Every person (except for an identical twin) has a unique set of inherited traits that makes him or her identifiable as an individual. This is true of all organisms that reproduce sexually. However, have you ever tried to distinguish one bean plant from another bean plant? Or one corn plant from another corn plant? Plants grown as crops have been selected to minimize trait variation because it is easier for farmers to manage a more uniform crop. In this activity, students germinate two types of corn seeds: popcorn and Indian corn. As a result of their observations, students will see that seedlings of the popcorn variety (which have been bred for uniformity) are very similar, while more variation can be observed among the Indian corn seedlings.
Understanding heredity in plants—especially crop plants—can be complicated. Many crop plants are propagated asexually, making it easy to grow a uniform crop. Crops like potatoes, garlic, and raspberries can reproduce simply by being split into pieces from which they will regrow. For example, one garlic plant produces many cloves. If these cloves are separated and replanted, they will grow into genetically identical garlic plants (like identical twins). Corn does not reproduce in this way.
Every corn plant has both male (tassel) and female (ear) parts. In order for kernels to form on a cob, each one must be fertilized by a grain of pollen. Pollen comes from the tassels at the top of the plant where it is easily picked up by the wind. For fertilization to occur, the pollen must land on a silk strand that sticks out the end of the ear. The pollen is transported down the silk to the egg, which will grow into a kernel after fertilization. As in all organisms that reproduce sexually, the sperm (pollen) and the egg each contribute half of their genes to the offspring (in this case, the corn that will grow from the kernel), creating a genetically unique individual with some traits from each parent.
If farmers select kernels from their best corn plants to save for seed, they know that half of the genes came from the mother plant. However, they do not know anything about the father plants. In the field, most corn silks are pollinated by surrounding plants, a process known as cross-pollination. Because pollen is blown in on the wind (open-pollination), the many kernels on a single ear can be pollinated by many different father plants. Planting the kernels from an open-pollinated ear of corn produces plants as similar as half-brothers or -sisters. In order to select for a more uniform and genetically similar crop, the source of pollen must be controlled—a difficult proposition on a windy day. The ears must be protected from chance pollination and then hand-pollinated with a tassel from the desired father plant. Then, the kernels will produce plants that are as similar as brothers and sisters.
Many farmers and gardeners plant hybrid varieties of corn and other vegetables. It takes years of controlled pollination to develop hybrids. First, a carefully selected plant with desirable characteristics is self-pollinated for seven generations, resulting in an inbred plant. Inbred plants have stunted growth and do not yield well, but when two different inbred plants are cross-pollinated, their progeny grow vigorously, yield well, and reliably express desired traits—these are hybrid varieties. Development of hybrids has greatly increased agricultural productivity, but seed saved from a hybrid is unreliable. For this reason, some gardeners and farmers prefer non-hybrid varieties.
