DNA (deoxyribonucleic acid) is the molecule of heredity found in our cells. It provides the information (genetic code) that makes us who we are. The code (referred to as A, C, G and T) in humans is approximately 3 billion letters long and is sometimes called the human genome.

Knowing the genetic code allows scientists to look for changes in the DNA sequence that may result in an inherited condition (e.g. Cri du Chat Syndrome, Cornelia de Lange Syndrome or Angelman Syndrome) or predispose an individual to a particular illness (e.g. cancer, heart disease or diabetes).

A chromosome is a large structure made up of a length of DNA. Human cells have 46 chromosomes including two sex chromosomes which provide the information that determines the sex of the individual (two X chromosomes in a female; one X and one Y in a male). Chromosomes are numbered from 1 to 22 based on size. Individuals have two copies of each chromosome pair; one inherited from our mother, and the other from our father.

A gene is a segment of DNA that usually contains instructions for making a specific product. The product of a gene is determined by the specific order of the letters in the DNA code.

As we inherit two copies of each chromosome (one from our mother and the other from our father) we have two copies of virtually all genes. The two versions of each gene may be the same, or slightly different.

An individual’s entire genome is divided up into chromosomes which are located within our cells. Each chromosome is one long piece of DNA. Along the molecule are regions of DNA referred to as genes. There are several thousand genes on each chromosome and approximately 25,000 genes make up the human genome. Each gene is made up of hundreds or thousands of nucleotide bases (A, C, G and T).

A fertilised egg cell has to divide many times to make a human being. This process requires the DNA present to be shared equally to produce two identical cells and so each cell copies it’s DNA before dividing.

Changes to the genetic code can occur when the DNA is copied. Some changes have no effect; some will alter an individual’s characteristics or cause an illness. A change can be as simple as a single letter change in the genetic code of a gene or large and complex involving many genes on a chromosome.

Large scale changes can occur if chromosomes break and are reconstituted in nonstandard ways (e.g. loss of genetic information known as a deletion) or the chromosome pairs are not divided up evenly between the cells. Both types of changes can be the underlying cause of a genetic condition.

Click the links below to read more about genetics in each specific syndrome:

Angelman

Cri du Chat

Cornelia de Lange