Why are traits acquired during the lifetime of an individual not inherited?
Based on inheritance, we divide all our body cells into two types: Somatic cells and germ cells. Somatic cells comprise most of our body cells which do not take part in the transfer of genes to offsprings. Germ cells develop only in the reproductive organ of the individuals. During reproduction, these germ cells carry genetic information to the next generation. If the mutation happens in the somatic cells, it does not transfer genetic material to the progeny. While if the mutation occurs in germ cell and the particular germ cell makes a new individual, then only the inheritance of new trait pass to the next generation.
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See also: What are the different ways in which individuals with a particular trait may increase in a population?