What does incomplete dominance refer to?

Incomplete dominance refers to a genetic scenario where the phenotype of a heterozygote is intermediate between the phenotypes of the two homozygotes. This means that neither allele is completely dominant over the other, and both contribute to the organism's appearance in a blended form.

In the context of the choices provided, this definition aligns perfectly with the idea that gene pairs are expressed differently and neither is dominant. For instance, if one allele leads to red flowers and another leads to white flowers, an organism with one of each allele might produce pink flowers. This demonstrates the key concept of incomplete dominance, where the traits do not exhibit a complete dominance relationship.

The other options, while they describe different genetic situations, do not accurately represent incomplete dominance. The notion of two dominant alleles coexisting refers more to codominance, where both traits are fully expressed. Similarly, one allele completely masking another illustrates complete dominance, where the dominant allele overwhelms the effect of the recessive one. Lastly, phenotypes identical to one parent do not relate to incomplete dominance as they suggest straightforward inheritance without the intermediate expression characteristic of incomplete dominance.