Unit – II B – 01

Mendel’s Experiment (Monohybrid Cross)

Logic of Mendel’s Experiment:

  • Initially, Mendel considered inheritance of one trait only. (Monohybrid). Then he studied two traits together (dihybrid) and then three (Trihybrid).
  • He started with pure line i.e. true breeding. He maintained a complete record by counting an actual number of offspring. He conducted ample crosses and reciprocal crosses to eliminate chance.


Monohybrid Cross:

  • A cross between two pure (homozygous) pattern in which inheritance pattern of only one of contrasting characters is studied is called monohybrid cross.
  • It is a cross between two pure (obtained by true breeding) parents differing in a single pair of contrasting characters.

Procedure of Monohybrid Cross Experiment

Step – 1: Selection of parents and obtaining Pure lines:

  • He selected pure line plants by ensuring that the selected male (pure dwarf) and female parent plants (pure tall) are breeding true for the selected trait or traits by selfing them for three generations.

Step – 2: Emasculation, Dusting and Raising F1 Generation:


  • Emasculation

Emasculation is a process of removal of stamens before the formation of pollen grains (anthesis). This is done in bud condition. The bud is carefully open and all stamens (9 + 1) are removed carefully. The stigma is protected against any foreign pollen with the help of muslin bag.

  • Dusting and Raising F1 Generation:

    The pollens from selected male flower are dusted on the stigma of emasculated female flower. During the pollination, it was assured that the pollen is mature and the stigma is receptive.This is artificial cross. Mendel crossed many flowers, collected seeds and raised F1  generation. All plants of F1 generation were tall.

  • Punnett Square for F1 Generation:

T (tall) is a dominant character t (dwarf) is a recessive character.



  • Reciprocal Cross:
  • He thought F1 Generation is tall because tallness character is given by female parent and dwarfness character is given by a male parent.
  • To counter check it he performed reciprocal cross. Now, He selected pure line plants by ensuring that the selected male (pure tall) and female parent plants (pure dwarf) are breeding true for the selected trait or traits by selfing them for three generations.
  • He got the same result as in the first case. From this, he concluded that tallness is a dominant character, while the dwarfness is the recessive character.
  • The plants obtained from the crossing of two individuals differing at least one set of characters are known as hybrids and the process of obtaining them is called hybridisation.
  • Punnett Square for (Reciprocal Cross) F1 Generation:

T (tall) is a dominant character t (dwarf) is a recessive character.

Step – 3: Selfing of F1 hybrids to produce F2 Generation:

  • Mendel allowed natural pollination in each F1 hybrid; collected seeds separately and F2 generation is obtained. The ratio of tall plants to dwarf plants in F2 generation is found to be 3: 1
  • Punnett Square for F2 Generation:

T (tall) is a dominant character t (dwarf) is a recessive character.

  • The ratio of tall plants to dwarf plants was around 3:1. Thus phenotype ratio (tall : dwarf) is 3: 1.
  • The genotype ratio (pure tall : hybrid tall : pure dwarf) is 1:2:1.


Checking With Other Traits:

  • Mendel performed monohybrid crosses and reciprocal crosses with all the seven pairs of contrasting characters separately and obtained similar results.
  • Only one of the two characters was expressed in F1 generation. In F2 generation the character which was shown in F1 generation was in large number and the other in small number and the ratio was found to be 3:1. This ratio is called monohybrid ratio.

Genotype ratio:

  • The ratio of pure dominant character to hybrid character to pure contrasting recessive character is called the genotype ratio.
  • In monohybrid cross experiment the genotype ratio for F2 generation is 1:2:1.

Monohybrid Ratio:

  • Monohybrid ratio is defined as the phenotypic ratio of different types of offsprings (dominant and recessive) obtained in F2 generation of a monohybrid cross.
  • In monohybrid cross experiment the phenotype ratio for F2 generation is 3:1.

Mendel’s Conclusions of Experiment:

  1. Characters such as a height of a stem, a color of seed etc. are inherited separately as discrete particles or unit. He called them a factor or a determiner. Now it is called a gene.
  2. Each factor exists in contrasting or alternative forms. For e.g. for the height of a stem, there are two factors one for the tallness and other for the dwarfness. These two forms of genes are called alleles.
  3. One of the factor is dominant and another factor is recessive. Only dominant factor expresses in F1 generation.
  4. In an organism, inheritance of each character is controlled by a pair of factors. One of the factors is contributed by the male parent and the other by female parent. Thus higher organisms are diploid (2n)
  5. From F2 generation Mendel concluded that in hybrid the two factors do not mix together but they just remain together.
  6. During gamete the formation, they separate or segregate and each gamete receives only one factor from each pair of factors. Thus gametes are haploid (n).

Diagrammatic Representation of Monohybrid Cross

Mendel's Experiment (Monohybrid Cross)



 

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