Genes and Genetic Conditions
The following information pages were developed in partnership with the Garvan Institute of Medical Research, NSW Health Pathology, and Wolper Jewish Hospital
This video introduces some key ideas that are used when discussing genetic conditions and carrier screening.
Genetic information is encoded in long sequences of bases (known by the letters A, C, T, and G) that join together to make long strands of DNA. Inside your cells, your DNA is tightly wrapped into chromosomes - 46 in all (23 pairs).
Your entire set of genetic information is called your genome. An identical (or near-identical) copy of your genome is found within almost all of your trillions of cells. Genes are sections of DNA within the genome. They contain information coding for molecules- usually proteins- that carry out the functions of cells. Two copies of each chromosome means we have two copies of almost every gene.
Differences in genes (variants) make us unique and can also influence our health. Most are harmless, but some can stop genes from working as they should. We call these gene faults. Gene faults can affect the way the cells work and can cause genetic conditions. Gene faults can be passed from parents to children.
Recessive genetic conditions
Many genetic conditions are caused by “recessive gene faults”. Most of the conditions tested for in genetic screening are inherited in this way. These conditions occur when children receive two faulty copies of the same gene- one from each of their parents. The parents are unaffected because they each have one faulty copy and one working copy.
A smaller number of conditions are inherited along different patterns...
X linked recessive genetic conditions
The X and Y chromosomes are different to the other pairs of chromosomes- children inherit either an X and a Y (chromosomally male) or two Xs (chromosomally female). There are some genes that are found only on the X chromosome. Recessive faults in these genes are more likely to cause genetic conditions in males who receive only one copy of the X chromosome, however in some cases, females can also be affected mildly.
One of the conditions included in carrier screening tests is called Fragile X. This condition, which affects cognitive development, has a complex X-linked inheritance pattern. In some rare cases, the results of screening for this condition may have implications for the health of the person being screened. You can read more about Fragile X here or contact us if you have questions.
Dominant genetic conditions
Some genetic conditions can be caused or influenced by a single copy of a gene fault. These are called “dominant” conditions. If someone has a dominant gene fault, there is a 50% chance they will pass it on to their offspring. One example of dominant inheritance is BRCA-associated hereditary breast cancer. Carrier screening does not usually test for dominant conditions. In some rare cases, gene faults that are mainly recessive can also have some mild dominant effects. This means that a carrier result can sometimes have minor implications for your own health.