4.2 Alleles

Alleles
Sometimes nature produces two three or even more variants (versions) of the same gene. These variants are called alleles. I will try to explain by a diagram..

We already know that in the DNA two copies of genes are present which may be the same or different. We therefore have two places where the blue or one of its variants may be present. We discussed the possibilities in the budgerigars of the two genes being bl/bl, bl+/bl or bl+/bl+. Things are a little more complicated as more than one type of the blue gene exists. These are called the Blue white face (same as simple blue), blue Yellow face type I, blue yellow face type II and the blue Golden face. All these genes are called alleles of the blue locus. The important thing to understand in this figure is that all these genes can only fit on the DNA in the place represented by the circle. Since there are only two circles on the DNA diagram, only two genes from the list of alleles given on the right can be present in any given bird. There is an order of dominance amongst these genes. The blue white face (bl) being the most recessive or the least dominant and the normal (bl+) or the green being the most dominant. If the two copies are the same, the bird will be a visual of that gene. For example if both copies are blue whiteface then the bird is a visual whiteface blue, if both are blue golden face then the bird is a visual blue golden face. If however the genes are different the bird will be a visual of the gene appearing higher in this list and a split to the other. As an example consider that a bird has a blue Golden face gene and another blue yellow face I gene. The bird will then be a visual blue golden face and split to blue yellow face as blue golden face is higher on the list.

To summarize alleles are different versions of the same gene. Only a maximum of two versions may be present in any given bird. 

Homework
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Which of the following budgerigars is genetically not possible.
1) A Yellowface type I split to whiteface
2) A Yellowface type II split to golden face
3) A Whiteface blue split to golden face
4) A normal split to yellowface II