Useful Notes on 4 Most Important Theories Formulated by Mendel

These two parents with contrasting characters are crossed. The red flowered parent produces only one type of a gamete; all of them carry only the R factor (During gametogenesis; there will be reduction division hence the gametes carry only one factor for every character.)

While the white parent also produces only one type of a gamete; all of them carry only the factor r with reference to flower color.

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When these two gametes fuse to form the F progeny genetically it will be Rr. This plant appears red phenotypically. According to Mendel when a pair of contrasting characters is crossed only one character makes its appearance in the F1 generation and the other does not show morphologically in spite of being present in the genotype.

The character appears in the F1 generation is called Dominant and the other recessive. The F1 plant is genotypically a hybrid or heteorzygote for it carries factors for both red and white. This is called monohybird (hybrid for one character)

When the F plants are allowed to self fertilize, two types of gametes are formed both on the male and female side. Since the F, genotypically has Rr during meiosis 50% of gametes would carry R and the rest carry r.

During self fertilization there is random mating of these gametes. The gamete carrying R can mate with either R or r, with the result three combinations are possible RR, Rr, and rr in the ratio of three – RR, Rr, Rr (red) to one rr (white). Of the three red plants, one is pure line and the other two are hybrids.

These results made Mendel to formulate the following laws:

1. Theory of Particulate Inheritance:

Characters from parents to offspring are transmitted in the form of discreet units or particles called factors (genes). These factors determine the characters in the individual. Two such factors are present in the individual’s body, while gametes have only one (in diploid organisms).

2. Law of Purity of Gametes:

Irrespective of the fact whether the individual is homozygous or heterozygous for a character, the gamete would always carry only one factor in the uncontaminated form for instance when a hybrid having two different factors in its genotype forms the gametes, they carry these factors in two different gametes without any alteration.

3. Law of Dominance:

When a pair of contrasting characters is crossed, only one character makes its appearance in the F1 generation.

This is known as the dominant character and the other, which does not show itself, in spite of being present in the genotype, is called the recessive character. According to Mendel, all characters exist in two alternatives in diploid organisms.

This is due to the fact that the two homologous chromosomes carry the two alleles. In the case of flower color, R and r are alleles. R is the original form hence dominant and r is the mutated form hence recessive.

4. Law of Segregation:

According to this, when a pair of contrasting factors are brought together as in a hybrid, in the next generation, they segregate to different gametes without undergoing any mixing or alteration or combining.

Thus when a cross of contrasting characters are made, even though it appears that the two characters are mixed; in the F2 generation, the two of them segregate as in seen in RR and rr.

According to Mendel (in the above Cross) the two characters brought together in the F, hybrid did not combine, but just remained together and at the time of next reproduction (to produce F2) they segregate.

The fact that there is no mixing of red and white in the F is proved by their reappearance in the F2 in their original parental form.