Transcript First Talk (powerpoint)

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