Gregor Johanm Mendel was the first person who studied Inheritance of characters. He performed experiments on Garden pea (Prisum Stavum) and kept a scientific record of his observation and worked almost eight years. His work was recognized in 1900 AD when De Vries of Holland, Correns of Germany and Tschermak of Austria performed experiments and got the same results.

For Genetical study, the specimen should have certain properties to ease out experimentation. For example Mendel selected Garden Pea (Prisum Stavum) for his work due to following reasons:

Mendal selected varieties of Garden Pea having many pairs of contrasting characters (Allelic pairs). For eg: Tall and dwarf plants, red and white flowers, green and yellow seeds, round and wrinkled seeds respectively.

Medal performed artificial cross-pollination between the flowers of two contrasting varieties of Pea i.e. Hybridization and obtained a mixed or Hybrid generation.

A hybridization cross in which inheritance of only one character is studied in which mono hybrid cross. Mendal selected height of the plants for his study.

Mendal selected two varieties, one with tall height and the other with dwarf height. Both of the varieties were Homozygous i.e. TT(tall), tt(dwarf). He performed artificial cross between the plants and obtained a Hybrid generation or First Felial Generation (F₁). When the seeds were planted, Mendal observed that all the plants of F1 generation were tall phenotypically. However as these plants were obtained as a result of hybridization therefore Genotypically plants were bybrid or Heterozygous.

From this observation Mendal concluded that in F₁ generation, one of the character (Tall in this case) appears while other (dwarf) remains hidden. The appearing character is called Dominant Character while the hidden character is called Recessive Character.

Mendal conclusion conceived a law which can be stated as:

The plants of F₁ generation were allowed to self fertilize their flowers. As a result seeds of F₂ or second felial generation were obtained. These when sown gave rise to a combination of Tall and dwarf plants in the ratio of 3:1 respectively.

From this chart, it can be easily concluded that:

Hybridization in which two different characters are selected for genetrical study is called Dihybrid Cross. Mendal selected Height and colour of flowers for study of inheritance. He selected tall plant with red flowers (TT RR) and Dwarf plants with white flowers (ttrr) and performed artificial cross-pollination. The resultant seeds of hybrid variety were sown and observed that all the hybrid plants were tall with red flowers.

It clearly indicates that tall height and red colour of flower is dominent in accordance with the law of dominance. Now plants of F₁ generation were allowed for self-pollination and seeds of F₂ of generation were obtained. These seeds when sown gave rise to different combination of height and colour. For example tall white and Dwarf red.

From above chart, it is clear that there is no effect of one character over the other. This led to the formulation of law according to which:

In some of the cases, no single character dominates the other. Instead of it, a blend of two characters results in F₁ generation. This phenomenon is called Incomplete or blending dominance.

To understand this phenomenon, we take the example of 4° clock plant. If true breeding homozygous red and white varieties of 4° clock plant are crossed, the resultant F₁plants produce flowers of Pink colour indicating incomplete dominance. When these hybrid plants are self fertilized then in F₂ generation red, pink and white flowering plants are obtained. This means that this type of inheritance obeys Mendelian laws of inheritance but there is mixing up of characters.

In some of the hybridization experiments, both of the contrasting characters simultaneously appear in hybrid generation without blending or making the effect of each other i.e. both the characters remain dominant. For example, if a homozygous white bull (WW) is crossed with a homozygous Red cow (ww) then the resultant F1 hybrid cattle appears Roan, showing that both the characters are separately dominant. Close search of skin of that animal shows that both type of hair i.e. red and white are separately present and there is no mixing of colour as in case of incomplete dominance.

Study of inheritance in human beings is quite difficult due to the fact that:

• Humans cannot be cross-bred at will.
• Self fertilization or fertilization with the nearest sibling is impossible ethically as well as religiously.

However some of the characters can be studied which obey the general Mandelian pattern of inheritance. Some of the examples are given below:

In Albinism, the black pigment or Melanin is totally absent from skin, hair and eyes in a living being (including men), Albinism is an inheritable trait and is related to a dominant gene (A). A person lacking this gene A and homozygous for recessive gene (a) will be an albino.

A normally pigmented person may be homozygous AA or Heterozygous Aa whereas an albino will always be a Homozygous recessive aa.

