Download B4 Revision

Variation Inheritance and Natural
Selection Revision
All species show variation: this can be divided into two groups
Blood group
Genetic Variation
Hair Colour
Diseases e.g.
Common cold
Environmental Variation
Variation can either be continuous: E.g. Height
Or discontinuous: e.g. Tongue rolling
DNA is found in the form of chromosomes which are located in
the centre of all cells… the nucleus
DNA is made up of 4 base pairs: Adenine always pairs with Thymine
Guanine always pairs with Cytosine
These chemicals are said to be
complimentary
DNA replication
In order to grow and reproduce, living cells need to duplicate
their chromosomes, as happens during cell divisions such as
mitosis. This is possible because DNA can replicate itself.
The order of the chemicals ACTG in a section of DNA makes a
gene
E.g.
ACCCTGGACCGGTAGTTCGTCCAGTTGCTA
A gene is therefore something
that codes for a particular
characteristic e.g. hair colour
If we change this order of chemicals we change the
characteristic that is being coded for.
ACCCTGGACCGGTAGTTCGTCCAGTTGCTA
ACCATGGACCGGTACTTCGTCCAGGTGCTA
We call a change like this a
mutation
Mutations can be bad e.g. Cystic Fibrosis
Mutations can be good e.g. Disease Resistance
1. Ultraviolet Light in sunshine
or Sun beds.
2. Chemicals in cigarette
Smoke.
3. Chemicals in the
environment.
4. Background radiation in the
environment.
5. Or they can just happen
spontaneously.
Mutations can happen for
many reasons
This Mutation can lead to variation : - Variation leads to
Evolution
Yeah that’s right DARWIN
BABY!!!
And his wonderful theory NATURAL SELECTION
Natural Selection
1) Each species shows variation:
Get off
my land
2) There is competition within each
species for food, living space,
water, mates etc
3) The “better adapted” members of
these species are more likely to
survive – “Survival of the Fittest”
Gutted!
Yum
4) These survivors will pass on their
better genes to their offspring who
will also show this beneficial variation.
A knowledge of genes allows us to do selective breeding. This
is where we decide what characteristics we want in the next
generation.
The Probability of characteristics being passed on can be
decided by a genetic cross diagram
Eye colour
Example 1: A homozygous
brown-eyed parent and a
blue-eyed parent:
X
BB
Parents:
Gametes:
Example 2: 2 heterozygous
brown-eyed parents
bb
Bb
X
Bb
B
B
b
b
B
b
B
b
Bb
Bb
Bb
Bb
BB
Bb
bB
bb
(FOIL)
Offspring:
All offspring have brown eyes
25% chance of blue eyes
Phenotype
Another method
Example 3: A heterozygous brown-eyed
father and a blue-eyed mother:
Genotype
Father
B
b
b
Bb
bb
b
Bb
bb
Mother