An allele (UK /ˈæliːl/ or US /əˈliːl/), or allel, is one of a number of alternative forms of the same gene or same genetic locus. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. However, most genetic variations result in little or no observable variation.
Most multicellular organisms have two sets of chromosomes; that is, they are diploid. These chromosomes are referred to as homologous chromosomes. Diploid organisms have one copy of each gene (and, therefore, one allele) on each chromosome. If both alleles are the same, they and the organism are homozygous with respect to that gene. If the alleles are different, they and the organism are heterozygous with respect to that gene.
The word "allele" is a short form of allelomorph ("other form", a word coined by William Bateson), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes. It derives from the Greek prefix ἀλλήλ, allel, meaning "reciprocal" or "each other", which itself is related to the Greek adjective ἄλλος (allos; cognate with Latin "alius"), meaning "other".
In a far of land there lived a beautiful butterfly
In a kingdom full of blossom covered trees
And winding through this paradise, a river danced along
And played with the constant summer breeze
The butterfly found a tiny branch at the riverside
And watched the water rushing who-knows-where
Carefully he sheltered from the power of the breeze
For surely it could toss him through the air
“I’m a butterfly,” he said, “trivial and small
And in the greater scheme of things I don’t mean much at all
So I’ll flap my wings to stretch myself and just enjoy the view
I’m a butterfly, what more can I do?”
One afternoon, the butterfly asked the river
“Madam, what makes you hurry so?
What splendid destination are you rippling toward?
Where, oh, where do you go?”
The river said, “I’m headed for the ocean
And summer breeze inspires me to race
My friend, you’d love the ocean, it’s remarkable to see
Won’t you join our friendly chase?”
“I’m a butterfly,” he said, “trivial and small
And in the greater scheme of things I don’t mean much at all
So I’ll flap my wings to stretch myself and just enjoy the view
I’m a butterfly, what more can I do?”
So the butterfly, he dreamed of the ocean
He longed to flutter high above the seas
But there are dangers in the sky for a tiny butterfly
So from his branch he asked a question of the breeze
“What makes you chase the river towards the ocean?”
And the breeze told him a most amazing thing
“I’m simply made up of the currents in the air
That start from the movement of your wing, your tiny wing
“You’re a butterfly my friend, powerful and strong
And I’m grateful for the way you’ve always hurried me along
When you flap your wings to stretch yourself, it might seem small to you
But you change the world with everything you do”
So he stretched his wings and took off from the safety of his tree