Transcript Variation and Genetics.

Variation and Genetics.
Variation is the different
characteristics that organisms have.
Some characteristics are
environmental. They are
not passed on from one
generation to the next
Other characteristics are
genetic. These are
passed from one
generation to the next.
Genetic Variation 1
Inside every cell
is a nucleus that
controls the cell.
Inside the nucleus
are threads called
chromosomes
The chromosomes are
made up from an
enormously long
molecule called DNA.
DNA acts as the code for
all the characteristics.
The DNA in a
chromosome is divided
up into sections called
Genes.
Each gene contains the
DNA code for a
different characteristic.
Normal human
cells have
46 chromosomes
- in 23 pairs.
They are in pairs
because we get half
of them from each
of our parents.
i.e. 23 from the egg
and 23 from the
sperm.
Pairs 1 – 22 contain the
genes that control the
body’s normal
characteristics (about
100,000 of them!)
The last pair are called
the sex chromosomes.
They determine what sex
you are..
This person is female
because they have two X
chromosomes
A man would have one X
and one Y chromosome.
Gametes
(Sex Cells)
… but gametes
(eggs and sperm)
only have 23 single
chromosomes.
… this is because
gametes fuse together at
fertilisation to produce a
zygote (fertilised egg).
This then grows into a
new person
46
Normal body cells
have 46
chromosomes (23
pairs)
46
23
23
46
Cell Division.
We need new body cells for growing and to replace old or damaged
cells.
It is important that the new cells have exactly the same set of
chromosomes as the original cells.
This is done by a type of cell division called Mitosis which occurs
throughout the body.
Gametes only have half the normal number of chromosomes in
them so they need a different type of cell division to produce them.
This type of cell division is called Meiosis. Meiosis only occurs in
the ovaries or testes in animals and the anthers or ovaries in plants.
Mitosis
During mitosis a cell divides once to
produce two new identical cells.
Original body cell
(46 chromosomes)
Two new body cells
(46 chromosomes each)
Meiosis
2nd division
During meiosis there are two cell
divisions which half the number
of chromosomes.
Gametes (eggs
or sperm)
23 chromosomes
each
Cell in the testes or
ovaries.
(46 chromosomes)
1st division
Boys or
Girls?
Sperm, however,
will have either
X or Y
Because gametes only
have half the normal
number of
chromosomes, each egg
will only contain a
single X chromosome.
XX
X
XX
X
XX
X
Y
XY
XY
XY
As any sperm can
fertilise any egg, this
gives an equal chance
of having a boy or a
girl
Key Points
This person’s
phenotype is
female.
XX
What someone looks
like is called their
Phenotype
X
X
Y
XY
X
Key Points
This person’s
genotype is XX.
XX
The genes that control
the phenotype are
called the genotype
X
X
Y
XY
X
Two sex
chromosomes
Key Points
Remember that gametes
only contain half the
normal number of genes
/ chromosomes
XX
Single sex
chromosomes
X
X
Y
XY
X
Key Points
XX
X
X
Y
XY
X
This is called a
Punnett Square.
It shows all the
possible new
combinations
the gametes can
make.
Genes and Alleles
A gene is a section of DNA on a
chromosome that contains the
code for a particular characteristic.
Genes
For instance, we have a gene for
eye colour, another for being able
to roll our tongue etc.
Genes and Alleles
Genes can come in different versions called Alleles.
These people have both got
genes for eye colour.
… but they have inherited
different versions (alleles) of
the gene.
… so they have ended up
with different phenotypes.
Because our chromosomes are in pairs we
actually have two copies of each gene.
Copy 1
Copy 2
These two copies of the gene could be the same allele (called homozygous)
Or they could be different alleles for the gene (called heterozygous)
One allele is usually stronger than the other. The stronger
(dominant) one will completely overshadow the weaker
(recessive) one if they are mixed together.
The allele for brown eyes is
dominant. The dominant allele is
always given a capital letter: B
The blue eye allele is recessive.
Recessive alleles always get a
lower case letter: b.
Phenotypes and Genotypes
Phenotype = brown eyes
Genotype =
BB ( Homozygous
dominant )
or
Bb (heterozygous)
Phenotype = blue eyes
Genotype =
bb ( Homozygous
recessive )
Hint: If someone has the recessive
phenotype they must be
homozygous for that allele!
Pure breeding?
This woman’s genotype has to be bb (i.e. two
blue-eye alleles). All her eggs will contain a
single b (one blue-eye allele) – so she is pure
breeding for blue eyes.
This woman’s genotype, however, could
be BB or Bb.
• If she is BB then she is pure breeding
for brown eyes.
• If she is Bb, however, half of her eggs will
have a brown-eye allele and half of them a
blue-eye allele. She is a carrier for blue eyes.
Pure Breeding 2
In rabbits the allele for brown fur is
dominant to the allele for white fur.
Gene = Fur colour
Brown fur allele = B
White fur allele = b
The white rabbit has to be bb
But the brown rabbit could be pure
breeding BB or it could be Bb.
How could you find out?
A Back Cross
(also called a Test Cross)
A back cross is used to find out if an organism
with the dominant phenotype is pure breeding
(homozygous) or not.
To do this you breed it with another
one with the recessive phenotype.
The mixture of phenotypes you get in the offspring
will tell you what the parent’s genotype was.
For instance:
We know that this rabbit
can only produce
gametes with the white
coat allele (b).
If this rabbit is pure
breeding brown (i.e.
BB). All its gametes
would be B
b
b
B
Bb
Bb
Bb
Bb
B
All the
offspring will
be brown.
On the other hand…..
This rabbit can still only
produce gametes with
the white coat allele (b).