Due to such reasons:

• When a normal person marries an albino (AA with aa) all the children will be Phenotypically normal due to the dominance of A (i.e. Aa)
• When an albino marries another albino, all the children will be albinos due to lack of dominant gene A (i.e. children will be all aa).
• When a heterozygous carrier (Aa) marries an albino, half of the children (50%) will be normal while remaining half will be albino.
• When two heterozygous carriers (Aa) marry, the children will obey mandelian ration i.e. one normal, two carrier and one albino (1:2:1)

Colour of the iris is the colour of eye. Normal colour of eye is black to brown under influence of a dominant gene, say B. Blue (or green) colour of eye is due to lack of this dominant gene A. So a person with black (or brown) eyes may be Homogenous black (BB) or Heterozygous black (Bb) while person with blue eyes is always homozygous recessive (bb).

• In case of marriage between Homozygous black (BB) with a homozygous recessive blue (bb) are the children will be heterozygous black (Bb).
• In case of marriage between two heterozygous (Bb), children will obey Mandelian ration i.e. one homozygous black(BB), two heterozygous black (Bb) and one homozygous recessive blue (bb).
• If two homozygous recessive (bb) marry, all the children will be blue eyed i.e. homozygous blue (bb).

In some of the traits, a single gene may have more than one allelic form, known as Multiple Allels. Very common example of multiple Allel is Human Blood Group.

Human red blood cells i.e. R.B.C have got certain specific proteins on cell membranes called Antigens, there are of two types of antigens i.e. antigen A and antigen B. A person having antigen A is labeled to have blood group A. Similarly presence of antigen B on RBC is called blood group B. If a person have got both A and B antigens is labeled blood group AB. Absence of antigens means blood group O.

It is due to the fact that there are 3 allels for blood groups i.e. I^A, I^B and alternative allel I, which synthesizes no antigen at all. Therefore a person with blood group A may be homozygous I^AI^A or Heterozygous I^Ai. Person with blood group B may be homozygous I^BI^B or heterozygous I^Bi. Blood group AB is always heterozygous I^AI^B. Person with blood group O is always homozygous recessive ii.

In human blood, there is another gene R responsible for specific protein called R^H factor. Presence of this factor renders blood to be Positive (+) for R^H. Its recessive allel n ^h does not produce any antigen and person with this allel therefore have got blood group R^H Negative. Different combinations of this factor as follows:

In living beings all homologous chromosomes show similar properties except one pair, this pair has got chromosomes with differ with each other and they determine the sex of that living being. Such chromosomes are called sex chromosomes while rests of the chromosomes are called Autosmes.

The difference in chromosomes which determine the sex may be of three types:

1. Difference in Number
2. Difference in shape
3. Difference in size of the sex chromosomes

Human cells contain 23 pairs (i.e. 46 in whole) of chromosomes. Out of these, 22 pairs are outosomes while 23^rd pair is composed of sex chromosomes. In female this pair of sex chromosomes contains two similar chromosomes while in male one of the chromosomes is similar in size, this smaller chromosome is labeled as Y chromosome, the other chromosome which is similar to each of the female sex chromosome is labeled as X. Therefore females are homozygous XX while males are heterozygous XY.

Ms Kusick estimated that about 200 genes are present on human X chromosome, which means that there may be inheritance of a number of characters due to this sex chromosome. Inheritance related to the sex chromosome is called Sex Linked Inheritance. In human beings two common and popular sex linked inheritance are as follows:

It is a human heridity disease in which blood fails to clot after an injury. It is due to deficiency of one of the blood clotting factor. Inheritance of this disease is sex linked i.e. related to X chromosomes. Man is the usual sufferer while females are commonly the carrier. Y chromosome is inert for this trait. Normal woman does not contain gene for Haemophilia but in case of carrier woman has got one haemophilic X chromosome i.e. XX. As it is a recessive trait therefore chromosomes of haemophilic gene (X) remains inactive in carrier female. In male XY pattern always means that the person is a sufferer because Y chromosomes is inert for this trait.

Each living specie has got specific or definite number of chromosomes, which is generally called Diploid and represented by 2n. Any change in the number of chromosome is called Abnormal Chromosomal number of Heteroploidy. It may be of following types:

It is represented by 2n-2.

