Every part of your body has been created due to your own unique genes. Genes are the fundamental part of life that are passed on from one living organism to another, determining what that organism is and looks like. Your genes were given to you by your parents -- they are a combination of the genes your parents have. There is a gene that determines what color hair you have, and there's another gene that controls what color eyes you have, and so on for the rest of your body. Genes not only control your appearance, but also make sure your body is working properly, from your tiny cells to your body as a whole. Genes are made up of a very small substance called DNA (which is short for "Deoxyribonucleic acid"), but nobody knew this until the 1940s because DNA is so tiny. Your DNA is in every cell of your body, with few exceptions.
Alleles are different versions of the same gene. For example, I said that there is a gene that controls what eye color you have. An allele can be the version of this gene that makes you have a certain eye color, such as brown or blue. Every gene in your body has two alleles. If you have at least one dominant allele for a gene, this is the version of that gene you will show. For example, brown eye color is a dominant allele. If you have just one brown eye color allele, you will have brown eyes. On the other hand, blue eyes are a recessive allele. In order for someone to have blue eyes, both of their two alleles must be for blue eye color. If you only have one allele for blue eyes and your other allele is for brown eyes, you will have brown eyes because brown eye color is a dominant allele. However, you will still have the recessive blue eye color allele and this recessive allele may be passed on to the children of this person.
An allele is a particular copy or state that a gene can have. So, any diploid organism (like any animal) has two copies of every gene. Each variant on these is called an allele.
Dominance is what happens when you have two different alleles of the same gene. If one of the alleles is dominant and the other recessive, then you will express the dominant allele. You must have two identical copies of a recessive allele in order to express it.
The human example that is most commonly given is eye color. So, the gene that determines the pigment in the front of the iris has two alleles, pigmented (usually brown), or un-pigmented (blue). The brown eye allele is dominant over the blue eye allele. Therefore, if you have two brown eye alleles, or one brown eye allele and one blue eye allele, you will have brown eyes. On the other hand, if you have two blue eye alleles, then you will have blue eyes.
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