A little history
As we know today, Gregor Mendel, best known for his pea experiments, has been at the root of genetics. He has shown by his experiments that if he crossed two peas (F1) with different characteristics such as the color of the flower, the size of the leaves ... Then the resulting (F2) kept the characteristics of only one parent. All the flowers of these young peas had the same color and size of leaves.
It's as if they had "lost" one of the properties. When he crossed these F2 peas between them, the F1 characteristics reappeared among the F3 generation. Mendel called these features shown by the F2 : Dominant. And the hidden features in F2 have been called recessive.
Currently we still call it dominant and recessive. However, we know that Mendel discovered "complete dominance". There are indeed other forms of dominance. We already know these forms so we will especially bring some provisions to remember.
We know that genes carry characteristics and that these genes are located on chromosomes. There are genes dealing with the color of the eyes, the color of the legs, the size of the bill ... The chromosomes are located in the cells of the body: They are stored in the nucleus of each cell. In each nucleus of each cell are the genes for the color of the eyes, the
color of the paws, the size of the beak ... However, the functioning of the genes "eye color" is manifested only in the eyes. In the legs, the genes "eye color" do not appear. Each cell "knows" where it is in the body and what genes it needs to activate.
The chromosomes go by pair, all the genes are found in pairs. For the eye color gene, we have two genes. This also applies to the color of the legs, the size of the bill ... These two genes for the color of the eyes can cause a color of the blue eyes. It is also possible for one gene to take care of the blue color and the other for a brown color. (This does not mean that the being will have one blue eye and the other brown, the brown eye is dominant on the blue, so both eyes will be brown).