The Secrets of Life (3) - Genes and Health and Development

There are 25,000 - 35,000 genes (exact number is not yet known) in a single human cell. A gene is a tiny segment of the chromosome that carries the genetic information for human growth and development. Despite tiny, it is made up of a very long sequence of chemical bases: Adenine (A), Guanine, (G), Thymine (T) and Cytosine (C). e.g. TAGGCCCAGTCACTG .......... These bases are grouped into 3-lettered words (e.g. TAG, GCC etc), each producing a particular type of amino acid for the cell. The sequence of the 3-lettered words is arranged specifically to enable the amino acids to assemble in the correct order for the cell to function. A gene is therefore a basic unit containing particular genetic instructions for performing a particular function for human growth and development (e.g. gene HBA1 - Haemoglobin Alpha 1 in chromosome 16 is responsible for producing a protein for the red blood cell). As chromosomes come in pairs, genes also come in pairs.
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

The cause of gene mutation can be as follows:
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 .............................