Mendel law of Inheritance

Gregor  Johann Mendel  (father of genetics) was one of the pioneer scientist who worked in the field of genetics.

He carried out several experiements (crosses) on the Pisum Sativum (garden pea) for several years like monohybrid cross, dihybrid cross, test cross, reciprocal cross, etc.  

He took 7 contrasting traits (character) in the pea plant-

S.No.	Traits	Dominant Trait	Recessive Trait	Chromosome number
1.	Flower colour	Violet	White	1
2.	Seed colour	Yellow	Green	1
3.	Height of the plant	Tall	Dwarf	4
4.	Shape of Pod	Inflated	Constricted	4
5.	Position of Flower	Axial	Terminal	4
6.	Colour of the pod	Green	Yellow	5
7.	Shape of the seed	Round	Wrinkled	7

On the basis on monohybrid and dihybrid cross mendel proposed four postulates namely:-    1. Postulate of paired factor.

                     2. Postulate of Dominance.

                     3. Postulate of Segregation.

                     4.Postulate of Independent Assortmant.

Later on these four postulates are combined into three laws (;also known as mendel’s law of Inheritance) that are:-

1.   Law of Dominance –   (based on the monohybrid cross)

·        According to the mendel the characters are controlled by the discrete stable unit called factor (later called genes/allele).

·        Factor always occur in pair.

·        In which each pair of factor come from either of the parent.

·        In the pair of factor one pair expresses while another remain hidden the former one (which express itself) is known as Dominant gene while the later one is known as the Recessive gene.

For eg;  T and t are the two gene governs tallness (T - for tall, t - for dwarf) in the pea plant both of them remains together as ‘Tt’ in the plant showing tall  phenotype means only ‘T’ is expressing while ‘t’ is not showing any character in the presence of T; but in the ‘tt’ genotype plant it shows dwarf phenotype.

2.   Law of segregation-   (based on monohybrid cross)

·       Also known as the ‘Law of Purity of the Gametes’.

·        In the pair of factor both the factor remains together without any kind of blending (mixing) and at the time of gamete formation both separate (segregate) from each other.

·       So, each gamete receive only one factor from the each pair from either of the parent.

Eg;  that T and t remains together shows the dominant tall phenotype not the intermediate height of the tall and the dwarf means that characters are not missing.

3.   Law of independent Assortment-   (based on dihybrid cross)

·       When the two or more characters are studied simultaneously then inheritance of one character is not influenced by another character.

·       So, both the characters are inheritated in the typical monohybrid manner.

·       So, we can say that the dihybrid cross is the multiple of the two monohybrid crosses.

Eg; if we study two characters height of the plant and the colour of the flower simultaneously then the inheritance of height while not be affected by the inheritance of the flower colour. That is both the cross will have 3:1 ratio (that is the ratio of the mendelian monohybrid cross) and if we can join both the cross means do a dihybrid cross the ratio will be multiple of (3:1)*(3:1) (i.e. two monohybrid cross) will be 9:3:3:1.

Meaning of the terms used above-

Gene – the functional segment of the DNA which code for the protein.

Allele – gene which are present on the same locus (place) on the homologous chromosome and express same character.   Eg; T and t express tallness so they are allele of each other.

Trait – the character which can inherit from one generation to another (germinal character).      Eg; tall/dwarf, round/wrinkled., etc.

Inheritance – transfer of character from parent to the offspring.