Applied Genetics (plant breeding) STB 421 (FIFTH CLASS/LECTURE)
Variation and pland-breeding
Breeders aim at producing for every crop as wide as genetic variability as possible. This variability along with that created by nature are exploited by the breeder fro crop improvement. The breeder can obtain variation by plant introduction and local collection. The breeder can produce a mutation crossing or hybridization and polyploidy.
The evolution of crops as been mainly due to natural selection spanning over several thousands of years. Harlan did say about barley that for the past five thousand years, no remarkable change has been noticed in egypt, this however shows that breeders had been working in three historic times.
Modern breeders have received end-products of a long period of natural selection, under the conditions of cultivation - with man's assistance however. Man acted contrary to natural selection by preserving types useful to him but which would have become extinct as a result of natural selection. Modern plant breeder therefore is not interested in domestication of species as they are in the domestication of genes.
Cell Division
Plant breeding involves reproduction. Reproduction involves the multiplication of cells. The development of sex cells (gametes) and the growth of a young plant into an adult all involve cell multiplication. In other to multiply, cells undergo division, one divides into two, two into four, four into eight and so on.
The word division is misleading in some way because it implies that the process always involve halving of the cell and its contents. In fact we now know that cell divison is accompanied or preceded by the formation of new cell component so that the product of cell division, the daughter cells, are essentially similar to the parent cell. Understanding cell division is largely a question of appreciating how this uniformity is preserved.
In any description of cell division, the chromosomes occupy a central position. As the vehicle of heredity they determine the characteristics of the cell and its progeny (offspring), and it is essential that they should be correctly distributed between the daughter cells. It is known that a cell normally has a fixed number of chromosomes and this occurs in pairs. The so called Diploid condition. Two types of cell division are recognized to the behavior of the chromosomes (Fig 1.)
Fig 1.
In the first of this, the daughter cells finish up containing exactly the same number of chromosomes as the parent cell. This is called mitotic cell division (or just mitosis) and it is the type of cell division which takes place during the organism growth.
In the other type of division known as the meiotic cell division (or meiosis), the daughter cells finish up with half the total number of chromosomes present in the parent cell. This kind of division generally takes place in the formation of gamete, though in a more complex plant it occurs in the formation of spores.