Alleles are alternatives of one gene. For example brown hair or blonde hair or brown eyes and blue eyes.
These are the same gene but just differ in how they are expressed. Some alleles are dominate or recessive to each other. The blue eye allele is recessive to the brown eye allele. For a person to have blue eyes, they must have two blue eye alleles.
A brown eyed person has either one allele for brown eyes or two. They can carry the blue eye but it is not expressed.
Other alleles are more complex. Some are co-dominate. The best example of this is the ABO blood type. A and B are co-dominate and O is recessive.
In the image above, each letter stands for an allele of one gene. A is the dominate allele of the gene and a is the recessive allele of the gene. It doesn't matter how they are written, AA, Aa, or aa are all possible expressions of one gene.
You would get one from your father and one from your mother.
If these stood for eye color, a person with BB (brown eyes) would have two copies of the dominate gene and have brown eyes. He would not give any other color to this children except B (brown).
A person with Bb (brown eyes) would show the dominate color but would have a recessive copy that would not show. This person could give B or b to his children. If the other parent also was Bb, it would be possible for a certain number of children to have blue eyes even though both parents have brown eyes.
A person with bb (blue) would have the recessive eye color. They could only give b (blue) to his children.
You can see that the probabilities are 3 to 1 (Brown to blue).
These are by chance and the couple could have all Brown eyed children or 2:2 or all blue eyed children.
DNA contains genes . Each gene controls a single character and each gene has its two alternative form called alleles.
Example. Plant height is controlled by a gene , and it has two alternative forms
1. Tall ( TT )