Allele vs Genotype vs Phenotype - The Breakdown

Often students find it hard to tell the difference between alleles, genotypes, and phenotypes, just because they are so intertwined. Here's a breakdown of how these tricky biology vocab words are related.. 

Alleles are versions of genes, there's tons of alleles and thus genes on our chromosomes. For each gene, whether it be eye color, hair color, or whether we are right handed or left, there are two alleles (sometimes more). The reason there are two is because we get one chromosome from our mom and one from our dad. Those chromosomes carry tons of alleles - each being a different version for a particular gene. So for every chromosome pair (remember humans carry 46 chromosomes, or 23 pairs - these pairs are made up of one chromosome from dad and one from mom) there are two alleles for every gene. 

Those two alleles make your genotypes ( written as Aa or AA or aa) for a particular gene. So you can either be homozygous recessive (written as aa) for a gene, homozygous dominant (written as AA) or heterozygous (written as Aa) meaning you have one dominant allele and one recessive. When you're heterozygous, what's expressed is the dominant allele (A, not a). When you're homozygous dominant or recessive then only the A or the a allele is expressed. Whatever is expressed makes up your phenotype.

It's easier to explain with an example. Lets say you have two alleles (A and a) that you acquired from your mom and dad. From your dad you got an allele for blue eyes, which is recessive (so the allele would be written as "a"). From your mom, you got an allele for brown eyes which is dominant (allele would be "A"). So your genotype then, for eye color  would be Aa - or heterozygous. Your phenotype is what is going to be expressed, which is always the dominant allele with a heterozygous genotype. The dominant allele here is brown eyes, so your phenotype would be brown eyes! Your phenotype is what is physically expressed, it is what's produced from expressing the gene/alleles. It can be eye color, height, hair color, or anything down to different proteins. In general terms, it's what you "see" as a result.