If this rabbit is
heterozygous (i.e. Bb) half
its gametes would be B
and half would be b
b
b
B
Bb
Bb
bb
bb
b
Now half the
offspring are
brown and half
are white.
Back Cross Summary
Pure breeding
(homozygous)
b
Hybrid
(Heterozygous)
b
b
b
Bb
Bb
bb
bb
B
B
Bb
Bb
b
B
Bb
Bb
All dominant
phenotype
Half dominant,
half recessive.
Other common test questions…..
1. Can two organisms with the dominant phenotype produce
offspring with the recessive one?
i.e. Could two brown rabbits have white offspring?
2. Can two organisms with the recessive phenotype produce
offspring with the dominant one?
i.e. Could two white rabbits have brown offspring?
3. If a couple already have two boys, what is the chance that
their next child will also be a boy?
1. Could two brown rabbits have white
offspring?
Yes – If they are both heterozygous.
Bb
B
B
Bb
b
b
On average, one in four of
their offspring will be
white.
This is called the 3:1 ratio.
2. Could two white rabbits have brown
offspring?
No, all white rabbits have to be homozygous
bb
b
b
bb
b
b
1.
If a couple already have two boys, what
is the chance that their next child will
also be a boy?
It is still a 50:50 chance.
A man produces about 100,000,000 sperm a day –
that’s 50,000,000 with an X chromosome and
50,000,000 with a Y chromosome.
So the fact that he has already used 2 sperm with a
Y chromosome makes no difference whatsoever.
Pedigrees
Having a pedigree means that the animal’s family tree is
known (also called a pedigree chart). It allows us to work
out which organisms are true breeding for a characteristic.
BB
How good is this
rabbits pedigree?
The parents are both
brown so their genotypes
have to be Bb
… so this rabbit could be
BB or Bb
BB
… so this rabbit could
also be BB or Bb.
This rabbit has to be
bb so it must have got
one b from each of its
parents
Bb
BB
Bb
B?
bb
B?
B?
B?
B?
Using Genetics
1. For centuries, man has been selectively breeding his
plants and animals to improve their quality.
2. Cloning is making exact copies of an organism. It is
often easy to clone plants, but more recently man has
been able to clone animals too.
3
Man is also now able to genetically modify
organisms to suit his needs. This is also called
genetic engineering.
Selective Breeding
By only allowing the
organisms with the
best characteristics to
breed, man can
“weed out”
unwanted alleles,
leaving only the ones
he wants.
As a result he is left
with pure breeding
pedigrees. We do this
with both plants and
animals as the next
couple of slides show.
Problems
• The problem with selective breeding is that the
number of alleles in the population gets steadily less
and less.
• This can lead to the problems of inbreeding as all
the animals or plants that are left are genetically
closely related to each other.
• Also, once an allele has been lost from a population
it is gone forever, so if tastes change, or a new disease
arrives the old “best” may not be good enough
anymore.
• These problems are even more exaggerated with
cloning and genetic modification.
Cloning
Cloning is the production of a new organism that is genetically
identical to the one that produced it.
Many plants clone themselves naturally by asexual
reproduction.
For instance, a potato plant will produce many potatoes
underground by mitosis – so they are all genetically identical. If
they are allowed to grow they will all be clones of the original
plant.
It was cloned potatoes that caused the Irish potato famine in the
1800s in which over 1 million people died and 2 million were
forced to emigrate. All the potatoes were susceptible to the
same disease.
Cloning Animals
(For this example, assume sheep have the same number of
chromosomes as humans.)
1. Take a body cell from a sheep.
6
7
2. Remove its nucleus (46
chromosomes)
3. Take an egg cell from the sheep.
1
2
3
4. Remove its nucleus (23
chromosomes)
5. Put the body cell nucleus into the
egg cell. (the egg now has 46
chromosomes – as if it had been
fertilised).
5
6. Implant the egg into the sheep’s
womb.
4
7. The lamb that is born is a genetic
clone of its mother.
Genetic modification
This is when a gene from an organism of one species is
inserted into an organism of a different one.
This generally produces genotypes that could never occur in
nature.
Examples of where these this technique has been used are:
• Making bacteria produce human insulin to treat diabetes.
• Making bananas produce human hormones to treat various
deficiency diseases
• Making crops resistant to a particular weedkiller (herbicide)
How is it done?
Example: Producing human insulin from bacteria
1. Cut a chromosome out of a human
cell and from a bacteria cell.
1
2. Cut up the chromosomes using
an enzyme.
3. Join the human piece containing
the gene for insulin into the bacteria
chromosome..
2
3
5. Grow lots of copies of
the bacteria.
4
4. Insert the joined chromosome
back into the bacteria. The
bacteria will now produce human
insulin.
A few little extra bits….
Genetics is full of terminology and rules. Here are a few of them….
The type of inheritance where you look at just one gene is
called Monohybrid inheritance.
Homozygous =
pure breeding.
Heterozygous = hybrid =
carrier (for the recessive
allele)
Offspring = f1 generation
Grandchildren = f2 generation
Sexual reproduction = 2
parents (using gametes
from meiosis)
Asexual reproduction
= 1 parent (using
mitosis)
The last two slides just show mitosis and
meiosis in a bit more detail – too much
detail for NCEA level 1, but useful if you
are going on to do Biology Level 2
Mitosis
1. The chromosomes copy
themselves and the
nucleus disappears
4. Two new identical
cells are produced
2. The chromosomes
split apart to opposite
ends.
3. The cell splits in
half
Meiosis
3. The cell divides.
1. The chromosomes
copy themselves
4. The chromosomes
are pulled apart
2. The chromosome pairs
are pulled apart.
5. The cells divide
again to produce 4
gametes.