Genes and human disease
The human body is made up of millions of cells each specialising in a particular function like the sensing light and smell or even the absorption of oxygen into the blood. Chromosomes are sub-cellular structures that exist in the nucleus of each cell that makes up the human body. There are 23 pairs of chromosomes existing in the human cell. Quite simply chromosomes are responsible for transferring genetic information from one generation to another.
Humans have two types of chromosomes- sex chromosomes and autosomes. There are two sex chromosomes, the X chromosome and the Y chromosome. They determine the sex of the individual. A male child inherits a Y from his father and X chromosome from the mother whereas a female child inherits X chromosomes from both her parents.
Chromosomes are made up of tightly packed lengths of DeoxyriboNucleic Acid commonly referred to as DNA, which in turn contain information needed to control the production of proteins. DNA controls which proteins will be produced in what quantity and when. Proteins are molecules that play an important role in determining the structure and function of the body’s cells tissues and organs. Proteins are made up of amino acids.
Genes are specific lengths of DNA that determine the order of amino acids used to make protein. Some proteins are needed in the functioning of all cells, others are not. In the second case, the gene coding for the needed protein is switched off allowing different structures to exist at the same time.
Alternative forms of the same gene are called alleles. Each person can inherit only one alternate form of a gene. The combined effect of alleles result in the physical differences in appearance among people like the colour of their eyes and hair, the shape of their nose etc.
Dysfunctional gene behaviour is commonly termed as a mutation. These mutations are responsible for causing illnesses. Moreover, if the gene mutations exist in the egg or sperm cell, children can inherit the defective gene from their parents. Diseases can occur due to a defect in a single gene or in a set of genes. According to the degree of gene mutation, diseases are categorised into the following:
- Chromosomal diseases: occur when the entire chromosome, or large segments of a chromosome, is missing, duplicated or otherwise altered. Down Syndrome is a prominent example of a chromosomal abnormality.
- Single-gene disorders: occur when an alteration occurs in a gene causing one gene to stop working. An example of a single gene disorder is sickle-cell anaemia.
- Multifactorial disorders: occur as the result of mutations in multiple genes, frequently coupled with environmental causes. An example of a multifactorial disorder is diabetes.
- Mitochondrial disorders: are rare disorders caused by mutations in non-chromosomal DNA located within the mitochondria. (The mitochondria are subcellular organelles.) These disorders can be found to affect any part of the body including the brain and the muscles.
Genes are also known to play a role in the occurrence of infectious diseases like tuberculosis and AIDS as well as some non communicable diseases like cancer and diabetes. This section includes a brief introduction to the role of genetics in some major diseases that burden the human population globally.