. Wander over here and plug in some numbers:

N = 13
k = 2
P = .0002
q = .9998

The probability of exactly 2 of 13 is 0.0000031131428598832475, or about one in 321218.

The probability of 2 or more of 13 is 0.0000031154274301448126, or about one in 320983.

[bldrnrpdx]

You are a rock star. Thank you.