Understanding inheritance

In the same way that we inherit traits from our parents, genes are passed from one generation to another in plants and other organisms as well. Maize, like humans and many other organisms, is diploid – each individual cell in the organism contains two sets of chromosomes, one from each parent (some plants, such as wheat, are polyploid; where each cell has more than two sets of chromosomes). During the process of meiosis, the cells divide to form gametes - cells with just one set of chromosomes — in preparation for fertilization. During fertilization, a gamete from one parent combines with the gamete of the other parent, to form a new individual.

In plants, pollen grains carry the male gametes (sperm) while the female gametes (eggs) are contained within carpels. In angiosperms, including maize, the part of the carpel that receives the pollen is the stigma. In maize the silk is the stigma and the tassels carry the pollen.

Each silk can accept one pollen grain.  That grain travels down the silk to fertilize the egg at its base.  One silk has at its base one egg, which accepts one grain of pollen. Therefore, each kernel is actually a separate individual with its own set of genes. This is why there can be ears of maize that have different colored kernels. Each ear of maize really contains a whole family – half-siblings that share the same mother (but may have been pollinated by pollen from different tassels).

Although it can happen that the pollen from a tassel fertilizes the silk on the same plant (self-pollination), in maize this is less likely than cross-pollination between different plants. Self-pollination often leads to a loss of vigor, called inbreeding depression. Conversely, when two different (often deliberately inbred) parents of maize are cross-pollinated, the result is often progeny that are bigger and healthier than either of the parents. This phenomenon is called hybrid vigor, or heterosis, and it is one of the reasons that most commercial varieties of maize on the market are hybrids. 

Definition: ploidy — the number of complete sets of chromosomes in each cell in an organism. Humans, a diploid organism, have 2 sets of 23 chromosomes (one from each parent) in every somatic cell (but just one set in each gamete, or sperm or egg cell). Many plants are polyploidy – they have 3 or more sets of chromosomes in each cell. This can be the result of chromosomal duplication in one or both of the parents.

Some examples of plants of various ploidy levels are:

Diploid: maize, tomato, rice
Triploid: banana, watermelon
Tetraploid: durum wheat, cotton, potato
Hexaploid: bread wheat, kiwifruit
Octoploid: strawberry, sugarcane

Definition: hybrid – in general, the offspring of two different varieties, breeds, or species. In the context of plant breeding and crop variety development, a hybrid is the heterozygous offspring of two homozygous (inbred) individuals. It has 2 different alleles at most genes, which can result in heterosis or hybrid vigor.