Mendelism and its biological significance


The below article will provide you with the detailed explanation of Mendelism and biological significance of Mendelism. You will also get information on the different laws formulated by Gregor Mendel.

Introduction


Gregor John Mendel is considered as the father of the genetic. He worked on pea plant and selected seven pairs of contrasting characters in the pea plant. Gregor John Mendel performed the following two hybridization experiments in pea plants:

  • Monohybrid cross

  • Dihybrid cross


Monohybrid cross

When the cross is made between a single pair of contrasting character trait, it is known as monohybrid cross. For monohybrid cross Mendel selected a tall (TT) and a dwarf (tt) of garden pea Pisum sativum . When Mendel crossed these pants, all the offsprings were tall. The initial cross between two varieties is known as parental or P1 generation and their offsprings are called first filial generation or F1 generation. This process of obtaining new plants (hybrid) is called hybridization. When these plants are allowed to self-pollination , the tall and dwarf plants are produced in the ratio of 3:1 (75% tall and 25% dwarf). The offsprings of F1 generation form F2 or second filial generation. The succeeding generation of this cross is called F3 generation.
All the offsprings of F1 generation would be tall. Although it contains both the gene of tallness (T) and dwarfness (t) but only the characters of tallness would be expressed in F1 generation. The character which was expressed in F1 generation was called as the dominant character by Mendel. Likewise, the character of dwarfness which would not be expressed in F1 generation was called as the recessive character.
These tall plants of F1 generation was allowed to self-fertilization (self-pollination). The tall and dwarf plants produced in the ratio of 3:1. It means F2 generation consisted of three types of plants:

  • Tall homozygous (pure) – TT – 25%

  • Tall heterozygous (hybrid) – Tt –50%

  • Dwarf homozygous (pure) – TT – 25%


On the basis of experiment, Mendel found that although the genes for tallness and dwarfness remain together in F1 generation they would be segregated in F2 and subsequent generations without mixing. Thus, he concluded that the genes of one trait do not affect the genes of another trait. Mendel also found that the gametes of F1 offsprings will always be pure. Mendel called it as the purity of gametes.

Dihybrid cross

When a cross is made between two pairs of contrasting character traits, it is known as dihybrid cross. For dihybrid cross, Mendel selected garden pea having two pairs of contrasting characters:

  • Round and yellow seeded (RRYY) plants

  • Wrinkled and green seeded (rryy) plants


dihybrid
(Image courtesy: www.wikipedia.org)

When these plants are cross-pollinated in F1 generation, they produced hybrid plant (RrYy) with round and yellow seeds, because of the presence of dominant genes "R" and "Y". When these plants (F1) were allowed to self-pollination, following four types of progenies were formed in F2 generation in the ratio of 9:3:3:1.

  • Round and yellow seeded – 9

  • Round and green seeded – 3

  • Wrinkled and yellow seeded -3

  • Wrinkled and green seeded - 1


Laws of inheritance or Mendelism

On the basis of monohybrid cross Mendel formulated two laws ( law of dominance and law of segregation) and on the basis of the dihybrid cross, Mendel formulated the law of independent assortment. The three laws are as follows:

  1. Law of dominance: The law of dominance states that when two homozygous individuals with one or more sets of contrasting characters are crossed, the characters that appear in the F1 generation are called dominant and those that do not appear in F1 generation are called recessive characters.

  2. Law of segregation: According to the law of segregation, the allele pairs separate or segregate during gamete formation and the paired condition is restored by the random fusion of gametes during fertilization. This law is also known as the law of purity of gametes.

  3. Law of independent assortment : The law of independent assortment states that if the inheritance of more than one pair of character is studied simultaneously the factors or genes for each pair of character assort out independently of the other pair.


Biological significance of Mendel's Law

The significance of Mendelism are as follows:

  • The laws of Mendel have practical application in plant and animal breeding.

  • With the help of laws of inheritance, it is possible to produce animals and plants of desired characters by crossing and breeding them.

  • A variety of disease resistant and high yielding crops, various new breeds of dogs, horses, cows, hens, etc., have been produced.


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