Simple genetics example
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Albino rabbits have white coats and
red eyes.
This is due to lack of the pigment
melanin
Albinos are actually rather rare in
the wild, but common in pet shops!
The original albino rabbit arose as a
result of a mutation – a very rare
event – due to a change in the DNA
of the coat colour gene.
Simple genetics example
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Brown rabbits are the normal colour
in the wild. The pigment melanin
gives them colouration which
enables them to blend in with their
surroundings, and protects them
from UV radiation.
Parents
We shall consider what
happens if we cross an
albino rabbit with a
brown rabbit.
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It does not matter which
one is the male, and
which is the female
The result is the same…
F1 offspring
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All the offspring are the
same: they look brown
Using F1 offspring as parents
If two of these rabbits are
allowed to reproduce…
F2 offspring
The result is:
 most of the offspring are
brown
 some are albino
F2 offspring
Actually the proportion is:
3 brown : 1 albino
(as a ratio)
75% brown, 25% albino
(as a percentage)
F2 offspring
We can also use these figures
to give the chance of an
individual rabbit being one
colour or the other
F2 offspring
There is a 3 in 4 (¾) or 75%
chance of any baby rabbit
being born brown
F2 offspring
There is a 1 in 4 (¼) or 25%
chance of any baby rabbit
being born albino
Some genetics terms
(dominant/recessive)
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Since all the F1 offspring are
of one type, we have a
straightforward situation.
We use the term dominant to
describe the characteristic
seen in the F1, and most of
the F2 – brown
Because they seem to recede
and are not seen in the F1, we
use the term recessive for the
albino forms
Some more genetics terms
(phenotype/genotype)
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Phenotype is a word used to
describe an organism by its
observable characteristics, i.e.
what it looks like, and we use
ordinary words to describe it
Genotype is defined at the
genetic level, i.e. what genes it
contains. This is usually not
directly visible, and we describe it
using a code
Some more genetics terms
(genes/alleles)
In rabbits, the two colour forms
are caused by 2 versions of a
gene.
We say that the coat colour
gene has 2 alleles: brown and
albino
Genetics conventions
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It is normal to choose a
particular letter to stand for
the gene we are considering
(could be A or B).
We use upper case
(CAPITALS) for the dominant
form and lower case for the
recessive form
Genetics conventions
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The original brown parent is
described using the genotype
BB (could be AA)
The original albino parent is
described as bb (could be aa)
These letters are in pairs
because chromosomes are in
pairs in adult cells
Genetics: explanations
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The brown rabbit (genotype BB) produces
gametes (sperms or eggs, depending on
its sex) containing only the allele B.
The albino rabbit (genotype bb) produces
gametes (eggs or sperms, depending on
its sex) containing only the allele b.
This is because meiosis separates
chromosomes so that only one of each set
is present in the sex cells
Genetics: explanations
The brown rabbits of the F1 all have
the genotype Bb.
Their phenotype is brown because they
contain a dominant allele B, and only
one dominant allele is enough to give
this colouration.
They have the same phenotype as their
parent (brown) but a different genotype.
Genetics: explanations
The white rabbits of the F2 all have
the genotype bb.
Their phenotype is white because they
contain a pair of recessive alleles bb,
and two recessive alleles are required
to give this colouration.
They have the same phenotype as their
grandparent (white) and the same
genotype. Only double recessive
genotypes can be forecast this way.
Genetics: more terms
Because rabbits with the genotype
Bb contain two different alleles they
are called heterozygous, whereas
rabbits with the genotype BB and
bb are both called homozygous
or true-breeding.
Genetics: predictions
In heterozygous brown rabbits the
recessive allele b is hidden, but it
could reappear in the next
generation…….