If some of the chemical bases in the sequence in the gene is changed, a gene mutation (基因突變) occurs. In this case, the normal genetic sequence in the gene is disturbed, causing the gene either not to function or function incorrectly, and hence the gene becomes faulty. Gene mutation may occur in several ways:
A. Spelling change - some chemical bases ('letters') in the genetic sequence are changed.
B. Insertion - extra 'letters' are inserted in the genetic sequence.
C. Deletion - some 'letters' are deleted from the genetic sequence.
D. Repetition - some 'letters' are repeated in the genetic sequence.
|
| Correct sequence | Faulty sequence |
| A. Spelling change | TAG GCC CAG CAG TCA CTG | TAG GCC CAG CAG TAA CTG |
| B. Insertion | TAG GCC CAG CAG TCA CTG | TAG GCC CAG CAG TCC TCA CTG |
| C. Deletion | TAG GCC CAG CAG TCA CTG | TAG GCC CAG CAG CTG |
| D. Repetition | TAG GCC CAG CAG TCA CTG | TAG GCC CAG CAG CAG CAG TCA CTG |
1. the faulty genes in the chromosomes are inherited from parents
2. the genes become faulty during the formation of the egg or the sperm
3. the genes become faulty during the lifetime of the individual (e.g. due aging or exposure to radiation etc.)
As genes are in pairs, sometimes a faulty gene in a chromosome may not cause any problem as there is still another working copy of the gene in the other pair of the chromosome. In fact, all normal human usually has several faulty genes in the body at any times, and the body still functions normally. Only that if both copies of the same gene in both pair of chromosomes are faulty, then genetic problem occurs. Sometimes, the fault in a gene is so severe that even there is a working copy of the gene in the other pair of the chromosome, it will still cause genetic problem. Such kind of faulty gene is said to be dominant.
Some examples of common genetic problems due to faulty genes are listed as follows:
| Disease | Gene | Symptom |
| Cystic fibrosis (囊腫性纖維化) | CFTA in chromosome 7 | Salty tasting skin, frequent chest infection and coughing or shortage of breath, poor growth and weight gain despite normal food intake |
| Haemophilia (血友病) | Factor 8 in X chromosome | Continued bleeding due difficulty to clot. |
| Thalassemia (地中海貧血症) | HBA1/2 in chromosome 16 | Ranges from no problem to severe depending on number of genes affected, anaemia (貧血) when severe. |
| Breast cancer (乳癌) | BRCA1 in chromosome 17 BRCA2 in chromosome 13 | Breast lump, change in breast size and shape, inverted nipple |
| Alzheimer (老人癡呆症) | PS1 in chromosome 14 PS2 in chromosome 1 | Difficulty with memory, learning, thinking and reasoning, and undertaking everyday tasks, usually associated with aging. |
| Parkinson (帕金森病) | PARKIN in chromosome 6 | Tremors, stiffness of muscles and slow movements |
The above gives some simplistic views on how changes in genes affect the health and development of individuals, in as much as changes in chromosomes do. It is recommended that affected individuals should seek professional advice from genetic counseling on details of medical issues, and implications on any possible inheritance issues when considering births of offspring .............................
