No Two Organisms of the Same Species Are Exactly Alike Eg People in Your Class Have Different

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6AVariation

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.

6BWhat 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.

Tall small

Pea pea

Plant 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)trueXtrue

Breedingbreeding

Tallsmall

Offspring(F1)all tall plants

Mendel called the tall characteristic dominant and the small characteristic recessive.

Similarly with guinea pigs:

Parents(P)trueXtrue

Breedingbreeding

Blackwhite

Offspring(F1)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)trueXtrue

Breedingbreeding

tallsmall

genesTTtt

Gametesall Tall t

Offspring(F1)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)trueXtrue

breedingbreeding

smoothrough

genesSSss

Gametesall Sall s

Offspring(F1)allsmooth coated

Genes all Ss

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?

F1hybridXhybrid

tallsmall

genesTtTt

Gametes T or t T or t

Offspring(F2)

GenotypeTTTtTttt

phenotypetalltalltallsmall

ratio 3 tall: 1 small

There is another way of working out all the possible combinations, a punnet square.

GametesTt

TTTTt

tTttt

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.

eggsperm

motherXXXYmother

cellcell

gametesall XX 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

2346

sperm

Fertilised egg

6CGenetics 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 AGirl 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.