Short essay on the Principles of Mendelian Inheritance (Mendelism)



(A) Principle of Unit Characters:

An individual is made up a large number of characters, each of which behaves as a unit.

(B) Principle of Paired Factors:

A trait or character is represented genetially in an individual by two factors present on the some locus in the two homologous chromosomes. The two factors may represent the same expression of the trait. (TT for tallness, tt for dwarfness) or both expressions as in hybrid (T/ for height/tallness) though only one is evident externally. Factors representing the different expressions of a trait are known as alleles/ allelomorphslallelomorphic pair. These days, the two similar factors of a trait/gene are also called alleles.

(C) Principle of Dominance:

In a hybrid, where both the contrasting alleles are present, only one factor/allele called dominant is able to express its effect while the other factor/allele called recessive remains suppressed. In a cross between pure/true breeding red flowered (RR) Pea plant and white flowered (rr) Peal plant, the F, generation is red flowered though it has received both the factors (R & r). It is because of the dominant nature of factor for red flower colour and recessive nature of the factor for white flower colour. On self breeding, the recessive trait reappears in the F2 generation showing that it is suppressed in F, generation and not lost.

(D) Law of Segregation (Principle of Purity of Gametes/First Law of Mendelism):

Mendelian factors or alleles of a trait are particulate entities which keep their identity in the hybrid, separate out at the time of gametogenesis randomly passing to different gametes for random pairing during passage to offspring.

In a monohybrid cross, say between a pure tall pea plant and dwarf pea plant, F, or hybrids are all tall. On self breeding them, both tall and dwarf appear in the ratio 3: 1 (787 tall to 273) dwarf in Mendel's experiment). Further self breeding shows that dwarf plants breed true. Amongst tall, one third are true while two third (one half of total progeny) behave as hybrids. This shows that phenotypic monohybrid ratio of 3 : 1 is genotypically 1 pure tall : 2 hybrid tails : 1 dwarf. Therefore, F1 plants or hybrids carry both the factors and alleles which separate out in gemetes, one half from each parent having the factor for tallness and the second half the factor for dwarfness.

Since gametes carry only one factor of a trail, the principle of segregation is also called principle of purity of gametes. It can be proved by taking those expamples in which the gametophytes or pollen grains carry specific genctic traits, e.g. starchy and waxy (= glutinous, lacking starch), pollen in maize and rice.

When true breeding starchy pollen bearing variety is crossed with true breeding waxy pollen bearing plant, the offspring or F, plants (diploid) produce pollen (haploid) through meiosis of both the types, starchy and waxy in the ratio I : I indicating that the two mendelian factor segregate in the ratio of 1 : 1 at the time of sporogenesis/gametogenesis and that a gamete/gametophyte contains single factor of a trait so that 50% of them contain one factor while the other 50% possess the second factor.

(E) Law of Independent Assortment (Second Law of Mendelism):

The two factors of each trait assort at random and independent of the factor of other traits at the time of meiosis (gametogenesis / sporogenesis) and get randomly as well as independently rearranged in the offspring. In monohybrid cross, the F organisms produce two types of male and two types of female gametes, so that there is possibility of four types of gamete combinations.

In a dihybrid cross, if the gene/factors are to assort independently, F, organism will produce four types of male (2 x 2, e.g. RY, R_y, rY and ry) and four types of female gametes with 16 types of gametic recombinations (4 x 4). Mendel crossed genetically pure yellow round seeded (YYRR) pea plant with green wrinkled seeded (yyrr) pea plant. F, plants were all yellow and round seeded.

In F2 generation, four types of plants appeared-315/556 or 9/16 yellow rounded, 101/556 or 3/16 yellow and wrinkled, 108/ 556 or 31/6 yellow rounded and 32/556 or 1/16 green wrinkled. The ratio of seed colour (416/556 or 3/4 yellow: 140/556 or 1/4 green) and seed texture (423/ 556 or 3/4 rounded: 133/556 or 1/4 wrinkled) or individual traits is 3 : 1 showing that the factors for each trait behave independent of the factors of the other trait.

Law of Independent assortment is applicable to only those factors or genes which are present on different chromosomes. Chromosomes have hundreds of genes which show linked inheritance or linkage. Mendel himself found linkage between flower and seed colour (white flower and white seed, red flower and grey seed) in pea.

Post Mendelian Discoveries are:

(a) Gene Interaction

(b) Incomplete Dominance

(c) Overdominance (super dominance)

(d) Codominance

(e) Multiple Alleles

(f) Blood group Inheritance

(g) Isoalleles

(h) Pseudoalleles

(i) Lethal genes

(j) Suppressor/Inhibitor Gene

(k) Epistasis

(l) Complementary genes

(m) Supplementary genes

(n) Duplicate genes

(o) Polymeric/Additive Genes

(p) Multiple factors

(q) Pleiotropic genes

(r) Qualitative Inheritance

(s) Quantitative/Poly Inheritance

(t) Pedigree Analysis