Abnormal chromosomal number is an error of Meiosis. The common phenomenon is called Non-Disjunction i.e. failure of a pair of chromosome to separate from each other during Diakinesis. The gametes thus formed are abnormal i.e. gamate having one chromosome less or more than the specific number. Fusion of such a gamete will result one chromosome less or more in Individual.

It is a trisomic male defect in which a male receives one X chromosome more than the normal number i.e. 22 + XXY or 47. Affected person may reach adult size but remains sexually immature, with infantile (body like) genital organs, non growth of beard and pubic hairs, baby face etc. Occurrence is 10 out of 10000.

Robert Brown discovered nucleus of the cell and later on analysis of nuclear material was started. Chromatin material was isolated from the chromosomes and on investigation, was found to be composed of different types of macro-molecules of proteins namely DNA (60 – 90%) RNA, Histones and some other proteins. It was also discovered that DNA forms the basic and essentially part of Genes i.e. the units of of heridity. Each molecule of DNA is further composed of sub-units called Nucleotides. Each Nucleotide sub unit is made up of Ribose sugar, Phosphate and nitrogen base. In chromosomes of higher plants and animals, histones are of 5 types bound to DNA-phosphate group by salt linkage. This complex of histone – DNA is called Chromatin.

F. Griggith, a genetic researcher studied the Bacterialheriditary material, during his work he selected two varieties of streptococcus Pnemonae i.e. one smooth capsulated but virulent variety and the other rough capsulated X non virulent. They were named or labeled as R-II and S-III varieties respectively.

He inoculated two identical laboratory mice with R-II and S-III varieties separately and fond that mouse inoculated with R-II was killed while other mouse remained will.

When both type of Bacteria were killed by means of heating and then injected in another group of mice, no harmful effects were observed. In another group of mice, heat killed R-II and living non-virulent S-III bacteria were simultaneously injected. It was observed that the mice were killed. F. Griffith was unable to understand this phenomena but concluded that R-II dead bacteria some how influenced living S-III bacteria.

Avery, Mc Carty and Macleod named this phenomena as Transformation and isolated the transforming substance. They found that only DNA possessed the transforming ability and if DNA is destroyed by means of enzyme Deoxy Ribonuclease, it losses power of transformation. It indicates that DNA is the hereditary material.

Hershey and Chase studies hereditary in Bacteriophage T₄ virus. They labeled virus T₄ with radioactive phosphorus P³² for DNA and sulphur S³⁵ for protein coat of the virus. After incubation in bacteria, viruses were ruptured and analyzed, P³² was found in DNA while S³⁵ in Protein coat. Radioactive isotope labeled virus were again incubated in Bacteria and their S³⁵ isotope containing cell membranes were removed. The newly formed virus was found to contain P³² isotope in their DNA clearly indicated that offspring virus received hereditary material i.e. DNA from their parent virus.

DNA is a very complex compound, consist of large or Macro-molecules. Each macro molecule is composed of sub units called Mucleotides. In term such nucleotide is composed of 3 components i.e.

1. Dexoy Ribose sugar
2. Phosphoric Acid
3. Nitrogenous Base

These are Amino acids in which nitrogen is the basic element, therefore they are also called Nitrogen bases. In DNA, they are of four types namely:

1. Adenine
2. Thymine
3. Cytosine
4. Guanine Nucleotides

These nucleotides arrange in a specific manner to form a single chain like molecule of DNA, each chain contain billions of these nucleotides.

Adenine and Guanine are double ring nucleotides and are called Purine Bases while Thymine and Cytosine are single ring nucleotides and are called Pyrimidine Bases.

Different molecules of Nucleotides are linked together by means of Phosphate in such a way that PO₄ binds with C₃ of one Deoxyribose and on other hand binds with C₅ atom of other Deoxyribose forming a long Nucleotide chain.

J.D. Watson and Francis H. Crick combinely studied the structure of DNA with the help of X-ray defraction technique and Data provide by Murice H.F Wilkins. All the 3 got Nobel prize for their pioneer work. It is quite intresting to quote that J.D watson solved this problem of DNA structure in a Dream.

The DNA molecule is a Double helical structure resembling a ladder, each pillar of ladder is a polynucleotide chain twisted around each other. The corss bars or Rungs of ladder are made up paired nitrogen bases. The pillar or upright of ladder is composed of Deoxy Ribose sugar and PO₄^-3 bonds. Nitrogen bases i.e. Purine and Pyrimidine bases are joined together in the following pattern:

Distance between two chains of Nucleotides is 20A° , the coiling of chains is right haded or clock wise, one complete turn is 34 A° distant.

