Biology Forums - Study Force

Okay can you define the three terms and give me an example for each. These are questions I dont know from my honors biology study guide! test tomorrow, so points will be awarded soon! thanks!

Simple dominance is when one trait is always dominant over the other one. Like brown eyes are dominant over blue. So if the allele for brown is present, they eye color will always be brown.

Incomplete dominance is when the two traits mix. Like in flowers, if red and white are incompletely dominant, then the offspring would be pink.

Codominance is where neither trait is dominant and both are present. Like blood type. A and B are co dominant, if an offspring recieves both alles they will have bllod type AB.

Simple dominance-the presence of the dominant allele complete hides the recessive allele;  TT and Tt = tall, tt  = short

Incomplete dominance-there is a "blending" of phenotypes;  RR=red, WW=white, RW=pink.

Codominance-both alleles are expressed; IA=type A blood, IB=type B blood, IAIB=type AB blood.

I always found the best way to visualize this is to understand whats going on.

In most cases with true dominance, the dominant gene codes for a functional protein while the recessive allele does not. The recessive phenotype comes from the absence of the functional protein. However, even if one dominant allele is present, the protein is still being made so the dominant phenotype is expressed.

For incomplete dominance, the proteins being expressed work against each other and are usually dependent on expression levels. For example, if a flower had a genotype of RR then both alleles are expressing proteins that code for red. If the flower had the genotype WW then both alleles are coding for a white pigment. Yet, with the genotype RW, the proteins work against each other and the resulting phenotype is a pinking color. Hence "incomplete" since neither true phenotype of either the red or white allele is being expressed.

For co-dominance, the proteins expressed by the genes do not interfere with each other and as long as one allele is present the expression will result in the true phenotype. For example, with blood surface proteins, type A allele expresses the A antigen, type B expressed the type B antigen, and type AB expressed both antigens, with levels independent of each other.

Sure my explanation may be a little long and drawn out, but I think if you read it along with the other ones it might give you a better understanding.