Genetics is the scientific study of heredity, or how particular qualities, traits, or genes are transmitted from parents to offspring.
These are the basic units of inheritance. Genes are made of a chemical called deoxyribonucleic acid, commonly referred to as DNA. Each gene contains instructions for making a specific protein, and each protein has a particular function in the body. As such, genes guide the development of the body e.g. determine eye and hair colour and instruct the body to make enzymes for biochemical processes.
Genes are packaged into structures called chromosomes that are found in the nucleus of every human cell. Chromosomes come in pairs, and a normal human cell contains 46 chromosomes consisting of 23 pairs. Every individual receives half their chromosomes from their mother and half from their father.
A mutation refers to a structural change in a gene. It can be an alteration to a gene’s size, arrangement, or molecular sequence. Genetic disorders can arise when one or both copies of a specific gene have undergone a mutation.
When an alteration in just one copy of a gene pair causes a genetic disorder, the disorder is referred to as dominant. An individual who possesses a gene for a dominant disorder usually manifests the disorder and is affected by it.
Since each parent passes only one copy of every gene to a child, a child conceived by a couple where one individual carries a mutation has a 50% chance of inheriting the abnormal copy of the gene and a 50% chance of inheriting the normal copy of the gene. The same risk applies to each conception, regardless of the outcome of previous conceptions.
Sometimes a dominant disorder appears in an individual whose parents do not manifest the disorder. This situation occurs when either:
a) A new mutation in a dominant gene causes a disorder to appear for the first time in a family.
b) The disorder has reduced penetrance, meaning that only a percentage of individuals who have the mutated gene will develop the disorder, e.g. DYT1 Generalised Dystonia.
Dominant disorders can occur in multiple generations in a family and there can be multiple affected family members in one generation. Except when there has been a new mutation, every affected individual has a parent who carries the mutation.
When alterations in both copies of a gene pair are required for a disorder to manifest, the disorder is referred to as recessive. With recessive inheritance, a single normal copy of a gene appears to compensate for the mutated one to prevent the disorder developing.
An individual who has one mutated copy of a recessive gene and one non-mutated copy is referred to as a carrier. Carriers are unaffected by the disorder but can pass the mutated gene on to their children. When both partners in a couple are carriers, their children will have a 25% chance of inheriting the disorder, a 50% chance of becoming carriers and a 25% chance of inheriting both normal copies of the gene. The same risk applies to each conception, regardless of the outcome of previous conceptions.
There is typically no “family history” with autosomal recessive disorders, since these conditions require both parents to be carriers. Tay-Sachs disease is an example of a recessive disorder.