In plant breeding, the most widely used methods are mutagenesis, mass selection, intraspecific hybridization (inbreeding and outbreeding), distant hybridization, and polyploidy.
To increase the diversity of the starting material, artificial mutagenesis is used, in which the frequency of mutations increases tens and hundreds of times.
Example: using X-ray radiation, valuable varieties of cereals were bred, characterized by increased productivity, a shortened stem, and the absence of awns. With the help of chemical mutagens, many varieties of potatoes, tobacco, tomatoes, peppers, and ornamental plants have been created.
After selection, mutant forms are usually involved in crossbreeding to obtain valuable combinations of traits.
In plant breeding, different types of crosses are used.
Inbreeding (closely related crossing) in plants is based on forced self-pollination of cross-pollinated forms and is used to transfer genes to a homozygous state and obtain pure lines.
When crossing pure lines, hybrids are formed that have a pronounced heterosis.
Example: in this way, hybrid corn seeds are obtained, which are used to sow most of the world’s area allotted for this crop.
Used in plant breeding and distant hybridization. The resulting hybrids, as a rule, are sterile, which is mainly due to violations of meiosis during the formation of germ cells. To overcome the non-interbreeding of species, special methods have been developed, one of which is polyploidy.
For the first time, GD Karpechenko succeeded in overcoming the infertility of an interspecific hybrid in 1924. The scientist obtained a sterile cabbage-rare hybrid with a diploid set of 18 chromosomes, of which 9 are “rare” and 9 are “cabbage”. The conjugation of these chromosomes did not occur. Karpechenko doubled the chromosome set. The polyploid hybrid had 36 chromosomes (18 “rare” and “cabbage”). The possibility of conjugation appeared and the hybrid became fertile.
In the 20s of the twentieth century. wheat-rye hybrids (triticale) were obtained. In 30 N. V. Tsitsin crossed wheat with wheatgrass, and I. V. Michurin obtained interspecific hybrids of fruit and berry crops.
Many agricultural plants are polyploid. Polyploidy in plants is caused by treating seeds with colchicine. With its help, triploid, tetraploid and even octoploid forms were obtained in more than 500 plant species.
Animal breeding has a number of features:
- animals reproduce only sexually;
- the number of individuals in the offspring is small;
- puberty occurs several years after birth;
- it is difficult to breed clean lines, since self-fertilization is impossible;
- some signs (milkiness, egg production) appear only in females.
The main methods (selection and hybridization) are also used in animal breeding. The selection applies only to individuals.
Closely related crossing (inbreeding) is used to consolidate economically valuable qualities in the breed.
Interbreeding (outbreeding) is carried out to consolidate the qualities necessary for a person, which are available in both breeds. In hybrids, an increase in vitality, productivity, and disease resistance is observed, that is, heterosis is manifested.
Example: when crossing two meat breeds of chickens, heterotic broiler chickens are obtained, which are characterized by fast growth and large mass.
Distant hybridization implies the crossing of animals that belong to different species.
Example: when a horse is crossed with other equids (donkey, zebra, etc.), viable offspring are obtained that combine the characteristics of both species.
The selection of individuals from animals can be difficult, because some important qualities are manifested only in females (fertility, egg production in chickens, milkiness and fat content of milk in cattle, etc.).
To determine these traits in males, the method of determining the quality of sires by offspring is used. First, small offspring are obtained from the producers and their productivity is compared with the mothers and with the average productivity of the breed. If the productivity of the daughters turns out to be increased, this indicates a great value of the sire, which should be widely used to further improve the breed.
Artificial insemination is widely used in animal breeding. With its help, you can get more offspring from a specific manufacturer.
To increase the number of offspring of a valuable female, embryo transplantation is used. In the female, the number of eggs is increased with the help of hormones. Then the eggs are fertilized, the embryos are removed and implanted in other females.