RULES FOR THE INHERITANCE OF TRAITS : MENDEL’S CONTRIBUTION

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Gregor Johann Mendel (1822-1884) worked out the first ever scientific experimental study on heredity.
Mendel, an Austrian Augustinian monk observed variations in the characteristics of garden pea plant (Pisum sativum) which he had cultivated in his monastery garden.
Mendel was curious to find out the results of crossing of pea plants with the variation in traits.
The visible contrasting characters that Mendel observed in the garden pea plants were given below:

MENDEL’S MONOHYBRID CROSS

Mendel selected the garden pea plant, Pisum sativum for his experiments.
He selected tall and dwarf plants and allowed them to grow naturally.
As pea plants produce seeds only by self pollination, he observed that tall plants always produced tall plants generation after generation under natural conditions.
Similarly, dwarf plants always dwarf plants generation after generation.
Hence, he termed the tall and dwarf plants as wild types or pure breeding varieties.
Then he crossed a tall plant with a dwarf plant, produced progeny and calculated the percentage of tallness and dwarfness in subsequent generations.
When a pure breeding tall plant was crossed with a pure breeding dwarf plant, all plants were tall in the first filial generation (F1) i.e., there were not any medium height plants or dwarf plants.
This means that only one of the parental traits were seen and not the mixture of the two.
When such a F1 tall plant was allowed to have self pollination, both the tall and dwarf plants appeared in second filial generation (F2) in the ratio of 3:1.
This indicates that both tallness and dwarfness were inherited in the F1 plants but only the tallness trait was expressed.
The first experiment of Mendel considering the inheritance of a single trait (Height of the plant Tall/Dwarf) is called Monohybrid Cross.

Expression of morphological characters as tall or dwarf plant, violet or white flower is called Phenotype.
The expression of gene (or Chromosomal makeup) of an individual for a particular trait is called Genotype.

MENDEL’S FIRST LAW

Law of Segregation: Every individual possesses a pair of alleles for a particular trait. During gamete formation, a gamete receives only one trait from the alleles.
A particular trait can be dominant or recessive in a particular generation.

The cross in which two pairs of characters are studied is called dihybrid cross.
Let us take an example between plants with round and green seeds and those with wrinkled and yellow seeds.
The genotype of round and green seeds is shown by RRyy and that of wrinkled and yellow seeds is shown by rrYY.
In the F1 generation, all plants produced round and yellow seeds; which means that wrinkled texture was the recessive character and so was the green colour of seeds.
When plants of Fl generation were allowed to self pollinated;
It was observed that most of the pants in the F2 generation produced round and yellow seeds.
Some plants produce round green seeds, some produce wrinkled yellow seeds and some produce wrinkled green seeds.
The ratio was 9:3:3:1; This is known as the dihybrid ratio.

MENDEL’S SECOND LAW:

Law of Independent Assortment: Alleles of different characters separate independent from each other during gamete formation.
In the above example; alleles of texture were assorted independently from those of seed colour.