Genetics (Heredity, Variation) Basic Concepts and Facts
A short introduction to Genetics:
Heredity is the process through which parental characters pass from generation to generation.
Due to heredity, a hen’s chick cannot be hatched into duck’s egg. Though some similarity is found among the family members but they are not exactly alike.
There are some dissimilarities between them which is called Variation.
The branch of biology which deals with mechanism of heredity and variation is called Genetics.
Gregor Johann Mendel is known as Father of Genetics.
There are different terms used in genetics Some terms used in Genetics are given below:
- Unit character
Unit character is known as the definite external character of an individual of a cross.
Each organism consists of its own type of character. For example, a sunflower plant has its own type of leaf, flower, stem, etc.
- Monohybrid and dihybrid cross
When only one character is involved in a particular cross, it is called monohybrid cross.
Dihybrid cross is the cross where two unit characters are involved
- Dominant and recessive cross
Dominant characters are those characters which express themselves by covering the other character in a cross. They dominate recessive character and express themselves.
Recessive characters are those characters which cannot express themselves in a cross.
They are covered by dominant characters.
- Allele or allelomorphs
Allele or allelomorphs are the pair of genes which control a pair of contrasting character.
- Phenotype and genotype
The morphological appearance of an organism is called phenotype of that organism.
And the genetic make-up of an organism is called genotype of that organism.
The organism which have their phenotype and genotype different are called Hybrid.
Gregor Johann Mendel:
Gregor Johann Mendel was born on 2nd of July in 1822 Ad in a village of Germany. After he finished his schooling, he went to Brunn of Austria. He joined Vienna University for his higher Education.
He did various types of crossing sweet pea (Pisum sativum) in the garden of church of Brunn.
Later on, His work was known as mendelism or law of In inheritance or Law of genetics and he is known as Father of Genetics today.
Mendel used seven pairs of contrasting character for his experiment.
Fortunately, in a monohybrid cross, Mendel elected pure tall and dwarf pea plants. He made artificial cross between them with the help of a small brush.
In the first filial generation all the pea plants are found tall. The plants of this generation were allowed to self-pollinate.
In second filial generation about 75% of pea plants were found tall and 25% short.
In third filial generation 25% of pea plants were pure tall and 50% were hybrid tall.
Mendel noticed the following things when he studied third filial generation
- Pure tall pea plants form tall plants only
- Hybrid tall pea plants form both tall and short plants in the ratio of 3:1
- Pure dwarf pea plants form dwarf plant only.
Mendel’s law or Mendelism
Mendel have done various experiment and as the result of his experiments he proposed three different laws. These laws are Known as Mendelism or Mendel’s Law:
- Law of dominance
- Law of purity of gametes or law of segregation or
- Law of independent assortment
Every organism shows differences from the other organism of the same species as well as of other species.
These changes occurs due to a number of reasons like environment and the genetic make-up.
In the universe, no one is exactly same and they show variation among themselves. Even the identical twins of a parent differ in some or minor aspects.
Variation is a morphological or physiological change in organism due to either genetic or environmental reason.
Variation is classified into two terms. They are:
|Somatogenic variation||Blastogenic Variation|
|These variation is acquired by an organism due to influence of environment||These arise due to change in the germ cells.|
|They are not inherited from parents to offspring||They are inheritable|
Continuous variation is that in which there is possibility of intermediate character as well.
For example, there is no distinct category of height. People are not only either short or tall.
There is the different possibility of height between very tall and very short.
Some other examples of continuous variations are skin color, physique, intelligence, etc.
This variation are highly influenced by environmental condition and food. Such variations are due to the interaction between the genotype and the environment.
This type of variations controlled by a very large number of chromosomes.
Discontinuous variation is that in which there is no possibility of intermediate character. For example, except some abnormality, we are either male or female; we have particular type of blood group, special fingerprint, etc.
These characters are not influenced by food and environment. Such variation are controlled by very less number of alleles.
Mutation is the sudden chromosomal change by which very new and different characters from parents to offspring.
This term is coined by a Dutch biologist De Vries.
Some examples of mutation in human beings are six fingered limb, a cut at the lip by birth, disability by birth.
Mutation is also found in different organisms.
Four horned rhino, five legged cattle, single horned ship are some of the examples of mutation in other organisms.
The organism who are victim of mutation are called mutants and the cause is termed as mutagens.
Radioactive rays, chemicals, etc. are some examples of mutagens.
Why did Mendel select pea plant for his experiment?
- It has a number of distinguishable contrasting character.Thus, different experiment can be done through it.
- A large number of progency (offspring) can be obtained from a single plant of pea.
- A pea plant life cycle is of short duration, so the result of experiment can be obtained in a short period of time
- Pea plant is convenient to handle.So, they can be grown anywhere.
- The hybrid pea plants obtained as a result of cross are also fertile.
- A pea plant is naturally self-pollinating and artificially cross-pollinating.So, it helps to maintain the purity of characters in offspring