Genes are responsible for the characteristic features (or traits) of an organism: plant or animal. The characteristics or traits of parents are transmitted to their progeny (offsprings) through genes present on their chromosomes during the process of sexual reproduction. This happens as follows.
Genes work in pairs. There is a pair of genes for each characteristic of an organism (one is dominant gene and the other is recessive gene). Each parent possesses a pair of genes for each characteristic on a pair of chromosomes. However, each parent passes only one of the two genes of the pair for each characteristic to its progeny through gametes.
Thus, the male gamete and female gamete carry one gene for each characteristic from the gene pairs of parents (which are located on the pair of chromosomes). But when a male gamete fuses with a female gamete during fertilisation, they make a new cell called zygote with a full set of genes (on a full set of chromosomes). This zygote grows and develops to form a new organism having characteristics (or traits) from both the parents which it has inherited through genes.
The two genes (or pair of genes) responsible for a particular characteristic chromosome pair are always present on the corresponding positions of the pair of chromosomes. For example, in the two genes for the same characteristic (length of plant stem), are present on the corresponding positions of the pair of chromosomes.
One gene of the pair is for ‘tallness’ and the other is for ‘dwarfness’. Please note that though the progeny inherits two genes (or a pair of genes) for each trait from its parents but the trait shown by the progeny depends on which inherited gene is dominant of the two.
For example, if a pea plant progeny (or hybrid) inherits the gene for tallness (T) from one parent and the gene for dwarfness (t) from the other parent, then it will show the trait of ‘tallness’ and become a tall plant because the gene for tallness is dominant over the gene for dwarfness. So, although the gene for dwarfness (t) is present in all the cells of the hybrid plants, it does not show its effect (because it is a recessive gene). If, however both the parent plants pass on one copy each of the recessive gene for dwarfness (t) making the genotype (tt), then the traits of dwarfness will appear in the progeny plant.
Please note that the genes for ‘tallness’ and ‘dwarfness’ are not to be considered two different genes. They are just the two forms of the same gene which controls only one characteristic feature of a plant: length of its stem. But there can be increase in length of stem making the plant tall or decrease in the length of stem, making the plant dwarf.