Introduction to Genetics

Genetics is the scientific study of heredity, or how particular qualities, traits, or genes are transmitted from parents to offspring.

Basic concepts in genetics:

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.

Understanding autosomal dominant inheritance:

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.


Autosomal Dominant Inheritance
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.

Understanding autosomal recessive inheritance:

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.
Autosomal Recessive Inheritance
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.

X-linked inheritance:

In X-linked disorders, there is a mutated gene on the X chromosome. Unlike autosomal inheritance, the possibilities of having affected children, depend on which parent transmits the altered gene.

Males receive a Y chromosome from their father and an X chromosome from their mother, whereas females receive an X chromosome from both parents. Men are affected by X-linked disorders because they have only one copy of the X chromosome and have no “backup” copy to make the necessary protein product. Women who have one mutated copy of the gene tend to be unaffected because their one functional copy compensates for their non-functional one.

The only established example of an X-linked Jewish genetic disorder is glucose-6-phosphate dehydrogenase (G6PD) deficiency which has an increased incidence in some people of Sephardi Jewish ancestry.