The human genome

Except for identical twins, each person's DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, which can form a 'bar code' that is different from one person to the next.

The genome is the entire genetic material of an organism. The human genome has great importance for medicine. We think there are about 20,000 genes in the human genome. However, they only make up about 1.5 per cent of the genome. The remaining 98.5 per cent was once called 'junk DNA', but scientists now think it is important in controlling gene expression. This means it controls when the genes are used to make proteins.

It is vital that the human genome is fully understood.

It enables us to:

  • search for genes linked to different types of disease
  • understand inherited disorders and their treatment
  • trace human migration patterns from the past

Scientists are searching for disease-associated genes. One example is a disease called phenylketonuria (PKU). It is caused by a change in the PAH gene. The PAH gene contains the information required to make the enzyme which breaks down phenylalanine in the liver.

Phenylalanine is found in food and, when the enzyme does not work properly, high levels of phenylalanine can harm the development of the nervous system in young children.

A person has the PKU phenotype (which means they have the disease) if they have two harmful PAH alleles. If someone has one harmful PAH allele and one normal allele, then they are said to be a carrier.

Scientists are able to create a pedigree chart, which is similar to a family tree diagram. The pedigree chart shows the inheritance pattern of the disease. This enables scientists to identify differences in the DNA of the affected and unaffected individuals. It is now possible to detect the presence of the genes by having a simple blood test.

Diagram of a pedigree chart showing ows the inheritance pattern of a disease

Genotype and phenotype

Altogether there are known to be about 500 versions of the PAH gene. These are called genetic variants. Most contain code for an enzyme that works, but some do not. Each person has two copies of every gene - these are called alleles.

The genetic variants that an organism has makes up its genotype.

The genotype for a single gene in an organism is created by the combination of alleles it has.

The phenotype of an organism describes its features. It is the result of the information in its DNA (genotype + gene expression) and any interaction with the environment.

genome + environment  \rightarrow phenotype