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.

46 chromosomes

Normal human cells have

- 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!)

A man would have one X and one Y chromosome.

This person is female because they have two X

chromosomes

The last pair are called the sex chromosomes.

They determine what sex you are..

Gametes (Sex Cells)

Normal body cells have 46

chromosomes (23 pairs)

46 46

… but gametes (eggs and sperm)

only have 23 single chromosomes.

23 23

… this is because gametes fuse together at fertilisation to produce a zygote (fertilised egg). This then grows into a

new person

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.

MitosisDuring mitosis a cell divides once to

produce two new identical cells.

Original body cell (46 chromosomes)

Two new body cells (46 chromosomes each)

MeiosisDuring meiosis there are two cell divisions which half the number of chromosomes.

Cell in the testes or ovaries.

(46 chromosomes)1st division

2nd division

Gametes (eggs or sperm)

23 chromosomes each

XY

XX

X X

Because gametes only have half the normal

number of chromosomes, each egg

will only contain a single X chromosome.

X

Y

Sperm, however, will have either

X or Y

As any sperm can fertilise any egg, this gives an equal chance of having a boy or a

girl

Boys or Girls?

XY XY

XX XX

XY

XX

X X

X

Y

Key Points

What someone looks like is called their

Phenotype

This person’s phenotype is

female.

XY

XX

X X

X

Y

Key Points

The genes that control the phenotype are

called the genotype

This person’s genotype is XX.

XY

XX

X X

X

Y

Key PointsRemember that gametes

only contain half the normal number of genes

/ chromosomes

Two sex chromosomes

Single sex chromosomes

XY

XX

X X

X

Y

Key Points

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 Phenotype = blue eyes

Genotype =

BBor

Bb (heterozygous)

( Homozygous dominant )

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

Gene = Fur colour

Brown fur allele = B

White fur allele = b

In rabbits the allele for brown fur is dominant to the allele for white fur.

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

For instance:

A back cross is used to find out if an organism with the dominant phenotype is pure breeding

(homozygous) or not.

The mixture of phenotypes you get in the offspring will tell you what the parent’s genotype was.

To do this you breed it with another one with the recessive phenotype.

(also called a Test Cross)

B

B

b b

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

BbBb

Bb Bb

All the offspring will

be brown.

B

b

b b

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

BbBbNow half the offspring are

brown and half are white.

On the other hand…..

bb bb

B

B

b b

BbBb

Bb Bb

B

b

b b

BbBb

bb bb

Back Cross SummaryPure breeding (homozygous)

Hybrid (Heterozygous)

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

Bb

B

B

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

bb

b

b

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.

How good is this rabbits pedigree?

BB

This rabbit has to be bb so it must have got one b from each of its

parents

The parents are both brown so their

genotypes have to be Bb

… so this rabbit could be BB or Bb

… so this rabbit could also be BB or Bb.

BB

BB

Bb Bb

bb

B?

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.

• 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.

Problems

CloningCloning 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.

4

4. Remove its nucleus (23chromosomes)

Cloning Animals(For this example, assume sheep have the same number of

chromosomes as humans.)

5

5. Put the body cell nucleus into the egg cell. (the egg now has 46 chromosomes – as if it had been fertilised).

1

1. Take a body cell from a sheep.

3

3. Take an egg cell from the sheep.

2

2. Remove its nucleus (46chromosomes)

6

6. Implant the egg into the sheep’s womb.

7

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

1. Cut a chromosome out of a human cell and from a bacteria cell.

2. Cut up the chromosomes using an enzyme.

2

3 4

3. Join the human piece containing the gene for insulin into the bacteria chromosome..

4. Insert the joined chromosome back into the bacteria. The bacteria will now produce human insulin.

5. Grow lots of copies of the bacteria.

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.

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)

Homozygous = pure breeding.

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

2. The chromosomes split apart to opposite

ends.

3. The cell splits in half

4. Two new identical cells are produced

Meiosis 1. The chromosomes copy themselves

2. The chromosome pairs are pulled apart.

3. The cell divides. 4. The chromosomes are pulled apart

5. The cells divide again to produce 4

gametes.