Inheritance Children tend to look like their parents Not exactly, but enough to let silly aunts say “doesn’t he take after his mum!” People used to think that this was like mixing paint – add a bit of mum and a bit of dad and stir well.
About 150 years ago Gregor Mendel showed that this is not what happens at all! He showed that information is passed from parent to child in precise packages, called Genes. Humans have many thousands of these things. Each gene has a precise chemical effect. Nowadays, you can think of a gene as a computer program for making some parts of you.
Dad gives you a copy of a certain gene and Mum gives you one as well. What do you want two for? Surely you can’t mix computer programs in the same way that you mix paint? Why Two?
The Inheritance of a Gene Everybody has TWO copies of each gene (except in some special cases) Which of the two copies do you get in each case? Answer — it’s random, ‘tossing a coin’ You get one of your copies from Mum You get one of your copies from Dad
Fine, but does it matter which of the two you get, since they are the same gene anyway? The very important answer is that they need not be the ‘same’, in the sense of being the same program. Many genes exist in more than one version (mutants) called alleles. The different versions can have different effects. Now it matters which one you get! So What?
Our (fake) Example for the Day Let us suppose that humans come in two varieties: normal humans with Green ears others with peculiar Pink ears.
We will suppose that there is a certain gene that, as one of its effects, controls the colour of your ears. It exists in two varieties (alleles), one ‘for green ears’ and one ‘for pink ears’. Let me call the two alleles G (for Green) and P (for Pink). The Gene
GPPPGG Genotypes There are two alleles for the ear gene. Call them G and P. Everybody has two copies of the gene. So the three possibilities for a person are
Phenotypes G is the ‘allele for Green Ears’, P is the ‘allele for Pink Ears’ I say that G is Dominant and P is Recessive. What that means is … GPPPGG Green Ears Pink Ears Green Ears Not one of each!
The six types of Marriage GP PP GG GP GGPP GP PP
GG GP GGGP GGPP GP GGGPPP GPPP GPPP The Possible Children
GG GP GGGP GGPP GP GGGPPP GPPP GP PP 10050 100 5010050 25 GG PP The Chances