Watson and Crick model is the most suitable and most probable explanation of DNA structure. It is also ideal for the phenomenon of Self Replication as expected during cell division especially during interphase.

The double helin is held together by means of weak hydrogen bonds During cell division three bonds break, starting from one end to the other just like unzipping of a zip. Thus purimes separate from pyrimidine leaving them unmatched. Now each nucleotide base forms new bond with a new apponent in such a way that:

This procedure is repeated time to time whenever there is need of DNA replication. Also newly formed chain is exactly alike and similar to the parent chain.

Ribosome, a living inclusion of cytoplasm is the site of cellular protein synthesis. They are found attached to Endioplasmic Reticulum or suspended in Cytoplasm. Each Ribosome is made up of Protein and RNA.

Ribonucleic Acid or RNA is also a polynucleotide with a fixed sequence of Nucleotides, Unlike DNA, RNA have got following properties.

• It is a single chain compound and composed of single strand of Nucleotide chain.
• In DNA, 1 of the oxygen atom is deficient while in RNA oxygen is found in the same ratio as of carbon i.e. C₅H₁₀O₅.
• In RNA Thymine is replaced by Urocil radicle or base.
• Due to unpaired and exposed nitrogen bases, RNA is capable to bind with any other nucleotide chain.
• RNA is of three types:

1. Messenger RNA or mRNA
2. Transfer RNA or tRNA
3. Ribosomal RNA or rRNA

Initiation of protein Synthesis is called Transpiration in the nucleus. The double helix of DNA unzips at any specific area (to be synthesized) temporarily, along with the open chain, mRNA chain is synthesized and copies the genetic information of DNA (or gene). Now this mirror Image mRNA separates and leaves the nucleus. It gets associated with the Ribosome.

Transfer RNA (or tRNA) in the cytoplasm picks up individual Amino Acids and carries them to the Ribosome. In Ribosome these amino acids bond with specific nitrogen base of mRNA in the specific coded sequence and therefore a mirror image or copy of the coded message of mRNA is obtained in the form of Polypeptide chain. TRNA molecule becomes free and again reaches the cytoplasm.

As we known that each biological function of a living being is under influence of a specific gene. Beadle and Tatum suggested one gene – one enzyme hypothesis i.e. with each enzyme of a specific reaction is synthesized or under controll of a specific gene. Any alternation in the gene will change the sequence of a reaction and therefore effect the phenotype. Genes are generally inert and stable but changes may occur in the amount, organization and contents of genetic material, such change is called Mutation. It may be obvious and visible by cytological technique. Some sequential changes (which may be invisible) are called Chromosomal Aberration. When these two type of changes pass from adult to off spring could alter the hereditary instruction.

Evolution is not a new topic for study but the history is so ancient that in every Era man was eager to know his origin, origin of other living beings, the changes which took place in them etc. These questions were answered in different Theories, few of them are as follows:

• Anexemander in 600 BC described that man actually originated in water and later on came to inhabit land. He changed the shape accordingly and acquired the new hanitat.
• Xenophanes , a Greek philosopher of ancient times was the first Paleontologist who studied the fossils and discovered that evolution had taken place.
• Empedocles in 500 BC gave a strange concept about origin of animals and plant. According to him animals and p0lants both grew or arose out of earth. Different organs of animal were separately born, later on they combined to form bodies and only those which were in perfect combination survived and the rest were eliminated.
• Aristotle in 350 BC described that living beings were imperfect initially, they changed their shape and habits according to the need and become more perfect and suitable.

In 1806 AD, French Biologist Jean Baptist de Lemark conceived a new theory of evolution in his book Philosophic Zoologique. This theory was later on named as Lamarkim.

In this theory Lamark explained that climatic factors and requirement of the living beings in accordance are responsible for evolution. Lamark explained this phenomenon in three steps:

• Effect of Environment on living beings
• Use of Disuse of different organs
• Inheritance of acquired characters

According to Lamark, if seeds of a plant are sown in different environment the plants soon adapt their new environment. If a number of generations are cultivated in that particular environment, soon the specie of plant will adapt accordingly and transmit the newly acquired characters to new generations, thus new variety of plant will be obtained.

