We have read Mendel’s Laws of Inheritance in sexually reproducing organisms through his experiments and observations of the garden pea-plant. The crossing of Filial Generation proved to Mendel that genes which carry information for a particular trait, can be dominant and recessive and the dominant gene is the one which expresses its characteristic in the offspring. Also, when meiosis occurs it is the single allele from each parent which combines to form the genotype of the offspring. 

A major factor is that Mendel only considered the mixing of a single gene till the formulation of these laws. When two varied characters were hybridised and observed it was seen that it is not always true that only one gene will dominate completely. 

Terminology used: 

Incomplete Dominance, Co-dominance, Monohybrid Cross, Dihybrid Cross

Taking the Dragon Fruit flowers of two different colors , which is - red and white. A cross was carried out but this time the offspring were neither red nor white but pink.

This led to the deduction that genes are merely information careers existing in pairs in the parent. This pair can either be homozygous (both alleles are similar: TT or tt for height) or heterozygous (both alleles are dissimilar: Tt).

Dominance:

In self-pollination of the heterozygous pair the resulting Phenotype can show mixed results for a particular trait. This phenomena (as seen in dragonfruit) is termed as Incomplete Dominance.

In the previous experiment the F1 Generation had traits resembling to either of the two parents. In incomplete dominance. it was a little bit of both and in Co-dominance, alleles for a single trait mix and are expressed at the same time. A perfect example is human beings who have the blood group ABO.

The alleles responsible for blood groups are I^A, I^B , t. If I^A, I^B , t is expressed the resulting blood group is A, B, O. But in the case of co-dominance both I^A, I^B are simultaneously expressed as blood grup AB.

Inheritance of Two Genes, Law of Independent Assortment

While studying a single gene or trait, Mendel carried out a Monohybrid Cross. This is the cross where the parent plants both are homozygous. To study, inheritance of two genes, Dihybrid Cross was done. This is the one in which the cross is carried out between plants differing in two traits.

For example round and yellow peas are crossed with wrinkled and yellow pea variety. It is a hybrid cross as the traits in question are texture and color of the pea.

Law of Independent Assortment: The above Punnett Square [graphical representation of the probability of various genotypes mixing in a cross, developed by Reginald C. Punnett] shows how two genes are mixing each other. Those are represented as:

R = round

r = wrinkled

Y = yellow

y = green

As per the Law of Independent Assortment the genes of color will not interfere with the genes of texture and hence, the genes assort or combine independently.

Hence, through series of experiments and exhaustive analysis, Mendel was able to lay down a general foundation for the working of genes, their mixing with one another and the resulting characteristic in diploid (having chromosomal pairs) sexually reproducing organisms.