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I have noticed on this forum some individuals have a grasp concerning the phenotypic expression of the blue gene in chickens, but the genotypic understanding is sometimes lacking. I hope the following information will be helpful to any interested party.


First, some genetic information about genes and alleles.  As many of you know, the DNA located within a chicken’s cell contains all the information for making and maintaining the chicken. You could say DNA is like a very large and complicated recipe for building a chicken. This very large recipe (all the DNA in a cell) can be divided into smaller recipes called chromosomes. If a recipe for a chicken would fill a book (the DNA), then a chromosome would be several pages in the book. On the chromosomes (pages), there are simpler recipes (paragraphs) for making certain characteristics of a chicken; these paragraphs are called genes. A gene would be a complete recipe for the expression of a specific characteristic (specific to the gene).


Sometimes it takes more than one gene to cause the expression of a characteristic.  For example, the silver partridge secondary color pattern in chickens requires three different genes to be carried by the female chicken. If the book of recipes for making a chicken had pages, each page would be divided into paragraphs. Each paragraph would have a specific location on the page.  The same is true for a gene; each gene has a specific location on the chromosome and this is called a locus. 


Book --- Pages ----paragraph (paragraph number)

DNA---chromosome---gene (locus)



To make things interesting, chromosomes normally are found in pairs within the cell. For each chromosome there is a matching and somewhat identical chromosome. It stands to reason that if chromosomes are matching then there are matching genes on the two matching chromosomes. Sometimes this is the case and at other times it is not the case. For each locus (position on a chromosome) of a gene there is normally (but not always) another gene found at the same locus on the matching chromosome. Sometimes the genes are not the same but are still found at the same locus on the chromosome. These genes are called alleles.  For example, the allele for blue color is found at the blue locus on a chromosome; the other allele found at the blue locus can be blue or it may be not-blue. A chicken can have two not-blue alleles at the blue locus or a blue allele and a not-blue allele or two blue alleles at the blue locus.  


In order to produce a blue chicken, the blue allele must have help from other genes. The easiest way to understand how the blue allele works is to think of the blue allele as a diluting allele. The chicken must first be genetically programmed to be black so the blue allele can dilute the black color (caused by other genes) to a blue.  At the blue locus, a chicken can carry a blue allele (B*B) on one chromosome  and a natural allele (B*N) on the other chromosome. When a chicken carries one blue allele, the expression of the blue allele will be a dilution of black color to a blue color.  The chicken will be blue.



If a chicken carries to natural alleles B*N/ B*N, the natural alleles have nothing to do with making the chicken black.  There are other genes that make the chicken black.  A possible genotype of a black chicken could be E*E/E*E (extended black at the E locus), with Ml*Ml-PG*PG/ Ml*Ml-PG*PG at the melanotic and pattern locus.  


If a chicken carries two blue alleles, then the expression by both alleles is a splash color.  Splash chickens have  a whitish to gray ground color with splashes of black or blue found on various areas of the bird. 

Edited by Wappoke - 6/4/16 at 6:56pm