Gregor Johann Mendel born in 1822 at Brunn in Austria was the first person to conduct systematic experiment to understand transmission of characters over generations.He worked on inheritance of Sweet Pea (Pisum sativum) plant from 1857 to 1865( 8 years ). He presented his paper before Natural History Society of Brunn. In 1866 the paper was published in the Proceedings of the society.No one paid attention to his work. Mendel passed away in 1884 without getting any recognition of his work. In 1900 three scientist Hugo De Vries (Holland) , Erik Von Tschermak (Austria), Carl Correns (Germany) came to same conclusion ns which Mendel arrived 35 years back and rediscovered his paper and named as Mendel's laws of inheritance in his honour. Since then he is recognized as Father of Genetics. Batson later on published a book entitled as Mendel’s principles of Genetics.
His work was based on hybridization that is artificial cross pollination with an aim to introduce specif Principles of Geneticic trait.Garden pea had many sharply noticeable alternating forms of traits such as the plants are either tall (2m) or dwarf (0.5m) . The plants bear either purple or white flowers, simple or constricted pods, round or wrinkled seeds,green or yellow pods etc.In addition to these the experimental plants was having some plus points for hybridization as it is self pollinating and annual so pure lines are easily available and results can be recorded in less time.. In his preliminary experiment he rfpaid attention to only one trait at a time and named such attempt as monohybrid cross. Later on he started paying attention on two trait and their alternative forms at a time and termed ss uch attempts as dihybrid cross.
Traits and Alternative forms in experimental plant sweet pea (Pisum sativum )
S N | Trait | Alternative Forms | |
Dominant | Recessive | ||
1 | Height of stem | Tall (2m) | Dwarf (0.5m) |
2 | Shape of seed | Round | Wrinkled |
3 | Colour of cotyledon | Yellow | Green |
4 | Colour of pod | Green | Yellow |
5 | Shape of pod | Inflated | Constricted |
6 | Position of flower | Axial | Terminal |
7 | Colour of seed coat | Grey | White |
8 | Colour of flower | Purple | White |
Steps in the experiment : Monohybrid cross: Trait : Hight of plant : Tall vs Dwarf
· Testing of plants for purity in Inheriting traits as for example a tall plant should to give tall plants for 2 to 3 generations a dwarf plant to dwarf.
· Parental plants were treated as Male parent( Whose pollen grains were used) and Female Parent ( whose stigma was used) although are bisexual . Tall Male parent whose pollen were dusted on stigma of Dwarf Female parent.
· From plants which were considered as female parents ,the anthers were removed before anthesis (Maturation of pollen grains) the process is known as EmascsulationThe emasc.ulated female flowers were kept covered by porous paper bags to avoid undesirab le cross pollination.
· Hybridisation or artificial cross pollination was performed by dusting the pollen grains from flowers of male plants on stigma of flowers on female plants(Emasculated).
· Fertilized flowers were left on plants to set seeds.
· Seeds were collected separately from each plant.
· Seeds were sown next year in fields to raise F1 (First Fi1ial) generation plants.
· F1 generation plants were left to self pollinate.
· Seeds were collected separately from each plant of F1 generation.
· Seeds were sown next year in fields to raise F2 (Second Fi1ial) generation plants
· F2 generation plants were left to self pollinate.
· Seeds of F2 plants were collected and subsequently F3 generation plants were raised.
· In this way upto F7 plants were raised .
In brief the experiment included following steps : 1) Emasculation 2) Bagging 3) Seed setting 4) Collection of seeds 5) Raising of F1 generation plants 6) F1 plants self pollinated 7) Seed setting of F1plants 8) Collection of seeds from F1 plants 9) Raising F2 generation plants →F3Plants →F4 plants→F5 plants →→
Results of the experiment ( Monohybrid Cross between Tall male plant and female dwarf plant ) and Vice Versa
1. F1 Generation – All plants were tall (Dwarfness disappeared)
2. F 2 Generation – Tall and Dwarf plants both appeared in ratio approximately 3:1
3. F3 Generation – Of the tall plants of F2 some plants gave tall and some segregated into Tall and Dwarf in 3:1 ratio and dwarf plant gave only dwarf plants.
