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Master Class Lecture “Genes and Inheritance” Dr John Pulham Gregor Mendel University of Aberdeen Master Class Genes and Inheritance 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. Why Two? 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? The Inheritance of a Gene Everybody has TWO copies of each gene (except in some special cases) You get one of your copies from Mum You get one of your copies from Dad Which of the two copies do you get in each case? Answer — it’s random, ‘tossing a coin’ So What? 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! 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. The Gene 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). 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 GG GP PP 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 … GG GP PP Green Ears Green Ears Pink Ears Not one of each! The six types of Marriage GG GG GP GP GG GP GP PP GG PP PP PP The Possible Children GG GG GG GP GG GP GP PP GG GP GG GP GP PP GP PP GG PP GP PP PP PP The Chances GG GG GP GG GG GP GP 100 50 50 100 GP GG GP GP PP GG PP PP PP GG GP PP GP PP PP 25 50 25 50 50 100