What is the name of the inheritance pattern in which both alleles are expressed equally?  

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caledon eNotes educator| Certified Educator

When we say "expressed equally," this actually raises a lot of questions about the interaction between the genes, their products, and the effects of those products on the organism. Consider, for example, that a gene could have an error or mutation that causes it to never be transcribed properly at all; in this case, we would not consider the two alleles to be equally expressed. However, what if the gene's product is simply broken or inefficient? In this case we could still say that they are being expressed equally, but they will not be equally visible in terms of their effect on the organism. What is usually meant by "equally expressed" is that both alleles code for products that are created in roughly equal proportion and the effects of both can be recognized in the organism, as compared to organisms that are homozygous for one allele or the other. 

There are two terms that describe an equal expression of alleles; incomplete dominance, and codominance. One of the most common demonstrations of these effects is flower pigmentation.

In codominance, we can think of the allele products as working together to make something new; in the case of flowers, the example would be a red allele and a white allele making a pink flower. Note that they probably aren't combining the red and white pigments to create a pink pigment, but that the red and white pigments are equally distributed so that the cumulative effect is pink.

In incomplete dominance, we can think of both alleles as "fighting" for dominance, but neither one wins; instead they each carve out their own territory where they, and they alone, are dominant, but this is not consistent throughout the organism. In the case of the flowers, this would create a flower with red and white splotches, spots or some other distinct pattern effect.

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