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6A Variation
No two organisms of the same species are exactly alike eg
people in your class have different heights, weights, eye colour
etc.
They are said to show variation.
Continuous variation
With continuous variation, the differences are easily measured
and there is usually a wide range of measurements.
Height, shoe size and tail length in mice are examples.
If you graph examples of continuous variation they will look like
this:
Discontinuous variation
Some differences are hard to measure. People usually have or
do not have a characteristic eg some people can roll their
tongue while others cannot.
Some examples of discontinuous variation are cattle with or
without horns, human blood groups and tongue rolling.
If you graph examples of discontinuous variation they look like
this:
What is a species?
If the differences between organisms are very great then
they probably belong to a different species. All domestic dogs
look different but they can interbreed.
However dogs and cats cannot interbreed.
A species is a group of organisms that can interbreed together
to produce a fertile organism.
6B
What is Inheritance?
Inside the nucleus of each cell there are chromosomes that
carry genetic information. This information is passed on
through your generations from parent to offspring. Half you
genetic information comes from your father and half from your
mother. There are 42 chromosomes in most of the cells in your
body.
In 1865, Gregor Mendel published a paper about his breeding
experiments with peas.
Experiment 1
He collected seeds from pea plant that only produced tall
plants and seed from pea plants that only produced small
plants.
Plants that only produce one kind of offspring are called true
breeding.
Pollination
n
Tall
Pea
Plant
small
pea
plant
He grew both kinds of pea plants and when they flowered he
transferred pollen from a small plant to a tall plant and vice
versa. When he has fertilised the flowers, he waited for the
seeds, planted them and observed what height they grew to.
This generation of plants (F1) were all tall. This can be written
thus:
Parents(P)
true
Breeding
Tall
Offspring(F1)
X
true
breeding
small
all tall plants
Mendel called the tall characteristic dominant and the small
characteristic recessive.
Similarly with guinea pigs:
Parents(P)
true
Breeding
Black
Offspring(F1)
X
true
breeding
white
all black guinea pigs
This time the black colour is the dominant characteristic and
white is the recessive characteristic.
From his experiment Mendel decided that for every
characteristic there were ‘two factors’ that carry information.
We now call these factors genes.
His experiment can be rewritten as follows:
Parents(P)
true
Breeding
tall
genes
TT
tt
Gametes
all T
all t
Offspring(F1)
X
true
breeding
small
all tall pea plants
Genes
all Tt
The gene for tall is labelled ‘T’ because the dominant gene is
tall. We use ‘t’ to represent small. The gametes only have one
copy of the gene because when two gametes meet during
fertilisation the new organism will have the correct number of
chromosomes. The F1 can no longer be called true breeding,
instead we call this mixed gene organism a hybrid.
Similarly for guinea pigs with rough and smooth coat:
Parents(P)
true
breeding
smooth
genes
SS
ss
Gametes
all S
all s
Offspring(F1)
Genes
X
all smooth coated
all Ss
true
breeding
rough
Phenotype and Genotype
The appearance of an organism is known as its phenotype eg
red or white flowers, blue or brown eyes, smooth or rough fur.
The genes present for this characteristic are called the
genotype eg Tt means that the pea plants will be tall.
What happens when two hybrids interbreed?
F1
hybrid
genes
Gametes
X
tall
hybrid
small
Tt
Tt
T or t
T or t
Offspring(F2)
Genotype
TT
Tt
Tt
tt
phenotype
tall
tall
tall
small
ratio
3 tall : 1 small
There is another way of working out all the possible
combinations, a punnet square.
Gametes
T
t
T
TT
Tt
t
Tt
tt
Three of the F2 are dominant and one shows the recessive
characteristic. This ratio is the expected ratio for this type of
cross.
Monohybrid crosses
All the crosses we have looked at so far are called monohybrid
crosses because they have only involved one characteristic.
The different forms a gene can take are called alleles.
Tall and small are alleles of the height gene. O, A, B and AB are
the alleles for human blood group. Eye colour is also another
example:
We have found that if we cross two F1 hybrids we can predict
or expect their offspring (F2) will be in the ratio of 3 dominant
characteristic to 1 recessive characteristic. However when we
actually carry out these crosses, the predicted numbers rarely
occur.
eg if there are 100 F2 pea plants we would expect 75 to be tall
and 25 to be small. In reality you might not get this.
One investigator, Hurst did a similar experiment to Mendel’s.
He found in the F2 he had 1,310 yellow seeds and 445 green
seeds.
This works out to a ratio of 2.94 : 1.
Why are his results not exactly 3:1 like Mendel predicted?
It’s because it is chance which two gametes meet during
fertilisation.
The inheritance of gender
What decides whether we are male or female?
It depends on your chromosomes. Most human cells contain 46
chromosomes or 23 pairs.
22 pairs are the same in both sexes, but one pair is different.
This pair decides whether a baby is a girl or a boy.
Both sexes have chromosome pairs 1 to 22 the same. In
females pair 23 is two X chromosomes but in males pair 23 is X
and Y.
egg
sperm
mother
XX
XY
mother
cell
cell
gametes
all X
X or Y
This means that all female eggs contain one X chromosome but
that half the sperm will contain an X chromosome and the
other half will contain a Y chromosome. The sperm decides the
sex of the child.
Gametes should always contain only half the information for a
new offspring so that when two gametes meet the two halves
of information will make one complete set.
egg
23
23
sperm
46
Fertilised egg
6C
Genetics and Society
For centuries, animal breeders have attempted to improve
their animals and crops by selecting certain animals and plants
to breed from. This is known as selective breeding.
By selective breeding we have produced many new varieties of
crop plants that grow more quickly, produce more seeds and
are more resistant to diseases. Thus selective breeding is
useful.
Other examples of selective breeding are cattle:
Chromosome mutations
Changes to a chromosome are called mutations. They can
produce changes in the characteristics of an organism. The
mutation can occur naturally. The two chromosome sets below
are of two human females:
Girl A
Girl B
Girl A has a normal set of chromosomes. Girl B however has one
extra copy of chromosome 21, giving her 47 chromosomes in
every cell instead of 46. This mutation results in the girl having
Down’s syndrome.
Looking at the table below:
Woman’s age Chance of Down’s syndrome
(per 3000 births)
20
1
25
1.5
30
3
35
6
40
30
45
60
We can see that as a woman gets older her chance of having a
Down’s syndrome baby increases.
Amniocentesis is the removal of amniotic fluid for medical
examination. The fluid contains cells from the developing
embryo. The chromosomes in these cells can be examined for
normal cells.
Useful chromosome mutations
Most mutations are not useful to humans. However those that
are useful are encouraged
Example 1
In some crops like bananas and water melons, mutations result
in fruit without seeds. These crops produce seedless fruit.
Example 2
Mutations in some apple crops produce fruit with an extra set
of chromosomes. These apples produce more vitamin C than
ordinary apples.
Example 3
Sheep now keep their ‘baby’ coat as adults because we wanted
the soft wool for clothing.
Influencing the rate of mutation
Mutations normally occur naturally and are rare. However
certain mutations can be caused by chemicals like mustard gas
and by radiation such as X rays.