Back To CourseLife Science: Middle School
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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.
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.
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.
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.
There is also a condition in humans that is caused by incomplete dominance. Humans who have genotype HH have normal blood cholesterol levels, while humans that have genotype Hh have blood cholesterol levels that are twice as high as normal. These individuals may be prone to blocked arteries and heart attacks as early as their mid-thirties. But, this is not as bad as humans who have genotype hh because this is a very serious condition that can cause such blockages and heart attacks, but in individuals as young as two years old! Therefore, the heterozygous individuals are somewhere in the middle of the two homozygous genotypes, a blended version of the disease.
Sometimes, our phenotype, or traits we exhibit, may look similar but are based on different genotypes, or genetic makeups. We see this in complete dominance of alleles, or when the dominant allele determines an organism's phenotype no matter what. If orange fur is dominant and white fur is recessive, once you have an orange fur allele your fur color is going to be orange regardless of what the other allele is that you inherited.
But, with incomplete dominance an organism's phenotype falls between the two alleles. So now, the only way an individual can have orange or white fur is if they are homozygous, and have two identical alleles. Otherwise, if they are heterozygous and have two different alleles, we will see a blending of the two in the phenotype. This doesn't mean that the blending is a permanent change in the genotype, though. Individuals who have a blended phenotype still pass on only one allele to their own offspring, meaning that even if they themselves are yellow, they can still produce orange or white children, depending on what those children get from the second parent.
Alleles: Alleles are the different versions a gene can exhibit.
Phenotype: A phenotype is the traits that we exhibit.
Dominant allele: A dominant allele is genetic information that determines the organism's appearance.
Recessive allele: A recessive allele is genetic information that an organism carries, but that does not affect their appearance.
Complete dominance: Complete dominance is when an organism's phenotype is determined by a dominant allele.
Incomplete dominance: Incomplete dominance is when an organism's phenotype falls between the two alleles.
Homozygous: Homozygous means that they have two identical alleles
Heterozygous: Heterozygous means they have two different alleles.
Genotype: Genotype is the genetic makeup.
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Back To CourseLife Science: Middle School
35 chapters | 241 lessons