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.
If you've ever donated blood, you have surely been asked your blood type. Are you A, B, AB, or O? Did you ever stop to think about what this means? Or the fact that these are alleles on your genes? When we talk about genes and alleles, which are just different versions of genes, we generally refer to genes with only two alleles. For example, red or white flower color.
But some genes can have more than two alleles, which makes for some interesting genetic inheritance patterns. Let's look at this concept more closely using human blood type as an example.
In humans, there are three different alleles that can create four different phenotypes, or traits that are expressed. The alleles are A, B, and O, and the phenotypes are A, B, AB, and O. The letters correspond to the carbohydrate found on the surface of that person's red blood cells. So if a person's red blood cells are coated with carbohydrate A, we say that person has blood type A. If their red blood cells are coated with carbohydrate B, then they have blood type B. But, they could also have red blood cells that contain both carbohydrates, in which case their blood type is AB. And finally, they may have red blood cells that are coated with neither carbohydrate, in which case their blood type is O.
Since you get one blood type allele from each parent, there are several possible combinations that you could inherit. If you got two A alleles (one from each parent), then you would have a phenotype of blood type A, as well as genotype, or genetic makeup AA. If you got two B alleles, then you would have blood type B and genotype BB.
But here's where things get a bit complicated, because both the A and B alleles are dominant, meaning that they determine an organism's phenotype. For example, if blonde hair is dominant and brown hair is recessive, then it only takes one blonde hair allele for you to have blonde hair. You could very well have a brown hair allele too, but it would only show up in your genotype, not your phenotype.
Because both A and B are dominant, if you get one A allele from one parent and one B allele from the other, then your blood type and genotype would be AB. This is an example of codominance, or when both alleles are expressed in the phenotype.
But what's really cool is that the O allele is recessive, meaning that it is genetic information that does not affect an organism's phenotype unless you have two of those alleles. If you carry an O allele, you can still pass it on to your children, but if you have just one O allele, it won't affect your blood type because A and B are dominant. So if your genotype is AO, then you have blood type A, and if your genotype is BO, then you have blood type B. The only way to have blood type O is to have genotype OO, because there is no dominant allele to mask the recessive allele.
You might be wondering why organisms would have codominance and multiple alleles. Well, there are some advantages to having such a variety of combinations and phenotypes. The reason it is so important to know your blood type is because it determines what type or types of blood you can receive and what others can receive from you.
Your body doesn't like foreign invaders, because these can be harmful things that hurt or kill you. So if you have blood type A and you receive a donation of blood type B, your body will produce certain proteins called antibodies. These proteins are designed to identify and destroy pathogens and viruses in the body, and when a mismatched blood type appears, those antibodies can't tell the difference. They bind to those unknown carbohydrates and kill the blood cells, which is exactly what you don't want to happen to your new blood!
But for those who have blood type AB, this is not a problem at all because they already have both A and B carbohydrates on their own red blood cells. This makes people with this blood type 'universal recipients' because they can receive blood from any other blood type without fear of producing antibodies in response.
In contrast, people with type O blood are known as 'universal donors.' This is because their red blood cells have neither A nor B carbohydrates on them, so this blood type does not stimulate the antibody response in recipients. Their blood is safe for everyone, but because their bodies don't recognize either the A or B carbohydrates themselves, they must receive blood from only type O donors.
Here we can see that those with type O blood can be helpful as donors, while those with type AB blood are lucky as recipients. And without the variety of alleles and codominance, none of this would be possible!
It is easy to discuss inheritance patterns in terms of two alleles per gene, but some genes come with more than just two options. This increased number of alleles means an increase in both the possible genotypes, or genetic makeups, as well as phenotypes, or the traits that are expressed in an organism.
One of the first things we learn in genetics is how dominant alleles determine an organism's phenotype, while recessive alleles are genetic information that does not affect an organism's phenotype (unless an organism has two recessive alleles). But sometimes there is a codominance of alleles, which is when both alleles are expressed in the phenotype.
Blood type is an example of both multiple alleles and codominance. And we can use it as an example of how both can be useful genetically. The variety and codominance in blood types allow for some people to receive any type of blood, others to donate to anyone else, and everything in between.
Alleles: different versions of genes
Phenotypes: traits that are expressed
Genotype: genetic makeup
Dominant: dominant alleles determine an organism's phenotype
Recessive: recessive alleles are genetic information that does not affect an organism's phenotype unless you have two recessive alleles
Codominance: when two dominant alleles are expressed in the phenotype
Antibodies: proteins that are designed to identify and destroy pathogens and viruses in the body
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Back To CourseLife Science: Middle School
35 chapters | 241 lessons