Genetics

MENDELS LAWS OF INHERITANCE

The first scientific study leading to formulation of laws of inheritance was carried out by Gregor Johann Mendel.

Mendel conducted cross-hybridization experiments on garden Pea plant (Pisum sativum) and studied the transmission of characters.

Mendel selected garden pea as experimental material because

  1. It is easy to cultivate.
  2. Pea plant had several true-breeding lines.
  3. The plant had a short (seasonal) life cycle so that transmission of traits from one generation to next could be studied in short period of time.
  4. Flowers are naturally self-pollinated and cross-breed manually.
  5. Characters chosen by Mendel had distinctive contrasting traits like tall and dwarf or green and yellow seeds.
  6. Mendel took one or two characters at one time for his breeding experiments while his predecessors often studied all the traits simultaneously. Mendel took only those traits for his studies which did not show linkage, interaction or incomplete dominance.

Mendel selected seven pair of contrasting characters (given in table) & luckily all were dominant or recessive. There was no blending or incomplete inheritance or linkage.

S.NoCharacterDominant traitRecessive trait
1HeightTall(T)Dwarf(t)
2Flower colorViolet (purple) (V)White(v)
3Flower PositionAxial (A)Terminal(a)
4Seed shapeRound (R)Wrinkled (r)
5Seed colorYellow (Y)Green (y)
6Pod colorGreen (G)Yellow (g)
7Pod shapeInflated (I)Constricted (i)

 Mendel was lucky in selecting those traits, the genes of which did not interact. They were either present on different chromosomes or showed complete recombination. He did not combine pod shape and plant height in any of his dihybrid crosses the genes of which are close together on chromosome 4 and do not show frequent recombination.

He did not attempt to explain all the variations found in his results but left them as such, e.g., linkage of fl ower and seed colour.

He performed reciprocal crosses and studied the inheritance of a character for three or more generation.

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