From time immemorial, the phenomenon of the transmission of parental characters into the offspring has fascinated the imagination of human mind. How transmission of characters take place from generation of humankind. How transmission of characters take place from generation to generation was remained a mystery of life until the work of Mendel was re-discovered.
First scientific explanations on the fact were given by Gregor Johann Mendel. Mendel has explained the entire mechanism of inheritance while conducted the experiments on cross hybridization among different varieties of garden pea. He conducted his experiment for seven ears and published the work under the annual proceedings of the “Natural history society of brunn in 1865)”. But his experimental papers was heavily criticized and it was heard heaved to left benefit. Mendel died in 1884 and up to almost 16 years no one paid attention to his work.
In 1900 three scientists named Correns, de Vries and Tchermes separately worked on Mendel’s experiment and came to same conclusion that Mendel got since then Mendel’s work was came to light. The findings of Mendel were enlightened n terms of laws of inheritance.
Laws of Inheritance:
Mondel’s 1st law of inheritance:
Mondel’s a first law of inheritance is the law of segregation or principles of unit character. This law states that, “Paired factors or alleles segregate from each other during formatting fametes and pass into different gametes so that a gametes and pass into different gametes so that a gamete carries either of the dominant or the recessive factor but never the both. ” This law is the derived conclusion of three basic principles drawn from. Mendel’s monohybrids cross experiments.
i. Principle of unit character.
ii. Principle of dominance.
iii. Principle of purity of gametes or segregation.
i. Principle of unit character:
Phenotypic or visible characteristic an individual organism is determined by at least two mendelia factors called as genes. Principle of unit characters signify that, each character of an organism is carried and transmitted through a specific unit factor called genes. Thus genes are unit of character. One character is governed by a pair of genes which called unit of character.
In Mendel’s experiment he selected a number of separate traits in same species of graded pea and determined that each trait carried by a unit of character. For example Tallness phenotypic trait is carried by “TT” factors or genotype and dwarf ness is carried by “tt” genotype.
ii. Principle of dominance: (Monohybrid experiment)
Mendel in his monohybrid experiment selected a pair of contrasting characters or alleles in pea variety one is “Tall” and other is dwarf with genotype “TT” and “tt” respectively. He made artificial crosses between these two varieties. In first felial (F1) generation be got the offsprings all having relines character. No dwarf plant is generated.
Thus F1 plant is a hybrid tall having both the factors ‘T’ which tallness and’t’ for dwarfness in their gneotype and phenotypically expressed as tall plant.
Principle of dominancy says that, the character in hybrid which is phenotypically expressed is dominant allele over the character which has not phenotypic expression or suppressed. In above cross rall character is called as dominant allele and dwart is called recessive allele.
Mendel further suggested that, the factor for domination of a character when come in combination of recessive factor the dominant factor masks over the recessive factor. When the recessive factors remain alone, only they ability to be expressed phenotypically. One allele is dominant over its co allele in a hybrid.
iii. Principle of purity of gametes:
Principle of purity of gametes was derived from Mendel’s monohybrid experiment. While Mendel made cross between Apure tall variety of pea plant and a pure dwarf variety, he got all tall plants in F1 generation which are hybrids containing both dominant and recessive alleles, and called as heterozygotes. For F2 second fellial generation he took F1 hybrids as the parental generation and made crossed between the gametes produced y the hybrid.
From above experiment it was found out that when F1 hybrids tall produce its gametes the alleles are segregated from each other. The gamete of F2 generation carries either one of the factor for tallness or dwarfness. The genotype of F2 gametes is either “T” or “t”. This is due to the separation of maternal and paternal genes during meiosis. Thus gametes are produced from a hybridis always genetically pure. A gamete may carry either recessive or dominant factor, not the both. Therefore in F2 recombination ratio of dominant and recessive characters appeared in th eratio of 3:1.
Thus from Mendel’s monohybrid experiment first law of inheritance, the law of segregation was derived. Later the caseis varified in number of plants and animals and it ecome universalized that no gamete will have either homologous chromosomes or both allelic forms.
(II) Mendel’s second law of inheritance of law of independent assortment:
The law of independent assortment was derived from dihybrid cross experiment done by Mendel on pea plants. The law states that.
When he gametes are formed from the parents having two pairs of contrasting characters the members of the different pair of factors or genes segregate quite independent of one another and the factors are recombined independently to produce all possible recombination among the progeny.
When two pairs of contrasting characters are taken in a parental generation then only dominant character appears in F1 generation. But upon inbreeding or self pollination in F2 generation the characters are assorted independently i.e. segregated and recombined in independent manner irrespective their previous pair in heir parents.
Mechanism of independent assortment was clearly explained in the following experiment done by Gregor Mendel. In the dihybrid cross between two pea varieties i.e yellow-round seeded and green wrinkle seeded he assumes that alleles of yellow-roundseed are “YY” and “RR” and alleles of Wrinkle-green seed are “yy” and “n”.
From above experiment following conclusions are draw to deduce the law of independent assortment.
i. In F1 generation the offspring obtained are all hybrids showing round seeded with yellow cotyledon having genotype. Here all four Mendelian factors fro yellow and green colouration round and wrinkle shape. But in combination only dominant alleles are phynotipically expressed.
ii. While these hybrids were subjects to be self pollinated during formation of gametes the factors are segregated in mendelia fashion so that four types of gametic recombination are obtained. Genotypes of F2 gametes are YR, Yr, yR and yr.
iii. During F2 generation when these four types of gametes unite in fertilization then there are four possible recombination appeared in offsprings. The factors randomly and independently recombined to produce four types of phenotypesin sixteen recombinations in the ratio of 9:3:3:1.
Thus in the above cross alleles of one set behaved independent with respect to those of the other set at the time of combination during fertisation.
iv. In fact, round character appeared in combination with green and wrinkled character appeared in combination with yellow characters.
Thus law of independent assortments justifies an applicable to all type of dihybrid crossed in plants and animals.