You and Your Genes - Testing and Treating Genetic Diseases

Different versions of the same gene are called alleles. They can be either recessive or dominant. Genetic testing can determine whether a person is carrying the alleles that cause genetic disorders. But there are limits to the testing, and the subject raises a number of ethical issues.

Alleles

The chromosomes in a pair carry the same genes in the same places. But there are different versions of the same gene. Different versions of the same gene are called alleles. For example, the gene for eye colour has an allele for blue eye colour and an allelle for brown. For any gene, a person may have two of the same alleles or two different.

Recessive and Dominant Alleles

Alleles may either be recessive or dominant. Recessive: a recessive allele only shows if the individual has two copies of it. For example, the allele for blue eyes is recessive. You need two copies of the allele to have blue eyes. Dominant: a dominant allele always shows, even if the indiviudal only has one copy of it. For example, the allele for brown eyes is dominant. You only need one copy of it to have brown eyes. Two copies will still give you brown eyes. A person who has brown eyes but also holds a blue eye allele is called a carrier because even though they have brown eyes, they still carry the allele for blue eyes and can pass that allele on to future generations.

Genetic Terminology - Higher Tier

When describing an organism it is important to distinguish between the genotype and phenotype. Genotype: describes the genetic make up of an organism (the combination of alleles). Phenotype: describes the observable, physical characteristics that an organism has. This is often related to a particular gene.

Cystic Fibrosis

Cystic Fibrosis (CF) is caused by a recessive allele. People with CF produce abnormally thick and sticky mucus in their lungs and airways. As a result, they are more likely to get respiratory infections. Daily physiotherapy helps to relieve congestion, while antibiotics can fight infection. The disease blocks tubes that take enzymes to the gut meaning food is not digested properly, leaving the person short of essential nutrients. 

Inheriting copies of the CF Allele

You need to inherit two copies of the faulty allele to be born with CF. If you have just one copy, you are a carrier, but will not experience any symptoms. If two carriers have a child together, there is one in four chance of passing on the disorder.

Huntington's Disorder

Huntington's disorder is caused by a dominant allele, written as H. The symptoms usually develop in middle age, and include tremors, clumsiness, mood changes, memory loss and the inability to concentrate.

Inheriting copies of the Huntington's Allele

You only need to inherit one copy of the faulty…