Similar is the case of animals, they are also influenced by the environment. Animals adept according to the environment and animals may use certain body organs more while others are discarded. Use and disuse of body organs leads to acquire a more suitable new shape and therefore evolution takes place. For example long neck of Giraffe and vestigial limbs of snakes.

When a living being acquired a suitable character, the character is transmitted to the off springs. This is called inheritance of acquired characters. Consider the example of Giraffe, Ancestors of giraffe were short height and short neck. They start browsing on leaves of trees instead of grass. To fulfill the need to reach the leaves, they start to stretch their neck and forelimbs. Gradually new generations developed long neck and forelimbs due to inheritance of acquired characters.

According to Geneticists, environment could effect the somatic cells but they could not effect the germ cells, therefore environmental adaptations could be inherited.

German scientist Weismann removed the tail of several generation of mice but failed to get any tail less new variety of mice i.e. although the tail less character was induced for several generations but it never became a part of inheritance. This experiment was also contradicted by supporters of Lamark that deprivation of tail was not an environmental stimulus and the animal never participated actively or willingly in this experiment. Lamark’s theory has not been able to hold any ground due to insufficient scientific data.

Charles Darwin, a British Biologist conceived a new theory of evolution on more scientific grounds. He worked almost for 20 years on evolution and wrote a book Origin of Species by Means of Natural Selection in which he described the possible way of evolution in a stepwise manner, the explanation is much scientific then Lamark’s theory. Stepwise mechanism of evolution is as follows:

We are familier with the fact that variations among the species appear on the basis of genes i.e. variations are genetically controlled. Darwin’s theory is better suited to explain evolution due to this fact with is not explained by the others. According to his theory Better variations means better combination of Genes. Therefore individuals with better genotype are the fittest i.e. capable to exist while others perish. It means that generations by generations, nature is selecting the individual with better genotype. Due to gradual minor variations in genotype for survival, an individual after successive generations may become distinct new specie. For example if two plants of same specie are cultivated in two different ecological areas then after generations a number of minor variations may accumulate in them to make them two different species.

Darwin’s theory was strictly condemned by the fundamentalists of Christianity. In recent era, objections were put forwarded by Molecular biologists and Geneticists. According to them, Darwin failed to explain or differentiate heritable and non-heritable variations. Spontaneous changes or Mutations whenever occur, also plays role in the process of evolution. These changes were not mentioned by Darwin’s Neutral Variation, which are neither harmful nor harmless also occur but reason for their occurrence is observe.

Earth is full of certain evidences which prove that evolution had take place some of the evidences are as follows:

In living things, especially humans, certain vestigial structures, which were functional in the past again, become well developed and functional. This phenomenon is called Atavism or Reversion. Some of the examples are:

• Canine teeth become large and prominent like carnivores.
• Posterior auricular muscles become well developed and functional by which human being becomes able to move his ears.
• Appendix becomes large and functional as in herbevores.
• Coccyx vertebrae grow enormously to form 8 – 9 inch long tail.
• Hair appear profusely over face and body.

The organs, which are present in a rudimentary or function less form in an animals are called vestigial organs. For example, pelvic girdle in wholes, hind limbs in snakes. Presence of these organs in vestigial form indicates that these organs were functional in the past but due to disuse they become function less. This concept is further supported by the study of fossils, records show that these organs were well developed and functional in the past, they show similarity in the structure and functions in different groups of animals. Some of the vestigial organs of human beings are as follows:

• Semilunar fold of human eye is a representative of nictitating membrane of fish and amphibians.
• Vermiform Appendix which is large in herbivores.
• Auricular muscles, which are strong and serve to move ear in quadripods.
• Coccyx vertebrae which form tail in outer animals.

Gradual development of different body organs and change in the shape along with origin of new species in different era can be easily studies from the Fossils of ancient living beings. Age of fossil is estimated by Geological techniques by which we can identify the time period of origin of that particular specie and comparison with the fossils of that specie in other times easily shows us the development and progress with the time. Study of fossils shows that the ancient animal was actually originated in water from where it migrated to terrestrial habitat, invertebrates originated before vertebrates and gymnosperm were formed before angiosperms. Different era show a particular type of fauna and flora which clearly indicates evolution has taken place.