Incomplete Dominance of Gene Alleles

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  • 0:01 Different Versions of Genes
  • 1:14 Incomplete Dominance
  • 3:33 Real-World Examples
  • 4:40 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Genetics are complicated, and sometimes alleles affect an organism's appearance in tandem instead of one dominating over the other. This can produce a blended appearance, but without blending the actual alleles themselves.

Different Versions of Genes

Much of what makes you 'you' is the genetic information you get from your parents. But, even when two people inherit genes from the same parents, like you and your siblings, you can get different genetic information. This is because even though you may get the same genes, you might get different versions of those genes called alleles. For example, blue eye color versus brown eye color, or white flower color versus purple flower color.

Sometimes, the trait you exhibit, or your phenotype, is the result of one allele taking charge over the other. In this case, the dominant allele determines an organism's appearance, while the recessive allele is genetic information that an organism carries, but does not affect their appearance. For example, if brown eye color is dominant and blue eye color is recessive, you could have brown eyes, but still have an allele for blue eyes that you could eventually pass on to your own children.

When we have a clear allele dominator like this, where the dominant allele determines an organism's phenotype no matter what, we call it complete dominance. More often than not, though, things are not this cut and dry. What may happen instead is that an organism's phenotype falls between the two alleles. We call this incomplete dominance.

Incomplete Dominance

To be very clear, while the outcome of the phenotype is blended, this is not a permanent blending of the two alleles genetically, and each individual allele still has the same chance of being passed on to the next generation of offspring. Let's look at an example to see how this works.

Say that you are a bear. The genes from your parents dictate what color fur you will have, and from their genes you have the possibility of getting alleles for orange fur (O) and white fur (o). Now, if we had a situation of complete dominance, and orange fur was the dominant allele, if you got even one orange allele, your fur would be orange. It wouldn't matter to your phenotype if you got a white fur allele from the other parent because the dominant allele (orange fur) is what is shown, while the recessive allele (white fur) just sits back in the genetic shadows.

But, with incomplete dominance we get something a little different. Here, there is no dominance of one allele over the other in the phenotype. Let's say that both of your parents are still homozygous, meaning that they have two identical alleles: one of your parents has two orange fur alleles and the other has two white fur alleles. For this situation, all of their children will be heterozygous, meaning they have two different alleles. Each offspring will have one orange allele and one white allele, so their genotype, or genetic makeup, would be Oo, and instead of all their children being orange-furred bears, as they would with complete dominance, they are all a more yellow color since the fur has less orange pigment.

But, what about the next generation (the children's children)? This is where things get really interesting. If both parents are heterozygous, meaning they have genotype Oo, then the possible outcomes for the offspring change. Now, we find that the offspring could be homozygous, either for orange fur (OO) or white fur (oo), or that they could be heterozygous with yellow fur (Oo). So, we see that while the first generation of children were all yellow bears, both the orange and white fur alleles were preserved for future generations of orange, white, and yellow bear babies.

Real-World Examples of Incomplete Dominance

So, you're probably not going to find any orange or yellow bears, but incomplete dominance does come in all shapes and sizes in the real world. For example, pink roses are the result of a phenotypic blending between red and white roses. When the two colors come together the outcome is a pink rose, but that pink rose can still pass on each individual allele to produce either red or white offspring.

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