The chromosomal doctrine of inheritance, how do you understand it

The chromosomal theory of inheritance is a further development of Mendel's theory by the American biologist, J.P. Morgan. He did a lot of experiments with fruit flies and proposed that the hereditary factors are located on the chromosomes. 1909, the Danish geneticist Johnson first called Mendel's "hereditary factors" as genes. Chromosomes are found in the nucleus of the cell, and during cytokinesis (the period when the cell is not dividing), they exist in the form of chromatin; when the cell divides, they become short, thick, rod-like structures called chromatids (because they are darkly stained). Chromosomes are actually chromatin thickened into a tight, highly helically curled filamentous structure inside. The nucleus contains all the information necessary for further development and determines whether the cell will develop into a man, a horse or a pigeon, and whether the pigeon will be large or small, gray or red, a good or poor flyer, and so on. These developmental or developmental information and instructions are found in the filamentous structure of the chromosomes. Chromosomes contain a series of bases, called genes, in a sequential order. The distribution of genes on the chromosomes is like strings of beads on a necklace, and these strings of genes represent all of the inherited traits, and are therefore called the "basic units of heredity".