• Genes carry our genetic code.
• This code is stored on DNA, which is organised into chromosomes and kept in every cell of our body.
• Our genetic code is passed down to us from our biological parents, and it makes us who we are.

• Genes tell our bodies to make proteins.
• Proteins are found in every one of our cells and play critical roles in our bodies.
• The sequence or code of each gene tells our body how to make a particular protein.
• Proteins are made up of long strings of building blocks called amino acids. There are 20 different amino acids that can be used to make different proteins.  
• Amino acids are joined together one-by-one to form proteins according to the instructions in each person’s genetic code.  

• We all have variations in our genes.
• For example, genetic variation is the reason some people have blue eyes while others have brown or green eyes.
• Variations may be passed down from our biological parents, or they may be new changes (variants) that occur in us by chance. Another name for a change in the genetic code is ‘mutation’.  
• Sometimes a variant in our genetic code changes the way a protein is made. If this protein is essential for our body, a health condition may develop. 

Some of the genetic variants that can cause health conditions are:

Pathogenic variants 

• A pathogenic variant is a change in a gene that either causes a health condition or makes it more likely that a health condition will develop. The finding of a pathogenic variant in a genetic or genomic test may also be called an ‘informative result’.  
• There are several types of pathogenic variants. 

Nonsense variants  

• A nonsense variant is a change in the genetic code that adds an early ‘stop’ signal to the gene. This tells the cell to stop adding amino acids to the protein chain earlier than usual.  
• The result is a shortened (truncated) protein instead of the usual full-size protein. This shorter protein may be destroyed by the body or it may not work.  

Missense variants  

• When there is a missense variant in a gene, one amino acid building block is replaced with another in the protein that is produced from the gene.
• Although it is only a small change in the protein, the change may have a big impact on the way the protein works. 

‘Gain of function’ and ‘loss of function’ variants 

• As described above, when there is a change in a gene, it can affect the way the product of the gene – the protein – works.
• When the protein is more active than usual, the genetic change is called a gain of function variant.
• When the protein is less active than usual, or does not work at all, the change is called a loss of function variant.
• Both of these types of variants can cause health conditions. 

Mosaicism

• Mosaicism is a special type of genetic variation.
• Usually, all the cells in a person's body (except the eggs or sperm) have the same genetic makeup.
• People with mosaicism have a mixture of cells with different genetic information.
• This means some cells in the body may have a variant that causes a health condition, while the rest do not.
• Mosaicism may be seen across the whole body or only in a certain part of the body.
• When mosaicism only occurs in the eggs or sperm cells, it is called gonadal or germline mosaicism.