Scales of Mutations

According to the scale of change, mutations can be classificed into two levels.

(1) Chromosomal mutation,

If parts, or the whole chromosome is changed, it is called the chromosomal mutation. The change is in large scale, and many genes are affected. Usually, this is due to the error in the regulation of meiosis. It is much possible to occur in elder women. If the ovum carrying error undergoes fertilization, the genetic disease would be resulted. But the chance for these genetic diseases to be transmitted to further generations would be small.

The general chromosomal mutations are as follows :

The change		Explanation
***	The change in number of ploidys	It is due to the failure in separation of homologous chromosomes in meiosis.
(1)	Polyploidy ( Normally, the tetraploid).	It normally occurs in plants. If the plant is treated with cochicine, cochicine would inhibit the production of spindle apparatus in meiosis. Then, the zygote becomes a tetraploid. Owning to the fact that each gene in the cells of a tetraploid possesses 4 alleles, there is a higher change to have a well-adapted gene to dominate the life of the individual. Therefore, tetraploids are always stronger and easier to adapted to environment and survive., as the seedness water melon. In seedness water melon, the ovary is stimulated to grow after fertilization, but the zygotes fails to develop. Therefore, there is no seed.
***	A specific pair of chromosome has one more member. This is called	trisomy.
(1)	The 21^st pair somatic chromosomes has one more member.	It is called the Down's syndrome (or Mongolism) ( 香港唐氏綜合症協會 ) ; ( 最新資訊 ) ;( 唐氏綜合症資訊網 )
(2)	The male has one more X chromosome.	It is called the Klinefelter's syndrome. This disease will cause mentally retarded and sterility ( Reference : )
***	A individual has one chromosome less.	It is called the monosomy.
(1)	To have one S chromosome less,	It is called the Turner's syndrome 。
***	To change part of a chromosome,	the total number of chromosome does not changed.
(1)	Deletion	A segment of the chromosome is missing.
(2)	Inversion	A segment of the chromosome is inverted.
(3)	Translocation	A segment of the chromosome is moved and added to another chromosome.
(4)	Duplication	A segment of the chromosome is duplicated. This offenly occur and form the junk DNA. 98% of the DNA belongs to the junk DNA. The use of junk DNA is unknown.

(2) Genic mutation,

If the change occurs at genes level, it is called the genic mutation.

If a nitrogen base in a gene is changed, it would result in the change of one amino acid in the product polypeptide. If the primary conformation of the polypeptide is changed, the conformation at other levels would be changed also. As a result, the property of the polypeptide would be changed and the function and feature of the polypeptide would be changed also. This can bring to the change in genetic trait and thus, is a mutation and called the genic mutation.

(23.05.2007)