RULES FOR THE INHERITANCE OF TRAITS : MENDEL’S CONTRIBUTION
- 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.