Have you ever heard someone say, "There is always an exception to every rule." Well the truth is that for the most part, this is true. Not every rule has an exception. In the world of math, there are no exceptions. For example, 2 + 2 = 4, is a rule that has no exceptions. We can't discover tommorrow that 2 + 2= 5. In science, however, there are numerous exceptions, and in genetics there many of them.  In this section of Genetics R Us, we are going to look at the exceptions to the rules that Gregor Mendel laid down during the late part of the 19th century.
As we discuss this section, it's important to keep in mind that it is not be implied that Mendel was wrong. Mendel was, and still is, correct on what he said. However, what we now know is that under certain conditions his "laws" no longer hold true.
Mendel wasn't perfect, but then again, no one is.
Science has the job of explaining those exceptions. If Mendel had encountered any exceptions in his rules in his day, then he'd probably would have simply ignored it. We, on the other hand, cannot simply ignore such exceptions. Science must explain the unknown. With that in mind, let's begin
If you remember, Mendel's first law of inheritance states that there is no blending of traits. Children only show one of two traits.
However this is not the case when we look at the plants known as Antirrhinum majus. (Holy smokes, can you say that name.) This plant is commonly known as the snapdragon. Snapdragons are beautifal plants. (Well, at least in my opinion they are.) Snapdragons generally blossom in spring to early summer, fall.
     The snapdragon disobeys Mendel's laws
Like apples, snapdragons come in many different colors. There are red, yellow, orange, and purple snapdragons. The reason for this is that the snapdragon, like all plants, have a gene that encodes a protein that is responsible for its color. This gene comes in many different forms, called alleles. The red snapdragon and its allele, are the wild types. Let's see what happens when we mate a red and white snapdragon.
Snapdragons come in many different colors. The red snapdragon is the wild-type and under scientific investigation
If we mate a red and white homozygous snapdragon, what we should expect is that all of kids should come out as red. This is what Mendel's first law of inheritance would predict.
However this is not what we get. Instead, what we get is a pink snapdragon. In other words, there is a "blending" of colors. The wild-type red color is no longer dominate.
If you think that is strange, then look at what happens when two child pink snapdragons are mated. The result is that we get pink, white, and red snapdragons.
Here's a summary of what was just stated. This blending of traits is what is referred to as incomplete dominace. The wild type trait and its allele, for some reason, is not completely dominate in a heterzygous plant.
Incomplete dominace is something that is not quite well understood. Today, many researchers around the world are trying to understand how it works. Maybe one day you yourself might uncover the secret of imcomplete dominace. You might even help shed light on our next exception to Mendel's rule, co-dominace.
It's said that blue blood runs in the veins of those who are English. While all human blood is certaintly red, (not blue), what does runs in the veins of some people is something called co-dominace. Before we look at co-dominace, let's take at look human blood, and the different types of blood that exist.
Your blood is very important. The blood is part of the transport system. Your blood carries important chemicals and other substances to different parts of your body. Your blood is composed of blood cells, that are of two types, red and white blood cells. White blood cells are apart of your immune system that protects you from invaders, such as viruses, or bacteria. Red blood cells are the cells that we want to look.
If you ever went to the doctor, you probably were asked what type of blood do you have. It turns out that people have different types of blood. Here's why!!! On top of your red blood cells sits a glycoprotein. A glycoprotein is a protein that has a sugar attached to it. Your immune cells, (white blood cells), use this protein so it won't attack your red blood cells.
The name of this glycoprotein is isoagglutinin. It is made a gene which is called the isoagglutinin gene. Let's call this gene, I. This gene comes in three different alleles (different forms). I(A), I(B), and I(O). Different forms of the isoagglutinin protein is why the allele exist in the first place.
Your parents determine what type of blood you have. You can have type A, B, AB, & O blood. The AB type blood is an example of co-dominace. Both proteins encoded by each I(A) & I(B) allele, both sit on top the blood cell of a person with AB blood type.  In other words, both traits are expressed in a person.
Of course you cannot "see" your blood type as you can see flower color, but the point is that both traits, (indirectly), are expressed in a child. Unlike incomplete dominace, there is no mixture as we saw in our pink snapdragons. Another example of co-dominance is sickle-cell anemia, a common disorder that very common in my community, the African-American community. There are many other examples of co-dominace. Like incomplete dominance, co-dominace is not quite well understood, so the race is on to unlock the secrets of this genetic mystery.
Questions,  then e-mail me geneticsrus@.yahoo.com
Also, check out Howstuffworks! This guy named Marshall Brain has created a business that explains how things works. The material is presented in a cool, and easy manner that makes it one of the most popular sites around.