Semi-allelic genes are genes that have different alleles, or variations, but they are not completely dominant over each other. These types of genes are also known by synonyms such as co-dominant, incompletely dominant, and partial dominance. Co-dominance refers to the situation in which both alleles of a gene are expressed in the heterozygous individual. Incomplete dominance is a type of inheritance where the parent's phenotypes blend together to create a new phenotype in the offspring. Partial dominance indicates that the phenotype of the heterozygote falls between the phenotypes of the homozygote dominant and homozygote recessive. So, these synonyms for semi-allelic genes describe different aspects of the same phenomenon.