MENDEL'S LAW OF INHERITANCE

Mendel's Laws of Inheritance :- 

 Theoretical model of Mendel with the chromosome theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary material, and create what is now known as classical genetics, which was extremely successful and cemented Mendel's place in history.

Mendel's Laws of Inheritance

Mendel postulated three laws, which are now called after his name as Mendel’s laws of heredity. These are:

1. Law of dominance and recessive

2. Law of segregation

3.Law of independent assortment

1. Law of Dominance :- 

Definition:- 

When two homozygous individuals with one or more sets of contrasting characters are crossed, the characters that appear in the F1 hybrids are dominant characters and those do not appear in F1 are recessive characters.

- If there are two alleles coding for the same trait and one is dominant it will show up in the organism while the other won't

Explanation : 

The dominance and recessive of genes can be explained on the basis of enzymatic functions of genes.

-  The dominant genes - are capable of synthesizing active polypeptides or proteins that form functional enzymes, whereas the recessive genes (mutant genes) code for incomplete or non-functional polypeptides.

-  Therefore, the dominant genes produce a specific phenotype while the recessive genes fail to do so. In the heterozygous condition also the dominant gene is able to express itself, so that the heterozygous and homozygous individuals have similar phenotype. 

Importance of law of dominance :-

The phenomenon of dominance is of practical importance as the harmful recessive characters are masked by the normal dominant characters in the hybrids. 

- In Human beings a form of idiocy, diabetes, haemophilia etc. are recessive characters. A person hybrid for all these characteristics appears perfectly normal. 

- Thus harmful recessive genes can exist for several generations without expressing themselves. Exceptions to Law of Dominance is the Incomplete Dominance. After Mendel several cases were recorded by scientists, where F1 hybrids exhibited a blending of characters of two parents.

- These hybrids were found to be midway between the two parents. This is known as incomplete dominance or blending inheritance. It means that two genes of the allelomorphic pair are not related as dominant and recessive, but each of them expresses itself partially. 

- As for example, in four-o'clock plant, Mirabilis jalapa, when plants with red flowers (RR) are crossed with plants having white flowers (rr), the hybrid F1 plants (Rr) bear pink flowers.

-  When these F1 plants with pink flowers are self-pollinated they develop red (RR), pink (Rr) and white (IT) flowered plants in the ratio of 1 : 2 : 1 (F2 generation).

2. Law of Segregation (Purity of Gametes):- 

Explanation - 

The law of segregation states that when a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote (hybrid) the two members of the allelic pair remain together without being contaminated and when gametes are formed from the hybrid, the two separate out from each other and only one enters each gamete.

Example - 

Pure tall plants are homozygous and, therefore/possess genes (factors) TT; similarly dwarf possess genes tt.

-  The tallness and dwarfness are two independent but contrasting factors or determiners. Pure tall plants produce gametes all of which possess gene T and dwarf plants t type of gametes. 

- During cross fertilization gametes with T and t unite to produce hybrids of F1 generation. These hybrids possess genotype Tt.

-  It means F1 plants, though tall phenotypically, possess one gene for tallness and one gene for dwarfness. Apparently, the tall and dwarf characters appear to have become contaminated developing only tall character. 

- But at the time of gamete formation, the genes T (for tallness) and t (for dwarfness) separate and are passed on to separate gametes.

-  As a result, two types of gametes are produced from the heterozygote in equal numerosity. 50% of the gametes possess gene T and other 50% possess gene t. 

- Therefore, these gametes are either pure for tallness or for dwarfness. (This is why the law of segregation is also described as Law of purity of gametes).

- Gametes unite at random and when gametes are numerous all possible combinations can occur, with the result that tall and dwarf appear in the ratio of 3:1. The results are often represented by Punnett square.

3. Law of Independent Assortment:- 

Definition:

The inheritance of more than one pair of characters (two pairs or more) is studied simultaneously, the factors or genes for each pair of characters assort out independently of the other pairs. Mendel formulated this law from the results of a dihybrid cross.

Explanation:

The cross was made between plants having yellow and round cotyledons and plants having green and wrinkled cotyledons.

- The F1 hybrids all had yellow and round seeds. When these F1 plants were self fertilized they

produced four types of plants in the following proportion:

(i) Yellow and round

(ii) Yellow and wrinkled 3

(iii) Green and round 3

(iv) Green and wrinkled 1

The above results indicate that yellow and green seeds appear in the ratio of 9 + 3 : 3 + 1 = 3 : 1. Similarly, the round and wrinkled seeds appear in the ratio of 9 + 3 : 3 +1 = 12:4 or 3 :1. 

This indicates that each of the two pairs of alternative characters viz. yellow-green cotyledon colour is inherited independent of the round-wrinkled character of the cotyledons. 

It means at the time of gamete formation the factor for yellow colour enters the gametes independent of R or r, i.e, gene Y can be passed on to the gametes either with gene R or r.

Cytological explanation of the results: In the above experiment yellow and round characters are dominant over green and wrinkled characters which can be represented as follows:

(i) gene for yellow colour of cotyledons Y

(ii) gene for green colour of cotyledons y

(iii) gene for round character of cotyledons R

(iv) gene for wrinkled character of colyledons r

Therefore, plants with yellow and round cotyledons will have their genotype YYRR and those with green and wrinkled cotyledons will have a genotype yyrr.

 These plants will produce gametes with gene YR and yr respectively. When these plants are cross pollinated, the union of these gametes will produce F1 hybrids with YyRr genes.

 When these produce gametes all the four genes have full freedom to assort independently and, therefore, there are possibilities of four combinations in both male and female gametes.

(i)RY       (ii) Ry      (iii) rY        (iv) ry

This shows an excellent example of independent assortment. These gametes can unite at random producing in all 16 different combinations of genes, but presenting four phenotypes in the ratio of 9: 3: 3: 1.

Dihybrid ratio : 

RR yy - Round, yellow seeded ; Rr yy - Wrinkled and greed seeded