Genetics

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All human cells have 46chromosomes, 23 pairs.Different living things will have a different

amount.

The chromosomes are held in the nucleus of our cells.

• The chromosomes are made up of long coiled molecules of DNA.

• This chemical is divided into regions called genes.

• The DNA molecule contains four different molecules called bases. There are four different bases.

• Each gene is made up of a pattern of these four bases.

• The four bases are: cytosine, adenine, thymine, guanine (A, T, G and C)

• A and T always pair.

• C and G always pair.

FERTILISATION

• Gametes are the sex cells – the sperm and the egg.

• These cells only have 23 chromosomes.

• Because each egg and sperm have a random combination of genes this causes every offspring to have a different mixture of genes. This causes VARIATION

Gene control of characteristics• All characteristics are controlled by a

minimum of 2 genes. (one from mum one from dad)

• Different genes that control the same feature are called ALLELES

• Alleles can be dominant or recessive.• Individuals can be heterozygous or

homozygous.

THE ALLELES FOR SEX

• Gender (in mammals) is determined by sex chromosomes: XX (female) and XY (male).

• cross

THE ALLELES FOR EYE COLOUR

• B is the gene for brown eyes

• b is the gene for blue eyes

• Alleles:

• BB

• Bb

• bb

Dominant and recessive

• Some alleles for a feature are able to mask the influence of the other (weaker) gene when they are paired

• Stronger gene is DOMINANT and is represented by a capital letter e.g. H

• The masked gene is RECESSIVE shown as lower case of the same letter e.g. h

Inheritance

GENETIC DISEASES

Inherited diseases

• Cystic fibrosis• Sufferers of this disease produce a

thick, sticky mucus which coats their airways and lungs. If it is not cleared daily the person can get serious chest infections.

• The disease is caused by a recessive allele.

• If a person is heterozygous (Cc) then they are a carrier but have a normal phenotype, they don’t develop cystic fibrosis but can pass it on to their children.

• What happens if a carrier (Cc) and a ‘normal’ person (CC) want to have children?

• What are the chances of the child having the disease, being a carrier or not a carrier?

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But what happens if another two carriers wanted to have children?

• What happens if a carrier (Cc) and another carrier (Cc) want to have children?

• What are the chances of the child having the disease, being a carrier or not a carrier?

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Questions

T tall allele t dwarf allele

Predict the ratios of offspring from the following crosses.

1. TTxtt

2. TTxTt

3. TTxtt

4. TtxTt

5. Ttxtt

6. ttxtt

Sickle cell anaemia

• 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.

• The allele responsible for it is a recessive. What happens if two carriers were to have children?

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Where are the chromosomes found?

What are the sections of DNA?

What do genes code for?

How many pairs of chromosomes are found in a human nucleus?

Explain how sex is inherited.

• Genetic variation is caused by:• Mutation• Gametes• Fertilisation

• Mutations are usually harmful, but may be beneficial. Mutations can be caused by radiation or chemicals.

• Gene mutations occur because the DNA base sequence changes – altering or preventing the gene that is normally coded for.

Some mutations are useful, some are harmful.

• Cystic fibrosis

• Downs syndrome

• But what causes intelligence, sporting ability or good health.

• Nature or nurture?

Which are inherited, which are environment and which are both?

• Body mass• Red-green colour

blindness• Cystic fibrosis.• Nose shape

• Sickle cell anaemia

• Earlobe shape• Intelligence• Height • Eye colour