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An allele is one of the possible forms of a gene. Most genes have two alleles, a dominant allele and a recessive allele. If an organism is heterozygous for that trait, or possesses one of each allele, then the dominant trait is expressed. A recessive allele is only expressed if an organism is homozygous for that trait, or possesses two recessive alleles. Alleles were first defined by Gregor Mendel in the law of segregation.
When studying heredity one of the most common things to mess up is the terms gene and allele. It's really easy for somebody to say one when they mean the other. So let me first define what a gene is So that I can explain exactly what an allele is in that context.
So a gene is a particular region of your DNA that controls a specific trait. Examples of genes are, there's a gene on one of your chromosomes that controls whether or not some muscles are right across your tongue at a particular time during foetal development.
One version of that gives you the ability to roll your tongue. A different version of that gives you the inability to roll your tongue. Another example of a gene is a gene design guiding the make up of your earlobe. Some people have a version of that gene that gives what's called a detached or sometimes free earlobe shape. Other people have a version of that same gene that controls the earlobe shape that creates this attached earlobe shape. We call these different versions of a gene alleles. so it's kind of like saying genes are like articles of clothing and alleles are different brands. This is a shirt. There are various versions of the shirt gene, those various versions or brands are the different alleles. I have the shoe gene. Right now I'm using the dress shoe allele. If I went out running I would put on the sneaker version or allele for that particular shoe. I don't talk about the shoe allele of the shirt gene. I don't wear those. Alright?
Now, something else to understand is taht alleles can be dominant or they can be recessive. A dominant allele is one that will show itself in a cell or in a person's body. So for example the detached earlobe, the free earlobe is the dominant allele for this particular gene. For the earlobe gene. Whereas the attached earlobe shape that allele is recessive which means you don't see its effects unless it's paired up with an identical attached earlobe allele.
Now when you're naming alleles, what you do is you pick a letter that represents the gene. Say for example if we're talking about earlobes, I'll use the letter e. Then you use the capital letter of that letter for the dominant. So I'll call this the big e allele and then I'll use this little e allele for the attached earlobe. Alright? So that's what an allele is. You can keep in mind that an allele is a version of a gene then this becomes a lot easier.