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Gene&Gene Mutation   
A gene is the basic physical and functional unit of heredity. Genes, which are
made up of DNA, act as instructions to make molecules called proteins. In humans,
genes vary in size from a few hundred DNA bases to more than 2 million bases. The
Human Genome Project has estimated that humans have between20,000 and 25,000
genes.
Every person has two copies of each gene, one inherited from each parent
. Most
genes are the same in all people, but a small number of genes (less than 1 percent
of the total) are slightly different between people. Alleles are forms of the same
gene with small differences in their sequence of DNA bases. These small
differences contribute to each person unique physical features.
 
Gene Mutation
 
A gene mutation is a permanent change in the DNA sequence that makes up a gene.Mutations range in size from a single DNA building block (DNA base) to a largesegment of a chromosome.
  
Gene mutations occur in two ways: they can be inherited from a parent or acquired during a person lifetime. Mutations that are passed from parent to child are called hereditary mutations or germline mutations (because they are present in the 
egg and sperm cells, which are also called germ cells). This type of mutation is present throughout a person life in virtually every cell in the body.
 
Mutations that occur only in an egg or sperm cell, or those that occur just after fertilization, are called new (de novo) mutations. De novo mutations may explain genetic disorders in which an affected child has a mutation in every cell, but has no family history of the disorder.
 
Acquired (or somatic) mutations occur in the DNA of individual cells at some time during a person life. These changes can be caused by environmental factors such as ultraviolet radiation from the sun, or can occur if a mistake is made as DNA 
copies itself during cell division. Acquired mutations in somatic cells (cells other than sperm and egg cells) cannot be passed on to the next generation.Mutations may also occur in a single cell within an early embryo. As all the cells divide during growth and development, the individual will have some cells with the mutation and some cells without the genetic change. This situation is called mosaicism.
 
Some genetic changes are very rare; others are common in the population. Genetic changes that occur in more than 1 percent of the population are called polymorphisms. They are common enough to be considered a normal variation in the 
DNA. Polymorphisms are responsible for many of the normal differences between people such as eye color, hair color, and blood type. Although many polymorphisms have no negative effects on a person health, some of these variations may 
influence the risk of developing certain disorders.
 
 
source=http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/genemutation

 


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