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an allele is a wierd thingy we learned about in science
well what exactly is it
An allele refers to one form of a gene or locus. To people just learning about alleles, the concept is sometimes hard to understand.
Most organisms are diploid, meaning that they have two sets of chromosomes. In humans, we have 46 chromosomes. These 46 are made up of two sets of 23 pairs, or homologous chromosomes. We will have one copy of each gene on each of the chromosomes in a pair. However, each copy is not necessarily the same. One may be a different form. The fact that this homologous chromosomes can have genes that differ is what necessitates the word "allele," so that we may differentiate between different forms in the same gene locus.
I don't know all the genes involved in eye color, so the example I'm about to give should not be treated as a fact...just something used to clarify.
Let's say that in this example there are only two eye colors possible, brown and blue, and let's also say that brown eyes are dominant over blue eyes. Often alleles are given a letter to symbolize them. Capital letters for dominant traits, and lower-case for recessive. So in this example, let's say that brown will be "B," and blue will be "b."
-side note, dominance refers to an allele's ability to...overpower...another allele. In this case, if one allele is for brown, and the other is for blue, the brown eye color will be expressed, while the blue eye color will be recessive and will not be expressed.-
So I have two alleles for my eyes, BB. This means that I have two alleles and that each is the same in form...each being the dominant brown color. When two alleles are the same, the trait is called homozygous. I am homozygous dominant for brown eyes.
My friend has brown eyes as well, but her two alleles are Bb. This means she has two different alleles for her eye color trait. One is brown, B, and the other is blue, b. As discussed above, the brown eye color is dominant, and blue is recessive. This means her brown allele will be expressed and the blue is not, resulting in her having brown eyes. When two alleles are different, the trait is called heterozygous. She is heterozygous with brown eyes.
Another friend of ours has blue eyes. Knowing that in our example blue eyes are recessive, the only time the blue color will be expressed is when there is no dominant allele, B for brown eyes. So he must have two alleles as bb. These two alleles are the recessive alleles for blue color. Because there is no dominant allele here, the blue color can be expressed. Again, he has two of the same alleles, so he is homozygous recessive with blue eyes.
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That is a super long explanation, but alleles can get very complicated. What I gave as an example is the most basic of cases. With some living things, what is expressed by dominance or recessive traits gets much more complicated. Also, not all things are perfectly diploid or diploid at all, so they might have more alleles than simply two.
Very good answer, biophil!
Thanks blue. I was hoping the message didn't get lost in the long explanation.