4. F4 Generation – Some tall plants of F3 generation gave Tall and some again segregated in 3:1 ratio.
Such Monohybrid crosses were made for all the seven traits and their alternative forms and in all cases the dominant form appeared in F1 generation, In F2 both Dominant and Recessive appeared in 3:1 ratio.
Genetic Expression and Analysis of Results of Monohybrid cross
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Parents Tall Male (TT) x Dwarf Female (tt)
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Parental gametes T t
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Hybridisation Tt ( Zygote →Seed) F1 Plants (All Tall )
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Hybrid Gametes(F1) Male → T(1) and t (2) Female→ T(3) and t(4)
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Self Pollination TT (1,3) Tt (1,4) Tt(2,3) tt (2,4) F2 Progeny ( Tall : Dwarf,3:1)
Tall Tall Tall Dwarf
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↕ ↕ ↕ ↕
TT TT Tt Tt tt TT Tt Tt tt tt F3 Progeny ( Tall : Dwarf, 7:3 )
Tall Tall Dwarf Tall Dwarf Dwarf ________________________________________________________________________________________________
Results of F2 generation show phenotypic ratio 3:1 and on the basis of observation of F3
generation the Genotypic Ratio F2 becomes 1:2:1 ( Pure Tall : Hybrid Tall :Dwarf )
Summary of F2 Data obtained by Mendel in Pea from different Monohybrid crosses
Trait | Parental cross | F1 Phenotype | F2 Phenotypes | Total | F2 Ratio | |||
Dominant | Recessive | |||||||
Seed Shape | Round x Wrinkled | Round | 5474 Round | 1850 Wrinkled | 7324 | 2.96:1 | ||
Cotyledon colour | YellowxGreen | Yellow | 6022 Yellow | 2001 Green | 8023 | 3.01:1 | ||
Seed Coat Colour | Grey x White | Grey | 705 Grey | 224 White | 929 | 3.15:1 | ||
Pod Colour | Green x Yellow | Green | 428 Green | 152 Yellow | 580 | 2.82:1 | ||
Pod shape | Inflated x Constricted | Inflated | 882 Inflated | 299 Constricted | 1181 | 2.95:1 | ||
Position of flower | Axial x Terminal | Axial | 651 Axial | 207 Terminal | 858 | 3.14:1 | ||
Length of stem | Tall x Dwarf | Tall | 787 Tall | 277 Dwarf | 1064 | 2.84:1 | ||
Colour of flower | Purple x White | Purple | ||||||
Mendel’s Conclusions
On the basis of the results of monohybrid cross Mendel Proposed that expression of each trait is by a pair of Unit factors and some thing is transferred from parents to offspring (Progeny ) which carried information for expression of the trait. This something is particulate in nature and was termed as Factor or determiner by Mendel. This Factor of Mendel is now termed as Gene. Genes are present linearly on specific loci (position) on homologous chromosomes in pairs. Each gene exist in two forms or alleles and govern expression of alternative phenotypes (Allelomorphs ) such as Tall and Dwarf.
- The F1 progeny from a cross of two pure lines contains one allele for the dominant phenotype and one for the recessive phenotype. These two alleles comprise the gene pair.
- Mendel on the basis of Monohybrid crosses for all the seven traits concluded that phenotypic form which appears in F1 generation is governed by Dominant allele which suppresses the expression of Recessive allele. This is known as Law of Dominance.This law states that in a cross between two plants pure for contrasting aspects ( Alternative forms ) of a trait the dominant one only expresses itself in F1 generation and recessive one remains hidden or is suppressed. As evident from a cross between a pure tall plant and pure dwarf plant , all plants were tall in F1 progeny.3. Law of segregation (Law of purity of gametes or law of Unit characters)
This law states that when a pair of alleles Alternative forms of a trait ) are brought together in a hybrid, they donot mix up or blend ,but remain associated with each other and separate or segregate from each other in next generation when hybrid forms gametes. As during gamete formation, each gamete receives only one of the allele (unit factor ) and after fertilization F2 plants can receive both alleles for tallness(Homozygous for tallness) or one allele for tallness and one for dwarfness(Heterozygous for tallness ) or both alleles for dwarfness (Homozygous for dwarfness). There in F2 progeny both tall and dwarf plants were resulted .
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