● Let us take the example of one such `color{violet}"hybridisation experiment"` carried out by Mendel where he crossed tall and dwarf pea plants to study the `color{violet}"inheritance of one gene"`.

● He collected the seeds produced as a result of this cross and grew them to generate plants of the `color{violet}"first hybrid generation"`.

● This generation is also called the `color{violet}"Filial 1 progeny"` or the `color{violet}(F_1)`.

● Mendel observed that all the `F_1` progeny plants were tall, like `color{violet}"one of its parents"`; none were dwarf .

● He made similar observations for the other pairs of traits – he found that the `F_1`always resembled either `color{violet}"one of the parents"`, and that the trait of the other parent was not seen in them.

● Mendel then `color{violet}"self-pollinated the tall"` `F_1` plants and to his surprise found that in the `color{violet}"Filial2 generation"` some of the offspring were `color{violet}"dwarf"` ; the character that was not seen in the `F_1` generation was now expressed.

● The proportion of plants that were dwarf were `color{violet}(1/4th of the F_2)` plants while `color{violet}(3/4th of the F_2 plants)` were tall.

● The tall and dwarf traits were `color{violet}"identical to their parental"` type and did not show any `color{violet}"blending"`d, that is all the offspring were either tall or dwarf, none were of in between height.

● Similar results were obtained with the `color{violet}"other traits"` that he studied: only one of the parental traits was expressed in the `F_1` `color{violet}"generation"` while at the `F_2` stage both the traits were expressed in the proportion 3:1.

● The contrasting traits `color{violet}"did not show"` any `color{violet}"blending"` at either`F_1` or `F_2` stage.

● Based on these observations, Mendel proposed that something was being `color{violet}"stably passed down"`, unchanged, from parent to offspring through the gametes, `color{violet}"over successive generations"`.

● He called these things as `color{violet}"factors"` Nowadays, we call them as `color{violet}"genes"`.

● Genes, therefore, are the `color{violet}"units of inheritance"`.

● They contain the information that is required to `color{violet}"express a particular trait"`, in an organism.

● Genes which code for a `color{violet}"pair of contrasting traits"` are known as `color{violet}"alleles"`, i.e., they are slightly different forms of the same gene.

● If we use `color{violet}"alphabetical symbols"` for each gene, then the capital letter is used for the trait expressed at the `F_1` stage and the small alphabet for the other trait.

● For example, in case of the character of height, `color{violet}"T is used for the Tall"` trait and `color{violet}"t for the dwarf"`, and T and t are alleles of each other.

● Hence, in plants the pair of alleles for height would be `color{violet}"TT, Tt or tt"`.

● Mendel also proposed that in a `color{violet}"true breeding"`, tall or dwarf pea variety, the `color{violet}"allelic pair"` of genes for height are `color{violet}"identical or homozygous"`, `color{violet}"TT and tt"`, respectively.

● TT and tt are called the `color{violet}"genotype"` of the plant while the `color{violet}"descriptive terms"` tall and dwarf are the `color{violet}"phenotype"`.

● As Mendel found the phenotype of the `F_1` `color{violet}"heterozygote"` Tt to be exactly like the TT parent in appearance, he proposed that in a `color{violet}"pair of dissimilar factors"`, one `color{violet}"dominates the other"` (as in the `F_1` ) and hence is called the `color{violet}"dominant factor"` while the other factor is `color{violet}"recessive"` .

● In this case `color{violet}"T"` (for tallness) is `color{violet}"dominant"` over `color{violet}"t"` (for dwarfness), that is recessive.

● He observed `color{violet}"identical behaviour"` for all the other `color{violet}"characters/trait-pairs"` that he studied.

● It is convenient (and logical) to use the `color{violet}"capital and lower case"` of an alphabetical symbol to remember this concept of dominance and recessiveness.

● NOTE: (We should `color{brown}"not use"` `color{violet}"T for tall"` and `color{violet}"d for dwarf"` because you will find it difficult to remember whether T and d are alleles of the same gene/character or not).

● Alleles can be similar as in the case of `color{violet}"homozygotes TT and tt"` or can be dissimilar as in the case of the `color{violet}"heterozygote Tt"`.

● Since the Tt plant is `color{violet}"heterozygous"` for genes controlling one character (height), it is a `color{violet}"monohybrid"` and the cross between TT and tt is a `color{violet}"monohybrid cross"`.