Mendelian Principles

Johann Gregory Mendel was an Australian monk, who conducted several experiments with pea plants in the garden. He found several contrasting characteristics in pea plants such as tall and dwarf plants, yellow & green cotyledons, and round and wrinkled seeds etc. These characters occurred repeatedly generation after generation because pea plants are self pollinated. In other words, these characters were breeding true. Mendel chose garden pea as his experimental organism as it is an annual plant with well defined characteristic and grown & can be crossed easily. Mendel was fortunate in his choice of a diploid organism. Through many generations of natural self-fertilization, garden peas had developed into pure lines. A single alteration in a trait was, therefore demonstrated by a visible difference between varieties. Furthermore, in seven pairs of contrasting traits Mendel chose to study, one form was dominant over a well defined contrasting traits Mendel chose to study, one form was dominant over a well-defined contrasting alternative. Mendel studied these characters in his study:

a)      Vines – tall or dwarf

b)      Unripe pods – green or yellow

c)       Pods – inflated or constricted between the seeds

d)      Flowers – auxiliary or terminal

e)      Cotyledons – yellow or green

f)       Seed coats – smooth or wrinkled

g)      Seed coats – grey or white ( The plants which had white flowers produced white seed coats, whereas the plants which had violet flowers produced grey seed coats)

Mendel’s success in his first experiments may be attributed to his good judgment in making crosses, as far as possible, between parents that differed only in one trait. When this not feasible, he considered only one trait at a time.

Mendel affected a cross between two plants with contrasting characters. He took a tall variety and crossed it with dwarf variety. He obtained the seeds of this cross and grew them separately. All the seeds produced only tall plants. Then, when these plants were self pollinated, the next generation had tall and dwarf plants in the ratio of 3:1 with the other contrasting characters also, he obtained the same results in the second generation.

Detailed investigations by S.Bixit on pea led to the location of Mendel’s seven selection characters on four different chromosomes; two on chromosome 1, three on chromosome 4 and one each on chromosome 5 & 7.

Each parent had two factors responsible for any character in them. Thus, the tall parent had TT and the dwarf parent had tt. The gametes had only one factor. The gamete from the tall parent had one T, and the gamete from the dwarf had one t. When the gametes joined in fertilization, the zygote had one factor from each parent i.e. T and t. The generations formed by crossing was called first filial generation or F1 generation. All these of F1 generation grew into tall plants even though they had one factor for tallness and one factor for dwarfness. Mendel explained this by saying that the factor for Tallness or T was dominant over the factor for Dwarfness or t. The dominant factor expressed itself while the dwarfness factor did not express itself and was called a recessive factor.

In the next cross between the F1 generations, 50% of the gametes produced by the parent had tall factor T and 50% had the dwarf factor t. When fertilization took place among gametes, 3 types of combinations were formed such as TT, Tt and tt. That is in the second filial generation, 25% of the plants had both dwarf factors. In these plants, the one that had both factors and 25% of the plants had both dwarf factors. In these plants, the one that had both dwarf factors were called homozygous for tallness and the plants that had both dwarf factors were called homozygous for dwarfness. The ones with one tall factor and one dwarf factor were called heterozygous. In external appearance, the tall plants formed 75% and the dwarf plants 25%. This is called the phenotypic ratio (3:1) though genetically, there were 3 types namely homozygous tall (25%), heterozygous tall (50%) and homozygous dwarf (25%). This ratio 1:2:1 is called genotypic ratio. As only one character (in this case is the height) was taken into account, it is referred to as Monohybrid ratio

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