Breeding (selection) is the science of creating new plant varieties, animal breeds and microbial strains and improving existing ones. Its theoretical basis is genetics; its main methods are selection and hybridisation. Modern breeding is supplemented by biotechnology — genetic and cell engineering. The exam asks you to tell the methods apart, explain heterosis, and know Vavilov’s theory and the steps of genetic engineering.
§1. N. I. Vavilov’s work
A variety (plants), breed (animals) or strain (microorganisms) is a group of organisms created by humans, with stable hereditary properties and similar traits.
On expeditions all over the world N. I. Vavilov collected a huge collection of crop plants and reached two key conclusions:
Centres of origin of cultivated plants — each crop arose where its genetic diversity is greatest. For example, the Southwest Asian centre, which includes Central Asia — wheat, rye, peas, onion, carrot; the Central American centre — maize, pumpkin, peppers; the South American centre — potato, tomato, tobacco; the Mediterranean centre — olive, cabbage; the Abyssinian centre — coffee.
The law of homologous series in hereditary variation: related species and genera have similar series of hereditary variation. Knowing the variants of a trait in one species, you can predict and look for the same forms in a related one.
The more diverse the starting material for breeding, the greater the scope for selection.
§2. Methods of plant breeding
Mass selection — many individuals are chosen and their seeds sown together; used mainly in cross-pollinated plants (rye, maize). The result is unstable, so selection is repeated.
Individual selection — the offspring of each plant are studied separately; in self-pollinated plants (wheat, peas) it gives pure lines.
Inbreeding (crossing close relatives; in plants, forced self-pollination) — homozygosity rises and pure lines form. But harmful recessive genes become homozygous and show, so vigour falls.
Outbreeding (crossing unrelated individuals) and heterosis — when different pure lines are crossed, the F₁ hybrids are more vigorous and productive than their parents. In later generations heterosis fades because homozygotes become more frequent. So heterotic seed must be produced afresh every year (interline maize hybrids).
Distant (interspecific) hybridisation — hybrids are usually sterile, because chromosomes of different species do not pair in meiosis. G. D. Karpechenko (1924) overcame the sterility of a cabbage–radish hybrid by doubling its chromosome number. The wheat–rye hybrid is triticale.
Polyploidy — multiplication of the chromosome number (using colchicine); polyploids are usually larger and higher-yielding.
Experimental mutagenesis — raising the mutation rate with radiation and chemicals, then selecting useful mutants.
I. V. Michurin created many fruit-tree varieties by distant hybridisation and the "mentor" method.
Cotton breeding is well developed in Uzbekistan: for example, the wilt-resistant "Tashkent" series of varieties created by S. Mirakhmedov is well known.
Worked example
Why was the hybrid of cabbage (2n = 18) and radish (2n = 18) sterile, and how did Karpechenko make it fertile?
A cabbage gamete has 9 chromosomes and a radish gamete 9; the hybrid has 9 + 9 = 18.
Cabbage and radish chromosomes are not homologous, so they cannot pair in meiosis and normal gametes are not formed → sterility.
The chromosome number was doubled to 36; now every chromosome has a partner.
Answer: meiosis proceeded normally and the polyploid hybrid (amphidiploid) became fertile.
Worked example
A heterozygous Aa plant was self-pollinated for 3 generations. What fraction of the third generation is heterozygous?
Each self-pollination of Aa gives half heterozygous offspring (1AA : 2Aa : 1aa), while homozygotes stay homozygous.
So the fraction of heterozygotes halves every generation: (1/2)ⁿ.
For n = 3: (1/2)³ = 1/8 = 12.5%.
Answer: 12.5% — this is why inbreeding quickly raises homozygosity and why heterosis fades.
§3. Animal breeding
Animals reproduce only sexually, have few offspring and mature late, so breeding them is harder.
Animals are selected by exterior (body build) and productivity, and breeding males are assessed by their offspring (for example, a bull by the milk yield of his daughters).
Inbreeding fixes desired traits; outbreeding — crossing breeds — gives heterosis (in poultry and pig farming).
Interspecific hybridisation: a horse and a donkey give a mule (hardy but sterile).
Uzbekistan is known for the Karakul sheep breed.
§4. Breeding microorganisms and biotechnology
Microorganisms multiply fast — one cell gives billions of descendants in a short time — so mutagenesis and selection produce strains that make large amounts of antibiotics, enzymes, amino acids and vitamins (for example, fungal strains producing penicillin).
Genetic engineering transfers a desired gene from one organism to another:
The enzyme restriction endonucleasecuts DNA at specific sequences, and the desired gene is isolated.
The gene is inserted into a vector — usually a bacterial plasmid; ligasejoins the fragments.
The recombinant DNA is introduced into a bacterial cell.
The bacterium multiplies and makes the gene product — for example, human insulin or growth hormone.
Genetically modified organisms (GMOs) include plants resistant to pests or herbicides, among others.
Cell engineering:
Tissue culture and micropropagation — growing a whole plant from a single cell or a piece of tissue; producing virus-free seed potatoes.
Hybridoma — fusing a lymphocyte with a tumour cell to obtain monoclonal antibodies.
Cloning — transferring the nucleus of a body cell into an egg cell whose nucleus has been removed (Dolly the sheep, 1996).
§5. Vavilov’s centres and the stages of breeding a new variety
N. I. Vavilov identified seven main centres of origin of cultivated plants:
South Asian tropical — rice, sugar cane, cucumber, citrus fruits;
East Asian — soya, millet, tea, many fruit trees;
South-west Asian (including Central Asia) — bread wheat, rye, pea, flax, grape, onion, carrot;
Mediterranean — cabbage, olive, sugar beet;
Abyssinian (Ethiopia) — durum wheat, barley, coffee;
Central American — maize, beans, squash, upland (medium-staple) cotton;
South American — potato, tomato, peanut.
A centre is the region of greatest genetic diversity of a species; breeders obtain wild ancestors and local varieties from there.
Vavilov’s seed collection is still kept as a gene bank. Such gene banks preserve disappearing local varieties and wild relatives of crops for future breeding.
Stages of breeding a new variety:
Setting the goal — which traits are needed (yield, disease resistance, early ripening).
Choosing starting material — varieties with the desired traits, wild species.
State variety trials — the variety is tested for several years in different regions and entered in the register.
Breeding one variety usually takes more than ten years. Today DNA markers speed this up: the desired gene can be detected by DNA testing while the plant is still a seedling.
Worked example
A breeder wants a potato variety resistant to late blight. Where should resistance genes be sought, and what method should be used?
The potato’s centre of origin is South America, where wild potato species are diverse.
Forms carrying resistance genes are found among the wild species.
They are crossed with cultivated potato by distant hybridisation; if the hybrids are sterile, polyploidy is used.
Resistant, high-yielding plants are selected among the offspring and then propagated vegetatively (by tubers).
Answer: look for genes in wild species of the South American centre, then use distant hybridisation and selection.
Check yourself
Where is the centre of origin of the potato and tomato?
Why does heterosis fade in the second generation?
Which selection method is used for self-pollinated plants?
How is the sterility of distant hybrids overcome?
What do restriction enzymes and ligase do in genetic engineering?
How are monoclonal antibodies produced?
From which Vavilov centre does maize originate?
Why are gene banks needed in breeding?
Common mistakes
Thinking heterosis persists in later generations — from F₂ onward it fades.
Confusing the roles of restriction enzymes and ligase.
Mixing up mass and individual selection: mass selection suits cross-pollinated plants, individual selection self-pollinated ones.
Treating inbreeding as purely beneficial — it also exposes harmful recessive genes.
Attributing the law of homologous series to Darwin or Mendel — it is N. I. Vavilov’s law.
Blaming mutations for the sterility of distant hybrids — the cause is that chromosomes cannot pair in meiosis.