Allelic differences

Remember that each individual cell in a maize plant (excluding the gametes) has one set of chromosomes from each parent. Each individual cell therefore has 2 copies of each chromosome and thus 2 copies of each gene or DNA segment. The different forms of these genes are called alleles. As an example, in humans, there are many possible alleles for the genes that result in eye color: blue, brown, green, hazel, and other variations. If an individual inherits the same allele type from both parents, that individual is homozygous for that gene. Inheriting different alleles from each parent results in a heterozygous state. The resulting eye color depends on the dominant or recessive nature of each allele. Brown eye color is a dominant trait, for example, and blue eye color is recessive. If a person inherits one brown-eye allele from one parent and one blue-eye allele from the other parent, the resulting eye color is brown.  The blue-eye gene is recessive to the brown-eye gene.

The real situation with recessive and dominant genes in determining eye color is a bit more complicated than this, as it is with many inherited traits. Inheritance works the same for all organisms, including maize. There are usually tens of thousands of genes working together to create the appearance and behavior of an organism. The term genotype refers to the genetic makeup of an organism, while the term phenotype refers to the result, or expression, of the genotype – what we can see or measure (e.g., ‘blue’ eyes, or a ‘tall’ plant). 

Studying inheritance is complicated by the fact that most traits are not governed by just one gene but rather a combination of many genes. The phenotype of a plant may not give the whole story of the genotype behind it – some genes may be masked by the effects of other genes (e.g. the brown-eye allele masks the existence of the blue-eye allele). Scientists have found that many plants have genes for traits that cannot be predicted by looking at the plant. For example, a very short plant may still possess some genes (alleles) for tall height and some for small height, but is short because it has many more alleles conferring shortness. This is one reason why studying genetics – and the actual sequence of DNA -  is so important.