Final answer:
A situation where there are more than two alternative forms of a given gene is known as multiple alleles (option 1). This concept allows for a diverse combination of alleles in a population, contributing to genetic diversity. Codominance, while related, involves the simultaneous expression of both alleles in a heterozygote.
Step-by-step explanation:
A situation where there are more than two alternative forms of a given gene is called multiple alleles. Gregor Mendel's initial studies postulated that only two alleles for a gene exist—one dominant and one recessive. However, this has been proven to be an oversimplification; a single gene can indeed have multiple alleles within a population, creating diverse combinations of those alleles in the population's gene pool.
Multiple alleles contribute to the rich genetic diversity observed in many traits, such as blood types in humans. For example, the ABO blood group system is governed by three alleles—IA, IB, and i.
Another concept, codominance, occurs when both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example is the MN blood groups, where both M and N antigens can be present in the red blood cells of heterozygous individuals (LMLN). This scenario is different from multiple alleles, as codominance refers to the expression patterns of alleles rather than the number of allele variations present in the population.