Genes are the instructions for the growth and development of our bodies. A genetic condition is caused by an altered gene or set of genes. The four broad groups of genetic conditions are single gene conditions, chromosome anomalies, mitochondrial conditions and multifactorial conditions.
Single gene conditions
Genes are paired – one copy of each gene pair is inherited from the mother and the other copy from the father. Around 6,000 known genetic conditions are caused by inheriting an altered gene.
Generally, the alteration means that the information contained in the particular gene is either changed or absent. The four main ways of inheriting an altered gene are:
- autosomal dominant – the alteration is present in every generation and may cause the condition in every person who has the alteration. This is because the altered copy of the gene is dominant over the healthy copy. Examples include Huntington’s disease and familial hypercholesterolaemia (genetically linked high cholesterol levels)
- autosomal recessive – the affected person has two copies of the altered gene (they have inherited an altered copy of the gene from both parents). They develop the condition because they do not have a functioning copy of the gene. Examples of autosomal recessive genetic conditions include cystic fibrosis, phenylketonuria (PKU) and sickle cell anaemia
- X-linked dominant – this type of condition generally occurs in females. The ‘X’ refers to one of the sex chromosomes, which decide sex. In most cases the mother provides an X, while the father provides either an X (female child, XX) or a Y (male child, XY). Women with an X-linked dominant condition have one altered copy and one normal copy of a gene that is on the X chromosome. An example of an X-linked dominant genetic disorder is a rare form of rickets known as hypophosphataemic or vitamin D resistant rickets
- X-linked recessive – this type of condition is more common in males. It is caused by an alteration in a gene on the X chromosome. As males usually have one X and one Y chromosome (XY), they do not have a second ‘healthy’ copy of the gene. Examples of X-linked recessive genetic conditions include Duchenne muscular dystrophy and haemophilia.
Genes are the body’s instructions for making different molecules (such as proteins or hormones). The estimated 23,000 genes that make up a human being are arranged along tightly bundled strands of a chemical substance called deoxyribonucleic acid, or DNA. The DNA strands are tightly packed into structures called chromosomes. These chromosomes are present in every cell of your body. Over 1,000 known conditions are caused by chromosome abnormalities.
A chromosome condition means there is a change in either the structure or the number of chromosomes. This can happen in three main ways:
- The altered chromosome is passed from the parent to the child.
- The anomaly happens when either the sperm or egg (germ cells) is created.
- The anomaly occurs soon after conception.
Chromosome anomalies can occur in various ways, including changes in the number or structure of chromosomes, or how they are inherited.
Changes in number of chromosomes
Most people have 23 pairs of chromosomes, or 46 chromosomes in all. When the egg or sperm is made, the pairs split so that each egg or germ cell only contains 23 chromosomes.
Occasionally an error occurs during the division. For example, the egg or sperm might be missing a chromosome (22 chromosomes) or have an extra one (24 chromosomes). This means that at conception the baby has either too few (45) or too many (47) chromosomes. An example of this type of genetic condition is Down syndrome, where a person has 47 chromosomes rather than 46.
Babies are rarely born with changes in chromosome numbers because most of these pregnancies end in miscarriage.
Changes in chromosome structure
Sometimes the information contained in a chromosome breaks up and the pieces reform in a different pattern. For example, a fragment of chromosome may break off and be lost during the formation of either the egg or sperm cell. A section of chromosome might also break away and ‘stick’ to another chromosome.
In other cases, a fragment of chromosome may copy itself or the ends of the chromosome may join to form a ring. Some changes in structure are ‘balanced’ (chromosome material is not lost or gained) and are unlikely to result in a genetic condition.
Uniparental disomy means the child inherited a particular gene pair (both copies of the gene) from one parent only. This can cause a condition if it is necessary for the child to have inherited one such gene from each parent.
Normally every cell in the body contains the same genetic information – all 46 chromosomes, designated as 46XX (female) or 46XY (male). A person who has chromosomal mosaicism has different numbers of chromosomes in different cells; for example, 46 in some cells and 47 in others.
Mitochondria are like little batteries that make energy within each cell. The energy source is a chemical called adenosine triphosphate (ATP). Organs like the brain, heart and liver can’t survive without ATP.
Genes within the mitochondria, as well as in the nucleus of the cell, instruct the cell on how to make the enzymes that are crucial to ATP production. If any of these genes are altered, this can affect enzyme production and interfere with the production of ATP. If one of the genes in the mitochondria is altered, then the condition is inherited only from the mother. This is because each person inherits their mitochondria only from their mother, and not from their father.
The symptoms of a mitochondrial condition, depending on the genes involved, can affect the:
- brain and spinal cord –which can cause intellectual disabilities, deafness, vision problems and seizures
- heart – which can cause cardiomyopathy (heart failure) and irregular heartbeat conditions
- musculoskeletal (locomotor) system – which can cause poor muscle tone and floppiness.
Multifactorial conditions (conditions involving several factors), such as many common birth conditions or diseases like high blood pressure, are conditions caused by the environment interacting with the action of several genes. (This is also sometimes called polygenic inheritance).
For example, the birth condition spina bifida is caused by the action of several genes and also depends on the amount of folate in the mother’s diet during pregnancy (the environment). High blood pressure is influenced by many genes, but also is influenced by a person's diet and salt intake.
If you, your child or another family member have been diagnosed with a chromosomal or genetic condition, or if you know that a condition runs in your family, it may be helpful to speak to a genetic counsellor.
Genetic counsellors are health professionals qualified in both counselling and genetics. As well as providing emotional support, they can help you to understand a condition and what causes it, how it is inherited (if it is), and what a diagnosis means for your or your child's health and development, and for your family.
Genetic counsellors are trained to provide information and support that is sensitive to your family circumstances, culture and beliefs.
If a genetic condition runs in your family, a genetic counsellor can explain what genetic testing options are available to you and other family members. You may choose to visit a genetic counsellor if you are planning a family – to find out your risk of passing the condition on to your child, or to arrange for prenatal tests.
Victorian Clinical Genetics Services (VCGS) provides genetic consultation, counselling, testing and diagnostic services for children, adults, families, and prospective parents.
The Genetic Support Network of Victoria (GSNV) is connected with a wide range of support groups throughout Victoria and Australia and can connect you with other individuals and families affected by the same condition.
Where to get help
This page has been produced in consultation with and approved by:
Victorian Clinical Genetics Services (VCGS)
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