|Punnett square showing pea cross|
in the Punnett square above you can see two pea parent plants. One is yellow and has a dominant Y (yellow) gene and a recessive y (green) gene, the other is green and has 2 recessive y (green) genes. When these two plants cross-pollinate the seed they produce will grow into 50% yellow peas and 50% green peas. But what if you only like yellow peas? Well then, you'd better not grow any green ones nearby or cross-pollination could happen and you'd end up with some green in the mix.
So why talk about genetics on a garden blog? Because of hybrids (and I don't mean cars). Hybrid seed is often listed as F1 or F1 hybrid. Basically this is a first generation cross of two plants that show good qualities. While hybrid seed has it's advantages; being consistent in phenotype, having the best qualities of both parents, and being widely available. It also has disadvantages for the home gardener. Primarily, the seed from two hybrid parents will likely produce a large amount of recessive seeds. This means that two parents with great qualities will likely produce 75% recessive gene (bad quality) seeds. Those seeds will then grow into plants that don't have the same color, flavor, heat tolerance, or disease resistance that the original hybrids did. Or they will simply be sterile and not produce usable seed at all.
The alternative to hybrid seed is open pollinated seed. This means that the plants that will grow are from a stable genetic background that has been bred over many generations to eliminate most recessive throwbacks. Open pollinated plants will also cross together to create new strains with very similar qualities of the original plants. The major advantages to OP seed is being able to save your own seed (saving you money), creating new strains, and each successive generation grown in the same area will show improved adaptation to that area. If your garden seems to suffer from something unknown fairly regularly, after several generations an open pollinated plant will begin to build a resistance and you will see each new generation producing healthier and better producing plants.
There are some exceptions when buying OP seeds that need to be understood. OP plants will cross-pollinate with ANY nearby plants that are genetically close enough to them. For example, carrots are only sold as F1 hybrids in my area because a common weed here is too close a relative to them to make the resulting OP seed safe for eating. Queen Anne's Lace is everywhere in this area, and is a close cousin to the carrot, but not edible. If I were to collect any seed from carrots I grew, it would most likely produce Queen Anne's Lace, and some half-QAL half-carrot hybrids. So in this instance I am forced to buy hybrid seed.
Seed catalogs often list plants as hybrid or OP and a little research can help you to decide which is best for you. There are many books on plant biology and genetics as well to help gardener's learn how best to breed plants. One thing to be careful of when collecting seed from plants is the crossing of different species within the garden. Pumpkins, butternut squashes, acorn squash... all of these are from the same plant family and it is possible to have them cross pollinate unintentionally. If you are serious about your seed production you can limit this by either only planting one type of each family in your garden, planting the different species far from each other, or by selecting a newly opened flower, hand pollinating it with the correct pollen and then gently bagging it to keep other pollens out until it begins to fruit.