Genes and Alleles
A gene is a piece of DNA that
codes for a particular protein.
Proteins are large molecules that make up
most of the bodies of living organisms.
Each protein is unique, different to every
other protein.
Humans make about 40 000 different
proteins.
The next slide shows some molecular
models of different proteins.
Each protein made by the body
gives the body a particular feature.
• Hair colour, eye colour, height, blood
group are all features produced by having
a particular protein.
• Other words that can be used instead of
feature are characterisic and trait.
A gene can have different forms
• Everyone has a gene for blood group, but
different people have different blood
groups (A, B, AB or O)
• This shows that one gene (blood group)
can take different forms, (A, B, O).
• We call the different forms of a gene its
alleles.
Or imagine this…
• A T-shirt is always a T-shirt, but it can
come in different forms….
Each form of a gene is an allele.
• Imagine cats can only be black or white.
(this is a very simplified example)
• This means that the gene for cat fur has
two alleles, one producing black fur and
one producing white fur.
What would happen if a black cat
mated with a white cat?
• The black cat has the black allele for fur
colour, and the white cat has the white
allele.
• Say the black cat is the female. In each of
her eggs are the single copies of each
chromosome.
• On one of those chromosomes is the gene
for fur colour. This gene is in the form of
the black allele.
What would happen if a black cat
mated with a white cat? (continued)
• The white cat is the male.
• Each of his sperm cells contains a gene
for fur colour.
• His gene is in the form of the white allele.
Here are the kittens
As you can see, the kittens are
black.
• But they do possess the white allele,
because it was in the sperm of their white
father which fertilised the egg of their black
mother.
• The black allele has been used to produce
the coat colour, the white allele has
remained unused.
• We say the black allele is dominant to the
white one.
What about the white allele?
• The white allele, the one that is not used if
the dominant allele is present, we call the
recessive allele.
I’m the proud
possessor of a
recessive allele
for white fur.
How can you get a white cat?
• As the black allele is dominant, the only
way you can get a white cat is by having
two white parents.
• To have white fur the parent cats can only
have white alleles.
• So both the alleles of any white cat wil be
for white fur.
• So all the eggs of the female carry the
white allele and all the sperm of the male
carry a white allele.
What must the parent cats of these
kittens have looked like?
Describe the alleles of these two
kittens.
More new terms
• When both alleles are the same, the
organism is homozygous.
• If the two alleles are both dominant alleles,
then we say the organism is homozygous
dominant.
• If the two alleles are both recessive
alleles, then we say the organism is
homozygous recessive.
What if the two alleles are
different?
• An organism with one of each allele is
called heterozygous.
• Because they have both a dominant and a
recessive allele you don’t have to say
heterozygous anything, just heterozygous
on its own.
Can you tell what
alleles I have?
• Partly. You are have
black fur, so have at
least one dominant
black allele, but we
can’t see whether or
not you have a
recessive white
allele just by looking
at you.
These kittens
are my
offspring.
So to have white offspring
you must have a white
allele to pass on to them.
So you are a
heterozygous cat.
Shorthand
• To write down the dominant allele using a
shorthand, take the first letter of the word
that describes it (= black).
• Use the letter in the UPPER case (B) to
stand for the dominant allele and in the
lower case (b) to stand for the recessive
allele.
So instead of writing black you now put B
and instead of writing white you now put b.
So I’m
Bb
And we
are bb
I’m bb too,
of course.
