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Inheritance Part 1

Inheritance Part 1:
The process of formatting a new individual of their own by living organism is called Reproduction. Life perpetuates on the earth due to reproduction. In any species, off springs resemble their parents and that is the reason, distinct characters of every species are maintained. Thus each species differs from individuals of other species.

Each species maintains their characters generation after generation, which is called as Inheritance. However, at a same time, offspring are not exact copies of their parents which are called as variation.

All characters of organisms are not inherited. The living organisms modify their phenotypes in response to environment condition. The scientific study of heredity and variation is called genetics.


Mendel’s work:
A single ovum and a single sperm are responsible for complete inheritance to off springs form their mother and father respectively. Due to fertilization, the first cell Zygote comes into existence as a new offspring.

Mendel’s Experiments:
Mendel has performed the experiment on Pea plants (Pisum sativum-L) to study the mechanism of inheritance. Pea plant was selected on the basis of its distinct characters such as
  • It is a small plant and easy to cultivate and easy to cross-fertilize.
  • It has bisexual flowers which show self fertilization. Thus chances of accidental mixing up of characters are not possible.
  • It is an annual plant and has more generations are studied over a period of time.
  • Pure races are easily available
  • It produces a large number of off springs.

Mendel has applied the mathematics of probability in his experiment and crossbreed pure plants with contrasting pair and trait. He focused on only one character at a time and ignored other characters in his experiments.

Mendel’s Mono hybridization Experiments:
Mono hybridization experiment includes the inheritance of only one character at a time such as the height of plant. This character is expressed in two ways – Tall plants (greater height) and Dwarf plant (Lesser plant).

Mendel has selected pure tall and pure dwarf plants as Parent [P] generation and artificially pollinated and fertilized. After fertilization, seeds obtained and represents as their off springs. New plants can be grown using that seed and named as F1 generation. All plants of F1 generation were tall as like their parents in P generation.

Mendel further allowed the tall plants of F
1 generation to self-fertilize and the seeds obtained named as F2 generation. Out of total 1064 plants in F2 generation, 787 plants were tall and 277 plants were dwarf. Mendel carried out same experiments with seven different characters and each time he obtained same results.


Mendel’s conclusion:
Each character has two different expression which is controlled by a it’s ‘Factor’. The ‘factor’ controlling different expressions of a character is called ‘alleles’. Mendel’s factors were renamed as ‘genes’ later.

Every organism has two genes for any one character. Both these genes may be of the same kind or of different kinds. Organism is called ‘pure’ or ‘homozygous’ when genes are of same kind. Organism is called ‘mixed’ or ‘heterozygous’ when genes are of different kinds.

At the time of gamete formation, the two genes controlling any one character are segregated into different gametes. Thus any gamete contains only one gene for that particular character.

Each gene responsible for a particular expression of a character is given a symbol such as T represents gene for greater height and lesser height gene is represented as t.


Principles of Dominance:
When organism contains two different alleles for a character, only one of the two expresses itself. The allele which is expressed is called dominant gene and the allele which is not expressed is called recessive gene.

Law of Segregation:
During gamete formation, a pair of alleles segregates into different gametes and any gamete contains only one gene for an expression of a character. This means that gamete is always pure for an expression of a character. This is called Law of Segregation or Law of Purity of Gametes.

An organism can be homozygous or heterozygous for a character, but its gametes will always be pure for a particular expression of that character.


Test cross:
A cross, arranged for deciding whether an organism is homozygous or heterozygous, is called test cross.

Glossary:
  1. Character: Feature of an individual such as Height.
  2. Factor: Unit responsible for inheritance and also called as gene.
  3. Expression: Look in which a character appears.
  4. Alleles: Two alternative forms of gene
  5. Phenotype: Apparent expression seen due to combination of genes.
  6. Genotype: Genetic constitution for concerned expressions of a character.
  7. Homozygous: Condition representing both alleles of the same kind
  8. Dominant: Gene that suppresses the expression of its allele.
  9. Recessive: Gene which is not expressed in presence of its dominant allele.

Self study:
Example-1: Name the scientist who first studied the mechanism of inheritance.
a)  Robert hook
b)  Robert Brown

c)  Mendel
d)  Lamarch

Answer: Mendel is the first scientist who has studied the mechanism of inheritance and also known as “Father of Genetics”.


Example-2:  Each species maintain their characters generation after generation is called…………..
a)  Variation

b)  Inheritance
c)  Hybridization
d)  Genetics

Answer: Each species maintain their characters generation after generation is called inheritance.


Example-3: Gamete is always pure for an expression of a character.
a)  Law of Segregation
b)  Law of combination
c)  Principle of Dominance
d)  All of above.

Answer: Gamete is always pure for an expression of a character is a Law of Segregation.


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