Lecture 8 st
October 1 2013
→ Genetic Basis
What is the basic structural and functional unit of our body?
Asingle cell, a zygote, is formed by the union of a sperm and an egg.
This single cell, as every other human cell, carries 46 chromosomes in its
Chromosomes operate in 23 pairs.
One of each pair is derived from the mother and one from the father.
Each chromosome carries a DNAmolecule.
DNAcontains instructions, known as genes.
Genes carry the chemical code for how the body will grow, develop, function and maintain itself.
The full set of genetic instructions for a living thing, controlling its development from a single cell into a complex
adult body is known as Genome.
→ Discoveries in the History on the Genetic Basis of Human Body
Heredity was a well-known fact before Darwin.
Biologists studied the earth’s fossils and noted that various species have undergone gradual changes over the
course of many generations .
Darwin explained how and why those changes occurred. Two points in Darwin’s theory is very important.
-Adaptation: Changes in characteristics occur over time to meet the changing demands of their
environment. These changes pass on to next generations.
-Just like physical traits, species’ typical patterns of behaviour reflect adaptation and passed to
Following Darwin’s ideas, other scientists continued to study the mechanisms responsible for heredity.
Mendel was working with plants. He was breeding peas with only purple flowers or white
flowers and observing which colour flowers would be produced.
He noticed that there is a pattern in the colour of flower. While 75 percent had purple
flowers, 25 percent had white flowers.
Mendel deduced that there are discrete units, known as genes, and they exist in
two versions (white and purple). Like chromosomes, genes work in pair, and one
of them is dominant and the other is recessive.
Dominant genes are expressed whenever they are present and recessive genes
are expressed only when they are matched with another recessive gene.
The existence of dominant and recessive genes means that not all genes are
The genotype is the genetic make up of an organism, determined at the conception.
Phenotype is the observable physical characteristics of an organism that result from both genetic and environmental influences.