Diagnostic testing involves testing for a suspected genetic condition. Examples include testing for:
- CFTR gene mutations for cystic fibrosis
- Trisomy 21 for Down syndrome
Predictive testing involves testing a person for a specific gene mutation that may have implications for them in the future. For example, the BRCA1 gene mutation increases the risk of developing breast cancer. Another example is testing presymptomatic patients for the CAG repeat expansion in the HTT gene, which leads to Huntington’s disease.
Carrier testing involves testing asymptomatic individuals for the genetic mutation associated with specific autosomal recessive and X-linked disorders to estimate the risk of passing it to their children. An example is testing for the cystic fibrosis gene mutation when other family members are affected.
Somatic testing involves analysing tumour cells to identify mutations. This usually involves next-generation sequencing (NGS) panels that test many genes. An example of tumour testing is identifying the HER2 gene amplification on breast cancer cells, which indicates a role for targeted therapies specific to this gene, such as trastuzumab.
Other scenarios where genetic testing may be used include:
- Prenatal testing (testing for genetic conditions in the foetus during pregnancy)
- Ancestry testing (for ancestry and family origin)
- Forensic testing (to identify individuals for legal or criminal investigations)
- Paternity testing (to identify a child’s father)
Ethical Issues
There may be ethical implications to genetic testing. Depending on the specific test and implications, formal consent and genetic counselling may be required.
For example, if a patient’s parent had Huntington’s disease, there is a 50% chance that the patient has a copy of the abnormal gene and will develop the highly disabling condition. The person must be fully informed about the implications of the results before being tested, not only for them but also for their family. Children are never offered testing for an untreatable adult-onset disease such as Huntington’s. The decision about whether to test for the condition would be delayed until adulthood, when they can make a fully informed decision.
Karyotyping
Karyotyping involves examining the number of chromosomes, their size, and their basic structure. It helps diagnose conditions such as Down syndrome (trisomy 21) and Turner syndrome (45 X). It has a low resolution and will not detect more subtle abnormalities.
Chromosomal Microarray Analysis
Microarray testing is used to detect copy number variants (CNVs), which are small chromosomal deletions and duplications that might not be visible on standard karyotyping. The patient’s DNA is compared to a reference to detect extra or missing genetic material. It can identify microdeletion or microduplication syndromes.
Examples of where microarray testing might be used include in children with:
- Developmental delay
- Autism spectrum disorder
- Congenital abnormalities
Specific Gene Testing
Specific gene testing helps detect a known mutation in a single gene when a particular disorder is suspected.
Examples of where it might be used are testing for:
- CFTR gene mutations for cystic fibrosis
- FBN1 gene mutations for Marfan syndrome
- HFE gene mutations for haemochromatosis
- HTT gene mutations for Huntington’s disease
DNA Sequencing
DNA sequencing establishes the exact nucleotide sequence to identify abnormal variants. It can be used on any length of genetic material, from a single gene to the whole genome.
Next-generation sequencing (NGS) involves analysing the DNA of many genes. There are three main types:
- Targeted gene panels (testing a selected group of genes)
- Whole-exome sequencing (the exome is the 1-2% of the genome that contains all the protein-coding exons)
- Whole-genome sequencing (testing the entire genome)
DNA sequencing can produce large amounts of data, which require careful interpretation and clinical correlation. It may detect variants of uncertain significance, where there are abnormal variants, but it is unclear what effect these have on the patient.
Last updated November 2025
Now, head over to members.zerotofinals.com and test your knowledge of this content. Testing yourself helps identify what you missed and strengthens your understanding and retention.
