Genes and Alleles

A gene is a unit of molecular information. This information, which controls cell development and other cellular activities, is coded in a linear sequence of different types of deoxyribonucleic acids (DNA). Genes are passed (or inherited) from parent to offspring.  This is the main reason that children have similar appearances to their parents.  In corn, the number of genes is at least 10,000. These genes are located on chromosomes, which are found in the nucleus of each plant cell. The specific location on the chromosome of a particular gene is referred to as that gene’s locus (plural is loci).

A corn plant has two copies of each gene, one obtained from the female parent and the other from the male parent. An organism, such as corn, with two copies of each gene is called a diploid (“di” is from the Greek word, dyo, meaning two; “ploid” means set of genes or chromosomes). The reproductive cells (egg and pollen cells) produced by a diploid corn plant have only one copy of each gene and thus are said to be haploid (one set of genes or chromosomes).

The two gene copies inherited from the female and male parents may be functionally the same or different. Different forms of the same gene are called alleles. Often, alleles are designated by letters. Thus, if there are two different alleles at a locus, then the three possible genetic constitutions (i.e., genotypes) at that locus could be designated as:

where A1 and A2 are different alleles. If A1 and A2 are functionally different alleles, then the A1A1and A2A2 genotypes will have different phenotypes. A phenotype is any trait that is observable, such as grain color or grain yield. If the A1A1 and A1A2 genotypes have the same phenotypes, then the A1 allele is completely dominant to the A2 allele and the A2 allele is completely recessive to the A1 allele.  This means that only one Aallele is required for full expression of the phenotype associated with the Aallele.  If neither allele is completely dominant to the other, then the A1A2genotype will have a unique phenotype compared to either A1A1 or A2A2. The A1A1 and A2A2genotypes are said to be homozygous because for each genotype the two alleles are the same. In contrast, the A1A2 genotype is heterozygous.