1) Law of Dominance or First law of Inheritance :
In a cross between two organisms pure for any pair ( or pairs ) of contrasting characters, the character that appears in F1 generation is called dominant and the one which is suppressed is called recessive. ( Thus there is always uniform expression in F1 ).
Each character is controlled by a pair of factors and when it is a pair of dissimilar factors only one factor expresses. Thus one is dominant and the other is recessive.
Law of dominance can be explained with the help of a monohybrid cross experiment.
some other examples of organisms and traits which show dominance are; in guinca pigs, black coat colour is dominant over white. In human beings, curly hair is dominant trait over straight hair and brown eye colour over green or blue. In jowar pearly grain is dominant over chalky grain and so on. However, there are many cases where the dominance is not complete or absent. We will learn about it later in this chapter under deviations from Mendelian inheritance. Phenomenon of dominance is significant as the harmful recessive traits are masked, not expressed in the presence of its normal dominant allele. For example, In humans a form of idiocy, and diabetes and hemophilia are recessive characters. Thus Law of dominance is significant and true but it is not universally applicable.
2) Law of segregation or Second law of Inheritance :
Members of allelic pair in a hybrid remain together without mixing with each other and separate or segregate during gamete formation. Thus gametes receive only one of the two factors and are pure for a given trait. Therefore, this is also known as Law of Purity of Gametes.
Law of segregation also can be explained with the help of monohybrid cross experiment.
Reappearance of recessive trait in F2 proves the law.
All sexually reproducing higher organisms are diploid (2n) i.e. with two sets of chromosomes and gametes are haploid (n) i.e. with one set of chromosomes. Therefore law of segregation is universally applicable.
Mendel's third law of inheritance is based on dihybrid cross.
Dihybrid Cross Experiments and Dihybrid Ratio :
Mendal established the principle of dominance and segregation on the basis of monohybrid crosses. However, in an organism, many characters are present an each is controlled by a pair of alleles. Does one pair of alleles affects / influences the inheritance pattern of other pairs of alleles? Or each pair is inherited independently as if in monohybrid cross? To find out answer, Mendal performed dihybrid crosses i.e. considered two characters / allelic pairs simultaneously and then trihybrid crosses.
Dihybrid cross :
A cross between two pure ( homozygous ) parents in which the inheritance pattern of two pairs pf contrasting characters is considered simultaneously is called dihybrid cross. Or it is a cross between two pure ( True breeding ) parents differing in two pairs of contrasting characters.
Dihybrid :
It is heterozygous for two traits and produced in a cross between two pure parents differing in two pairs of contrasting character.
Dihybrid ratio :
The phenotypic ratio of different types of offsprings ( with different combinations ) obtained in the F2 generation of dihybrid cross is called dihybrid ratio. It is 9:3:3:1 in all Mendelian crosses.
Mendel performed many dihybrid crosses with different combinations and every time he got consistent pattern of results and the same ratio in F2. From this he established the principle of independent assortment.
He followed the same three steps which is a classical experimental design that was followed by mant breeders after him and discovered different gene interactions which we will study in the second part of this chapter. Let us see his very first dihybrid cross experiment.
Step 1 : Mendel selected a variety of pea plant having yellow and round seeds as a female parent and another variety having green and wrinkled seeds as a male parent. He obtained pure lines by selfing. He comfirmed that yellow round is producing yellow round and green wrinkled is producing green wrinkled for three generations.
Step 2 : He performed artificial cross i.e. dusted pollen grain from flowers of pure male parent on stigma of emasculated flower of female parent. In due course fruits (pods) were formed; he opened all the pods and collected seeds. All seeds of F1 generation were yellow and round as expected. (Yellow is dominant over green and Round over wrinkled).
Step 3 : He sowed the seeds and raised the plants of F1 generation and allowed selfing. When pods were formed, he opened and collected seeds of F2 generation. He was expecting 75% yellow round and 25% green wrinkled seeds. But he was surprised to find that seeds were of four types i.e. F2 generation seeds showed four different phenotypes as follows-
Yellow round 315; Yellow wrinkled 108; Green round 101; green wrinkled 32. ( Total = 556 ). This is in 9:3:3:1 ratio.
Out of different types, two were parental combinations ( Yellows round, green wrinkled ) and two were new / recombinations ( Yellow wrinkled, green round).
Mendel performed ample dihybrid crosses and reciprocal crosses with different combinations. Every time he got same, pattern of results i.e. uniform expression- both dominant in F1 generation. In F2 generation always he got both dominant in large number; one dominant and the other recessive in less number; the other dominant and the first one recessive also equally less (approximately equal) in number and both recessive were least in number. Thus in all Mendelian dihybrid crosses the ratio in which four different phenotypes occurred was 9:3:3:1 which is called dihybrid ratio.
Mendel applied the principle of probability in statistics, i.e. 'Probability of two independent event occurring simultaneously is the product of their individual probabilities.'
Dihybrid ratio is a product of two monohybrid ratios. (3:1) X (3:1) = 9:3:3:1.
From this Mendel concluded that inheritance of seed colour and inheritance of seed shape are the two independent events. Therefore, both dominants occurred in large number and both recessive in the least. The different phenotypes / combinations occur in the ratio 9:3:3:1 as follows;
Yellow round 3x3 = 9
Yellow wrinkled 3x1 = 3
Green round 1x3 = 3
Green wrinkled 1x1 = 1
If we consider ratio of (proportion of) yellow and green in F2 , it is yellow 9+3 = 12; green 3+1 = 4. Therefore the ratio is 12:4 i.e.3:1. Similarly ratio of round (9+3) and wrinkled (3+1) is 12:4 i.e. 3:1.
From the appearance of new combination in the F2 generation, uniform pattern of results of dihybrid cross and dihybrid ration, Mendel established principle of independent assortment. He performed trihybrid crosses also and then concluded that, when a dihybrid (or a polyhybrid) forms gametes, each gamete receives only one allele from each pair (due to segregation) and the assortment (distribution) of alleles of different traits is totally independent of their parental combinations i.e. each allele of any one pair of alleles is free to enter the gamete with any allele from each of the remaining pairs of alleles.
Thus a dihybrid forms four types of gametes due to segregation and independent assortment. How many different genotype can be there? To find out all possible genotypes let us use the Punnett Square / Checker board method.