RULES FOR THE INHERITANCE OF TRAITS : MENDEL’S CONTRIBUTION

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.

DIHYBRID CROSS

  • 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.
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