Modern varieties are those developed after World War II. This period marks four main changes that transformed how plants are bred and selected:

1. The UK national diet became dominated by processed food stuffs sold by supermarkets, takeaways, canning and frozen food companies. Any fresh fruit and vegetables sold came from large-scale, often foreign, horticultural growers. Previously, most food was bought from individual (not chains) shops purchasing vegetables and fruits locally.
2. The farmers and large-scale horticultural enterprises now buy their seeds in bulk, using supply chains usually originating overseas. Retail seed companies selling to gardeners no longer grow their own seeds but also buy-in the seeds they sell, mostly from sources dominated by commerce. Plant breeding companies now target commercial seed growers, not gardeners.
3. National and international companies are now the main sources of new varieties, replacing small seed companies also acting as local breeders.
4. Our understanding of how inheritance works now includes at the molecular level; this knowledge is increasing used to breed new varieties.

The seeds and plants of modern varieties sold to gardeners through retailers are now bred by companies targeting large-scale growers. Modern varieties are mostly F1 hybrids. These derive from two different inbred parental lines which aren’t sold and can’t be re-created from the F1 hybrids. It can be easier to breed new varieties in this way by creating new varieties simply by combining panels of parental types in different combinations. F1 hybrids are also renown for their uniformity and vigour but in fact the latter is not always true; hybrid vigour is a reality for only certain crop species. Instead, companies have used F1 hybrids to gain monopolies by simply retaining the parents, which never have to be sold on the open market.

Seedlings in the following (F2) generation will be a diverse mixture resulting from a random re-assortment  of the genes of the two parental lines and nowhere near as good as the original F1 hybrids. This means growers, including gardeners, need to buy new seed of F1 hybrids every season, a ‘sneaky’ way of selling more seeds. Some F1 hybrids are even sterile. This may be achieved by using parents of different ploidies, for example, one diploid and the other tetraploid and the progeny are sterile triploids. Increasingly, a combination of a cytoplasmic male sterility gene and a restorer of fertility gene is now often used. This makes it easier to create F1 hybrids. Sterile F1 hybrids are also often marketed as being good, for example, because seeds would get in our teeth.

Modern varieties do, in the main, suit commercial growers; the plants are  high yielding and often disease-resistant; their uniformity makes mechanical harvesting possible. They are also bred for the produce being often very resilient with a long shelf-life, allowing long-distance transport to far away customers.

But gardeners have often shunned F1 hybrids, still growing heirloom and established varieties. Yet they still need new varieties because tastes change, new methods of growing develop, e.g., raised beds and no-dig, and the climate is changing. Also  useful genetic traits keep getting discovered – such as new resistances. The webpages below explain in more detail the important advantages and disadvantages of modern varieties to gardeners.