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It is the branch of biology which deals with the study of inheritance of characters from one generation to another.

Characteristics which are received by offspring from parents.

The process by which characters are transmitted from offspring.

Differences among the members of different species.

These are thread like structures found in nucleus. They are made up of DNA and protein. Genes are the units of heredity located on chromosomes.

Before cell division each chromosome doubled to form two sister chromatids which are attached with each other at centrosome.

Mendel selected pisum sativum (a garden pea) for his experiments for following reasons.

1. Normally it self pollinates when flower is still unopened.
2. It can be grown easily.
3. Has strongly contrasting phenotypes e.g., pea plant either tall or dwarf. The seeds are rounded and wrinkled.

It states that "The characteristics of a diploid organisms are controlled by alleles occurring in pairs. Of a pair such alleles only one can be carried in a single gamete."

In pea plant the allele for tallness is represented by T and allele for dwarfness by t. we shall assume that each parent plant contains identical alleles. TT in case of tall plant and tt in case of dwarf plants. The gametes produced by tall contain only one T allele. Those produced by dwarf plant will contain t allele.

Fertilization brings the T and t together so that all the F₁ offspring have genotype Tt. On self pollination one of the F₁ plant produce two types of allele T and t in two different gametes. They fuse randomly to produce offspring possessing all three genotypes TT, Tt, tt.

It states that "Each pair of contrasted character may be combined with either of another pair. In other words each of the two alleles of one gene may combine randomly with either alleles of another gene."

Mendel crossed a variety of pea having yellow round seeds with another variety having green wrinkled seeds. The result were yellow round. These when self pollinated four types of plants in the ration 9:3:3:1. Nine plant had yellow green seeds, three had round green seeds, three had yellow wrinkled seeds. While one had wrinkled green seed.

The different form of gene are known as alleles. Allele is short for allelomorph from Greek words allelo - "each other" and morph - "form".

The genetic constitution of an organism is known as genotype. E.g., Tt, Rr.

The outwards appearance of the organism i.e., the way the gene express themselves in the structure of the organism is known as phenotype.

When an organism contain two similar alleles, it is said to homozygous e.g., TT, tt.

When an organism contain two dissimilar allele, it is said to be heterozygous e.g., Tt.

There are chromosomes which determine the sex of an organism. There is one pair of chromosome linked with sex.

Number of chromosomes in each cell of Drosophila are 8 or 4 pairs. 3 pairs are same in both male or female cell. These 3 pairs are called autosomes. 1 pair is different in male and female. This pair is called sex chromosome. Composition of male and female sex chromosomes can be expressed as follows.

Pair of sex chromosomes = XY

Male can produce two different kinds of sperms.

Pair of sex chromosomes = XX

Female can only produce one type of egg.

Characteristics which follows the laws of inheritance are called hereditary characteristics.

Those characteristics which are not transmitted from parents to offspring are non hereditary characters.

Following are some important hereditary characteristics.

Some people have the ability to roll their tongue into a distinct u shape. This ability is due to presence of dominant gene. Those which lack this ability carry the recessive gene and are called non-rollers.

Ear lobes hang free in some people but in other these are attached to the side of head.

Free hanging lobes are present in those persons having dominant gene.

Person having attached lobes have recessive gene.

There are many variations of human eye colour. The brown eyes are dominant over blue eyes. Blue and gray eyes are due to recessive gene. Darker colours are due to dominant genes.

Dark hair colour is dominant over blonde. There is a dominant gene for dark colours and recessive gene is for blonde hairs.

Finger prints are determined by many genes. This is why none of two persons have same finger prints. Finger prints are obtained for personal identity and crime detection.

There are many disease which are transferred from parents to the offspring.

In patients suffering from haemophilia lack the ability to clot blood on injured area. Person in extreme case may bleed to death.

Gene for haemophilia is recessive and is located on X-chromosomes. Therefore the disease is sex linked and is more common is male than female.

This is a disease of RBCs. The blood cell become distorted and they are unable to tissue. The gene of this disease is carried by autosome and not on sex chromosomes. Therefore individuals of both sexes men and women have equal chance to suffer from the disease.

A colour blind person cannot differentiate between red and green colours. Gene for colour blindness is located on X-chromosome. It is sex linked disease. It is more common is male than female.

Following are some blessings of genetics on human beings.

By advancement of genetics better varieties of animals and plants are produced.

1. It is a technique of altering the genotype of an organism by inserting into its DNA gene from other organism. Following are some blessings of genetic engineering.
2. Insulin is obtained from bacteria by insertion of insulin producing gene into its DNA.
3. Gene causing diseases can be detected e.g., thalassemia.

It is application of biological processes for the benefit of mankind. It inlcude drugs production and agriculture.

Branch of biology which deals with the origin of new species.

Lamarck's theory of evolution can be explained by following heads.

1. Influence of environment.
2. Use and disuse of organ.
3. Inheritance of acquired characters.

The most important aspect of Lamarck theory is that traits acquired by the parent during life time pass on their offspring.

The ancestors of giraffe were horse like and lived in Africa in distant part. In the beginning they used to graze grass but when they could not find any grass to graze on they were forced to browse up on the leaves of the trees. In the result of their efforts the length of each of these parts of the body was slightly increased. This small increase in length was inherited to their offsprings. In this way accumulation of such small increase through the successive generations the fore limb and neck become enlarged.

According to the Lamarck the ancestors of snakes were the lizard like. The lizard like ancestors of snakes developed the habit of gliding over the ground and creeping through the narrow fissures and holes. As the result of this their bodies become elongated and limbs disappeared after large number of generations.

Darwin presented his theory in a book "on origin of species by means of natural selection". Following are the main points of Darwin's theory.

Most of the species have a tendency to produce far more offspring than needed to maintain the population. These species over produce to produce large number.

Since space and food are limited the offspring of each generation must compete with one another for obtaining necessities of life. This competition may be among the individuals of the same species or individuals of different species. Darwin called this struggle for existence.

Not two individuals of the same species are alike. Darwin thought that some of these variations pass on to the next generation. They are called hereditary variations.

Darwin realized that the individuals with variations that make them better adapted to their environment, survive and reproduce in greater number than those less fit. Thus there favourable variations tend to be passed on to the next generation. Over many generations favourable variations gradually accumulate in the species and unfavourable variations disappear. Eventually the accumulated changes become to great that the net result is new specie.

Variations are originated because of following reasons.

Change in amount or sequence of hereditary material is called mutation.

Gametes are formed by meiotic division. Crossing over takes place during meiosis which result in origin of variations.

The evolution of evolution can be given from comparative anatomy. It has been seen that different structures in different specie have close resemblance. Homologous organs exhibit similarity in basic plan of construction because of their common origin.

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