The same ratio as we started with. Brown did not become more common. Now these folks all pair up randomly and have 4 kids each. Since we aren't going to allow incest, the Bb folks will find a bb for a mate. If they have 4 kids each, then we have 44 bb and 4 Bb. Again the same eleven blue to one brown ratio. Whether an allele is dominant or not does not affect how common a trait is. Now of course traits can become more common over time.
If brown eyes gave an advantage, then it would start to become more common. Brown eyes would also become more common if a bunch of Africans moved in or many of the blue-eyed people were killed for some reason witch burning? And there are other ways too of getting more brown eyes in Europe. Or more blue eyes in Africa see South Africa for example. Dominant does not mean common. Why some gene versions are dominant and some are recessive. Search-Icon Created with Sketch.
I hope it helps! One way a protein can gum up the works is by getting in the way of the normal version. This can happen because each protein has many different jobs. Sometimes a broken protein can carry out one of its jobs just fine but it can't carry out its second job.
When this happens, the broken protein gets in the way of normal proteins that are trying to do the second job. Imagine a basketball team. Four of the players know how to catch and shoot the ball, but the fifth player only knows how to catch the ball -- she can't shoot it. Every time that fifth player catches the ball, the shot clock expires and the team loses possession of the ball. So even though there are four good players on the team, the fifth player like the broken protein prevents the team from winning.
This situation is called a dominant negative allele. An example of an allele like this sometimes happens in the p53 gene. The p53 protein is very important for making sure our cells don't grow too fast.
Certain alleles have mistakes in their p53 gene so that the cells grow too fast and cause cancer. Each p53 protein is like a member of the basketball team.
Some broken versions of p53 can bind other p53 molecules catch the ball but they can't carry out their normal job of making sure our cells don't grow too fast shooting the ball. So the entire bunch of p53 proteins is ruined by the one bad protein.
The broken p53 protein wins out, so it's dominant. Another way a broken protein can be dominant is if it does something new, or does its usual thing at the wrong time. When a protein does something it shouldn't, it's called gain-of-function. Let's go back to our basketball team. Just like before, we have 4 good players. This time, the fifth player shoots as soon as she gets the ball, no matter where she is on the court.
So she almost always misses. By shooting at the wrong time, she makes the team lose. Or imagine that the fifth player does something new. Every time she gets the ball, she drop-kicks it into the stands as if she's playing football.
By using this new skill, she makes the team lose. An example of a gain-of-function allele occurs in achondroplasia, or dwarfism. Dwarfism is caused by a broken version of the FGFR3 gene. The normal job of FGFR3 is to prevent bone growth. The broken version of FGFR3 is hyperactive -- it tells bones to stop growing even when they should be growing.
It's like the basketball player who shoots as soon as she gets the ball. The hyperactive FGF3 protein causes a person's bones to be much shorter than normal. Even if a person has a normal copy of FGFR3 around, the broken version sends a signal that's just too strong, and since it wins out, it's called a dominant allele.
As you can see, there are lots of ways to get a dominant or a recessive allele. Huntington's disease, for instance, is a dominant mutation where, if one is carrying that version of the Huntington gene, that mutation, that dominant mutation, will give the individual the disease regardless of what that person's other Huntington's disease gene allele is.
That other Huntington's disease gene allele can be perfectly normal, but the person still has the disease because of that one copy of the Huntington's disease gene that is mutated.
That is dominance. Christopher P. Austin, M. Featured Content. Introduction to Genomics.
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