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Origin of new characters

Heredity is the presence of characters in the off springs which are present in the parents. These characters are directly transmitted to children from parents. The children thus get some characters from father and some other characters from mother. Some children get more of the characters from father and less from mother and vice versa. So, any child is not the photocopy of the father or mother. The child is little bit different from the father as well from the mother.

In the sexually reproducing animals including human beings, at the time of gamete formation, there is meiosis. During this cell division meiosis, the chromosomes number is reduced to half. For example, in case of human beings, there are 23 pairs of chromosomes. It is said to be Diploid number of chromosomes, denoted as 2n. In the sperm and ova(eggs), there are only 23 chromosomes. This number of chromosomes is said to be Haploid number of chromosomes and denoted as n. At the time of fertilization, the chromosomes of sperm and egg together make Homologous pairs of chromosomes and thus make Diploid number of chromosomes. During the process of formation of Homologous pair of chromosomes, there is crossing over to make the new set of chromosomes. This new set of chromosomes are little bit different from the those of parents. This new set of chromosomes are responsible for the origin of new characters in the off spring or children.

The new set of chromosomes are formed because of meiosis. The pattern of transmission of characters depend on the way the new set of chromosomes are formed. The pattern of transmission of characters in the off spring is studied in Genetics. This term Genetics was first used by W Bateson in 1905. The foundation of Genetics was laid in 1865 by Gregor Johann Mendel. For his overriding contribution in Genetics he is called as Father of Genetics.

Biography of Gregor Mendel

Gregor Mendel was born on 22 July 1822 at Heinzendorf of Silesian village in Austria. He joined Augustinian monastery at Brunn in Austria in 1843. He was sent to University of Vienna to study Natural Science and Mathematics. He returned to Monastery in 1853. He conducted research on common garden pea plant (Sativum pisum) for eight years from 1956 to 1864. Mendel presented data and conclusion derived from his experiments in a paper entitled “ Experiments in plant hybridization” which was read before Brunn Natural History Society in 1865 and was published in Annual proceedings of Natural history Society in 1866. His work was not recognized in his life. Unrecognized and bitterly disappointed , he died in 1884. Mendel’s observations went unnoticed on account of

• He published his work in an obscure journal.
• Failure of he scientists to notice his work because scientific world was at that time busy in the controversy arisen by the Darwin’s theory of origin of species.
• His ideas were ahead of his time as the ignorance was prevalent in that period about cytological basis of heredity.

In 1900, three eminent biologists Hugo de Vries of Holland, Karl Correns of Germany and Eric Von Tshermak of Australia working independently on heredity discovered the same phenomena originally uncovered by Mendel. Thus the genius of Mendel came to be discovered after 16 years of his death. Mendel for his great contribution is now famous as “Gather of Genetics”.

Mendel’s experiment

Mendel selected garden pea (Pisum sativum) as plant material for his experiment since it had following advantages.

• Possess many varieties with well defined characters.
• Flowers are bisexual and can pollinated easily.
• Easily be grown and readily be crossed.
• Very short life span, many generations formed.
• Hybrid pea plants are fertile.
• Flowers completely closed by petal, so plants predominantly self pollinating and self fertilizing.

Reasons for Mendel’s success

• Studied the inheritance of one character at a time.
• Carried out experiments to F2 and F3 generations only.
• Maintained statistical records, analyzed carefully.
• Selected pure line parent plants.
• All possible precautions taken to prevent self pollination.
• Even reciprocal crosses conducted but no change in the expected ratio of off springs.

Seven pairs of contrasting characters of pea plant used by Mendel

Trait dominant recessive

1. Seed color yellow green
2. Seed shape round wrinkled
3. Pod color green yellow
4. Pod shape inflated constricted
5. Flower color purple white
6. Flower shape axial terminal
7. Plant height tall dwarf

These seven pairs of contrasting characters of pea plant mean that if one character is expressed the other is not expressed. If the character tallness is expressed the other character dwarfness cannot be expressed. In the same way, if dwarfness is expressed, the other character tallness is not expressed.

