The theory of the genetic basis of heredity was quite unheard of
until the time of Gregor Mendel who proved with his experiments on pea
plants that organisms inherited their traits from their parents. It was
Mendel who first proposed that genes were the basic units of all living
organisms. Children receive half their chromosomes from their father and
half from their mother and hence they share the character traits of their
parents. Each human being has two complete sets of 23 chromosomes, which
are responsible for controlling all the bodily functions. Each parent in
effect contributes one complete set of the 23 chromosomes to the child.
Genetic information is stored inside the chromosomes in the forms of
DNA strands and there are around 40,000 genes, which are responsible for
instructing the ribosomes to synthesis specific proteins. (for specific
functions). A particular gene is responsible for a particular character
trait and parents pass on genes to their children, which is the fundamental
principle behind inheritance. Furthermore every gene may have two or more
different forms known as alleles. The genetic makeup of a person called the
genotype is actually the combination of these inherited alleles.
A person may either have homozygous alleles or heterozygous alleles.
Alleles for a particular trait may either be dominant or recessive. The
phenotype (physical appearance) of the person is determined by the
expression of the dominant alleles. There are some cases where neither of
the alleles are totally dominant in which case the phenotype is a
combination of the phenotypes of both the alleles. So we see that the
phenotype or the ultimate physical trait of a person is directly dependent
upon the genetic information received from the parents.
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