Both my little sister Angelina and I were both born on the same day… But we aren’t twins.
I was born January 26th 1998; while Angelina was born January 26th 2002. Same day but four years apart? That’s crazy… what are the chances?
Well I’ll tell you.
Not only are we born on the same day, but we are also phenotypically similar.
We both are girls who have brown hair, green eyes, and are on the shorter side. With just these variables, an estimated probability can be made.
Because each of these traits are independent events, one is not relying on the other, the multiplication rule applies. For each of the probabilities it must be multiplied twice, once for each child.
The probability that we are born on the same is one out 365 since 1998 and 2002 were not leap years.
The probability that we are both girls is a one out of four since a boy-girl probability is one in two.
The probability that we will be born with green eyes based on our parents is 36% or 9 out of 25.
The probability that we will be born with brown hair is one in two.
The probability we would be short is one in four.
1/365 * 1/365 * 1/2 * 1/2 * 9/25 * 9/25 * 1/2 * 1/2 * 1/4 * 1/4 = 81/10,658,000,000
That is only 81 times out of 10 billion, 658 million times.
That is truly a rarity only seen once in a lifetime. I’m glad I am part of it.
