Final answer:
The term that describes when both alleles are expressed in offspring is 'codominance.' In this genetic situation, each allele in a heterozygote contributes to the phenotype equally and visibly, such as in the MN blood group system in humans where both M and N antigens are expressed on the surface of red blood cells.
Step-by-step explanation:
The word from the list that best fits the definition "both alleles are expressed in offspring" is codominance . Codominance is a variation of inheritance where both alleles for a gene are equally and simultaneously expressed in the heterozygote. For instance, in the human MN blood group system, individuals with one M allele and one N allele (genotype LMLN) will express both M and N antigens on their red blood cells. Between parents that are heterozygotes (LMLN), the offspring can exhibit a 1:2:1 genotypic ratio, yet each genotype will have a distinct phenotype, clearly observing the effects of codominance .
In contrast, incomplete dominance refers to a situation where the heterozygote shows a phenotype that is intermediate between the homozygous phenotypes, such as pink flowers from a cross between red and white flowered plants. The term genotype describes the genetic makeup of an organism, while phenotype refers to the observable traits expressed by an organism. The ABO blood group system is a classic example of both multiple alleles and codominance in human genetics.