S-Cool Revision Summary

S-Cool Revision Summary

In a monohybrid cross two plants or animals, which differ at only one gene, are bred together.

Homozygous and heterozygous

Chromosomes come in pairs. Each chromosome in a pair will have a gene at the same point on the chromosome. There can be more than one alternative form of the gene at that point. These alternative forms are called alleles.

Both chromosomes in a pair have one allele for the gene. If the two alleles are the same we say that the individual is 'homozygous' for that gene. It they are different the individual is 'heterozygous'.

Dominant and Recessive

HH = Homozygous dominant

hh = Homozygous recessive

Hh = Heterozygous

Genotype and phenotype

When you look at someone or at a plant, you can only consider what they look like. You can't work out which alleles they have for a particular gene. You are considering their phenotype. This is the outward effects of the genes - what you see.

Knowing their actual combination of alleles - for example, whether they are homozygous recessive - is to know their genotype. To know what genes they carry.


You would soon get confused about which plants or animals you are talking about. There are the parents, then their offspring, and their offspring, etc. etc.

So, to make it nice and easy we give each generation a name.

The first plants or animals bred together are called the Parental generation, or P1 generation.

Their offspring are called the First Filial generation, or F1 generation.

Their offspring are called the Second Filial generation, or F2 generation.

And so on. And so on.

You choose your best animals (or plants) and breed them together.

Then choose the best animals in their offspring (F1 generation).

Breed these ones again to give an F2 generation.

Carry this on over many generations until you have the 'perfect animal' - well, the one with the best characteristics or traits that you wanted.

You can easily imagine that one big reason for selective breeding is money.

You can save a lot of wasted money if you weed out weaker individuals. For example, you could selectively breed for disease resistance. You can also ensure that you get the maximum output and therefore are more efficient. More potatoes grown on each plant means more money.

As humans we don't consciously go in for selective breeding. We just follow our romantic feelings.

Usually this works out fine. However, there are occasions when people discover that one of their genes actually gives rise to an inherited disease.

Sufferers of this disease produce a thick, sticky mucus which coats their airways and lungs. If it is not cleared by daily massage and physiotherapy, and treated with antibiotics, the person can get serious chest infections.

The cause of the disease was discovered in 1989 as being a recessive allele. This allele is carried by about 1 in 20 of people.

Haemophilia is a famous blood disease. Its fame comes from the children of Queen Victoria and their offspring.

The symptoms are that blood fails to clot. The smallest wound or tooth extraction can prove fatal. A bump will not lead to a bruise but large, internal bleeding.

This inherited disease causes the red blood cells to change from their usual round shape to become pointed like a sickle.

This shape change means that they get stuck in blood vessels and cannot pick up oxygen properly from the lungs.

This is also known as Huntington's Disease. Chorea means dancing, that's where we get the word choreography.

The symptoms of Huntington's chorea are a series of uncontrolled, dance-like movements which do not appear until the sufferer is in their forties. There is also a severe mental damage which gets worse with increasing age.

Unlike the previous examples, Down's syndrome is caused by having an extra whole chromosome.

Therefore, Down's syndrome is a mutation in which an extra chromosome 21 is passed into the same egg cell during meiosis. (The other egg cell created during the same meiotic division will have no chromosome 21 at all).

If the egg cell with two chromosomes 21 becomes fertilised, the zygote will end up with three chromosomes 21. It will have a total of 47 chromosomes instead of the usual 46. This causes Down's syndrome.

There is both natural and artificial cloning. Both produce clones, plants that are genetically identical to the parent plants.

Natural Cloning

The cloning process occurs through cell division mechanism of mitosis. It therefore allows them to undergo this form of asexual reproduction.

However, these plants can also reproduce using sexual reproduction (that is releasing gametes). This is important as it allows for genes to be shared between different individuals and then on to their offspring. This avoids the loss of genetic variation, which is the main problem of cloning.

Artificial Cloning

A small piece of branch or stem is cut from a larger plant and is perhaps dipped into an auxin rooting powder. In a few weeks a new plant develops.

Little do these humble gardeners realise that they are carrying out a form of micropropagation. This is a high-tech version of the traditional cutting approach.

In micropropagation, cuttings are taken from a stem and cut into smaller sections. Each section is sterilised first before adding them to a growth medium containing rooting hormones. After each develops roots it grows into a plantlet. Finally, they are hardened up by being grown in a greenhouse.

Tissue culture is another new technique that has been used for cloning plants. Here, only a few plant cells are needed. These are then added to the growth medium and hormones. They will develop into a new plant.

While cloning does occur naturally within animals, it is less common. Cloning is usually restricted to cells dividing by mitosis, and cells splitting as is the case of identical twins.

This technique has already been used to produce large quantities of human insulin using bacteria. It has been a great help to sufferers of diabetes.

In genetic engineering the gene that you want is cut out of a human chromosome using special enzymes.

The gene is then fitted ('spliced') into a length of DNA from a bacterial cell and then reintroduced back into the bacterial cell.

The bacteria is tricked into carrying out the instructions on the human gene and producing the protein, insulin.

Once the bacteria has been cultivated so that it multiplies many times, enough insulin is produced so that it can be filtered off and collected.

This whole process is carried out on an industrial scale so that masses of insulin is produced in a continuous process.