Some basic terms used in inheritance studies

Gene or Factor

Mendel called it factor which carry the character from the parent to offspring. In the modern sense an inherited factor that determines a biological character of an organism is called gene. This is the function unit of hereditary material. It is a segment of DNA. Gene is equivalent to the factor of mendel.

Allelomorphs or allele

Allele is abbreviated form of allelomorphs, which means one form or the other. It indicates the alternative forms of the same gene. Each character has two determiners called factor. In pure tall or pure dwarf plant, same allele is duplicated like TT or tt. But in the hybrid tall plant both alleles are present like Tt. An organism having same allele or two identical alleles is known homozygous and an individual with different alleles is called the heterozygous. For example an organism with both TT or tt is called homozygous and an organism with Tt is called heterozygous.

Homozygous plants are genetically pure or that character. They give rise to offspring having same character on self breeding. The heterozygous hybrid plants do not breed true on self fertilization. The heterozygous plants on self fertilization produce both tall and dwarf plant.

Homozygous organism bear either dominant or recessive alleles and never both. Where as heterozygous organism bear both dominant(T) and recessive(t) alleles. The homozygous organism(plant) produces one types of gametes either T or t only. But heterozygous organism produces two types of gametes nearly half with T and another nearly half with t. The homozygous plants do not show vigor but the hybrid or heterozygous plants show extra vigor.

Genotype and phenotype

It is the genetic composition of an organism. It can be like TT, Tt or tt for a particular character. The genotype can be ascertained from ancestry or progeny of individual. On the other hand, phenotype is character which can be observed from the outside like the tall plant or dwarf plant. Phenotype is expressed like form, sex, color, behaviour etc. phenotypes of organism is expression or observable structural traits or characters produced due to interaction of genes and environment. In some cases, the phenotype is not visible from outside. For example, the blood groups like Blood group A, B, O and AB and Rhesus Factor like Rh +ve or –ve is not expressed outside.

Monohybrid and dihybrid cross

In the monohybrid cross, Mendel considered only one pair of contrasting characters like tallness and dwarfness of plant. Similarly, the other characters like round or wrinkled seed or yellow or green seed etc. can be considered but only one pair of the contrasting characters. The other characters are not taken into consideration. In case of dihybrid cross, two pairs of contrasting characters are taken into consideration at the same time like the yellow or green and round or wrinkled seeds.

Reciprocal cross

The reciprocal crosses involve tow crosses concerning the same characteristics but with reversed sex. For example, if in first cross, A is as the female and B is as male, then in the second cross A is as male and B is as female.

The interpretation of Mendel’s results

The principles of inheritance as given by Mendel are as follows.

1. Principle of Dominance

Out of two contrasting characters only one is expressed itself in an individual. The factor which is expressed is called dominant while the other which has not shown its effect is termed as recessive. In monohybrid cross, when homozygous (true breed) tall pea plant is crossed with homozygous dwarf plant, all the plants appeared in F1 generation are tall although they receive both factors (Tt) T from tall plant and t from dwarf plant. In the F1 generation, the character of t is not expressed. The character which is expressed in the F1 generation is called the dominant character and the character which is not expressed in F1 generation is called the recessive. The factors responsible for certain character is always found in the pair like TT or Tt or tt. To these Mendel’s factors, Danish botanist Johansson introduced the term gene in 1909.

Importance of principle of dominance - the phenomenon of dominance is of practical importance as the harmful recessive characters remain hidden by normal dominant character in hybrids. In human beings, a form of idiocy, diabetes and hemophilia are recessive characters.

2. Principle of segregation ( purity of gametes)

The law of segregation states that when a pair of contrasting factors or allelomorphs or genes are brought together in a hybrid(heterozygote) these factors do not blend or mix up but simply associate themselves and remain together and separate at the time of gamete formation.

3. Principle of independent assortment.