A bit of Genetics
October 15, 2006
I’m going to take a short break from the normal distribution and talk about the basic mechanisms of genetic inheritance.
If you took biology in high-school, this will all sound familiar.
Genes and Alleles.
The genetics I’m going to talk about are for heterozygous species (such as ourselves). This basically means that an individual has two copies of each gene. Some species are homozygous — but they’re mainly single-celled organisms and we all know how trashy they can get.
When a gene has alternative versions, they are called ‘alleles’. We are only going to look at the case where a gene has two alleles. I’m keeping it simple, stupid.
One of these alleles is dominant. That means if an individual has even one copy of that allele, then they will show the physical attributes coded by that allele.
The other gene is recessive. In order for an individual to display the characteristics of the recessive gene, both of the copies held by that individual must be the recessive allele.
The classic example of dominant and recessive alleles is eye colour. The allele for blue eyes is recessive. The allele for brown eyes is dominant. To have blue eyes, you need two copies of the blue eye allele. If you have one blue eye allele and one brown eye allele, then you’ll have brown eyes.
We will call the dominant allele ‘A’ and the recessive allele ‘a’. See? The lower case is recessive while the upper case is dominant.
OK, so there are three possible combinations of allele:
Notice that aA and Aa are the same; the sequence does not matter.
Yeah. You’ve been waiting for this bit. I know.
So, we’ve got two potential parents. They breed. Give them some privacy, please. We’ll wait while they finish.
OK, they’re done. That was a bit quick, wasn’t it?
So what genes will the child carry? Basically the child will get one randomly selected gene from each parent.
The following table shows the possible offspring types for each parent combination:
|aa||aa||aa, aA||aa, aA|
|Parent 2||aA||aa, aA||aa, aA, AA||aA, AA|
For example, if both parents are ‘aa’ then all offspring will be ‘aa’ because they are the only alleles available.
If one parent is ‘aa’ and the other is ‘aA’, then offspring will either be ‘aa’ or ‘aA’; ‘AA’ offspring are not possible because only one parent has the dominant ‘A’ gene to pass down.
By going through the possibilities, we can come to the probabilities.
If both parents are ‘aa’ then the probability of offspring being ‘aa’ if 100%.
If both parents are ‘aA’ then the offspring probabilities are:
- aa — 25%
- aA — 50%
- AA — 25%
We can now build a probability matrix to determine the probabilities of offspring genotypes from any parental genotypes.
I’ll do that in the next installment and show how this probability matrix can be used in a computer simulation of genetics.
If you’re interested in writing software, check out my other blog: Coding at The Coal Face
Written while listening to: