Farmers have been choosing which plants and animals breed for thousands of years. The selection pressure is not survival in the wild but a feature that people want, such as yield, disease resistance or milk. The result is the same: allele frequencies change.
In selective breeding (artificial selection), humans choose individuals with desirable characteristics and allow only these to breed. Over many generations the frequency of the alleles for the desired features rises. It differs from natural selection because the selection pressure is set by humans, and the selected features need not help the organism survive. Disease resistance is introduced into wheat and rice by crossing a high-yielding but susceptible variety with a resistant variety, then backcrossing repeatedly to the high-yielder and selecting resistant, high-yielding offspring. Maize is inbred for several generations to give homozygous, uniform inbred lines, then two lines are crossed (hybridisation) to give F1 hybrids with hybrid vigour that are also uniform. The milk yield of dairy cattle is improved by selecting high-yielding cows and bulls chosen by progeny testing, using artificial insemination. Selective breeding reduces genetic diversity.
Fig. 1: introducing disease resistance into a high-yielding cereal by crossing and repeated backcrossing
Section 01
Three breeding programmes.
Choose disease-resistant wheat and press Next generation four times: the orange rust spots become rarer each generation, and the ears shrink in the F1 and then return to full size. Then try maize hybrids (inbreeding, inbred lines, hybridisation) and dairy cattle.
Principles: decide on the desired characteristic; select individuals that show it most strongly as parents; cross them; select the best offspring and breed from them; repeat for many generations.
The genetics: each round increases the frequency of alleles for the chosen trait; unwanted alleles are lost. The breeder is acting as the selection pressure.
Wheat and rice, disease resistance: high-yielding varieties are often susceptible to fungal diseases such as rust in wheat or blast in rice. Resistance alleles are found in wild relatives or old local varieties. The resistant plant is crossed with the high-yielder; the offspring are backcrossed to the high-yielding parent for several generations, each time keeping only plants that are resistant, until the new variety has the high yield and the resistance allele.
Examiner trap
Selective breeding does not create new alleles. It can only increase the frequency of alleles that already exist in the breeding population or its wild relatives.
Stage 1 · Learn
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In the disease-resistant wheat example, what are the orange spots on the plants at the start?
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After the first Next generation, the simulator says the F1 carry the resistance allele and are backcrossed. Backcrossed to which plant?
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In the maize hybrids example, the inbred lines are described as uniform but small and weak. Why are they weak?
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In the dairy cattle example, bulls are chosen by progeny testing. What does this mean?
Section 02
Maize: inbreeding, then hybrid vigour.
Maize is naturally cross-pollinated by wind. Breeders control pollination to make two quite different kinds of plant.
Inbreeding: plants are self-pollinated for several generations. Each generation, more loci become homozygous, giving inbred lines that are very uniform but small and low-yielding (inbreeding depression), because harmful recessive alleles are now expressed in homozygotes.
Hybridisation: two different inbred lines are crossed. The F1 hybrids are heterozygous at many loci, so harmful recessives are masked: they are tall, strong and high-yielding (hybrid vigour).
Uniformity: because each parent line is homozygous, every F1 plant has the same genotype, so the crop is uniform: plants grow to the same height and ripen together, which suits machine harvesting.
New seed every year: if F1 plants are crossed together, the F2 shows segregation and is not uniform, so farmers buy fresh F1 seed for each crop.
Examiner trap
The vigorous hybrids are not the inbred lines. Inbreeding makes plants uniform but weak; it is crossing two inbred lines that gives vigour, and the F1 is uniform because both parents are homozygous.
Section 03
Dairy cattle: breeding for milk.
Milk yield is a continuous, polygenic trait strongly affected by the environment, and only cows produce milk. Breeders work around both problems.
Selecting cows: the milk yield (volume, and fat and protein content) of each cow is recorded; the highest yielders are chosen for breeding.
Selecting bulls by progeny testing: a bull's milk alleles cannot be seen in the bull, so he is judged by the milk yield of his daughters, compared with the herd average.
Artificial insemination (AI): semen from the best bulls is frozen and used to inseminate many cows in many herds, so one bull can father thousands of calves.
Embryo transfer: the best cows can be given hormones to release many eggs; the embryos are transferred into surrogate cows, increasing the offspring from each high-yielding cow.
Costs: the use of a few bulls reduces genetic diversity and increases inbreeding; very high-yielding cows can suffer more mastitis, lameness and fertility problems.
Examiner trap
A bull does carry alleles affecting milk yield even though he produces no milk. That is exactly why progeny testing works: his daughters show the effect of the alleles he passes on.
Stage 2 · Exam
Exam-style questions
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Which feature distinguishes artificial selection from natural selection?
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Why are F1 hybrid maize plants produced from two inbred lines uniform?
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Why do farmers growing hybrid maize buy new seed each year rather than saving seed from their crop?
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Which is a disadvantage of using artificial insemination with semen from a small number of bulls?
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