# 132 Mutation, Down syndrome, effect of radiation

Mutation is a unpredictable change in the genes or chromosome number, as a result of fault copying when DNA is replicated, faulty separation of chromosomes during cell division, or exposure to radiation or some chemicals.

Down’s syndrome is caused by a mutation. When ova are formed in the ovaries, the chromosome number is halve. During this division process (meiosis), one of the chromosome (number 23) sticks to its partner. This results in one ovum with 24 chromosomes and one with only 22, and the ovum with 24 chromosomes is still viable. If it is fertilized, the fetus formed will have 47 chromosomes instead of 46. 

3 chromosome 21 in Down syndrome.

The presence of the extra chromosome causes unusual characteristics in the baby. These usually include lowered life expectancy, mental retardation (although some Down’s children are very intelligent), early puberty, and a distinctive round face and short neck. 

Ạ child with Down syndrome. 

Effects of ionising radiation and chemicals on the rate of mutation

• Mutation are normally very rare. However, exposure to radiation and some chemicals, such as tar in tobacco smoke, increases the rate of mutation. 
• Exposure can cause uncontrolled cell division, leading to the formation of tumours (cancer).  

The development of cancer from mutated cells.

• Exposure of gonads (testes and ovaries) to radiation can lead to sterility or to damage to genes in sex cells that can be passed on to children.
• Some scientists argue that there is a higher incidence of leukaemia (a form a white blood cells cancer) in the children of workers at nuclear power stations. 

Video: DNA mutation, effect of radiation and chemicals on mutation