Heredity and Evolution

Chapter Summary


The passing of traits from parents to offspring is called heredity. It is the heredity which is responsible for many commonly observable facts, like siblings looking similar in overall appearance.

Mendel's Laws of Inheritance

Mendel proposed two laws of inheritance:

First Law: Law of segregation

Second Law: Law of independent assortment

Law of Segregation

Every individual has a pair of alleles for a particular trait. During gamete formation, a gamete receives only one of the contrasting traits from the allele.

A particular trait can be dominant or recessive in a particular generation.

Law of Independent Assortment

Alleles of different characters separate independent from each other during gamete formation.

Example: Seed colour will separate independent from seed texture.

Sex Determination in Humans

23rd chromosome in all eggs is X but that in a sperm is either X or Y.

When a sperm with X chromosome fertilizes the egg, the zygote develops into a girl child.

When a sperm with Y chromosome fertilizes the egg, the zygote develops into a male child.


The change in inherited traits in biological population over subsequent generations is called evolution.

All organisms we see today have evolved from a common ancestor.

Darwin's Theory of Evolution

Salient points of Origin of Species:

Organisms have unlimited capacity to reproduce

Intense competition for resources results in natural selection

Survival of the fittest

Molecular Origin of Life

Stanley L. Miller and Harold C. Urey, conducted the Miller-Urey experiment in 1953 to demonstrate how the life would have originated on the earth.

This experiment proved the hypothesis of J. B. S. Haldane that life originated from inorganic raw materials.