Due to decreasing use over the years, I have decided to disable the forum functionality of the site.

Forums will still be available to view but new posts are no longer allowed.

Need genetics help - chickens...

Jenks January 17, 2010 February 22, 2011

Forums

Here is the question from a group member: > I need some clerification on this and hope someone can help. My books say that Side Sprigs are " Dominant to normal Comb" can someone explain this to me in Hillbilly language. In my mind I read this to mean that if you breed fowl with side sprig you will more than likely get chicks with the side sprigs. Or does this mean that you could breed one bird with side sprig and one without side sprig and maybe get some or most without side sprig. I do believe that if both parents have side sprigs you will most likely get side sprigs. I myself do not use the birds with side sprigs. I am looking for some expert opinions on this as I don't know one way or the other. Don > Here was the answer received by the group: The research indicates that it takes two autosomal dominant genes to cause side sprigs. These two genes are complimentary. For example lets say the symbol Sg1 was for one gene and the other gene would be Sg2. These two are completely different genes that cause the side sprigs. They must work together to produce the side sprigs. If a male bird had two Sg1 genes and was Sg1/Sg1 but had the other side sprig genes that were recessive or sg2/sg2 then the bird would not have side sprigs. In order to have side sprigs, the bird must have at least one Sg1 gene and an Sg2 gene. Lets say a female bird has sg1/sg1 genes but has two Sg2 genes or is Sg2/Sg2. If the above male and female mate and produce offspring, each offspring will inherit an Sg1 gene and an Sg2 gene and will express the trait side sprigs. There is no symbol for side sprigs- I used the Sg's just for an example. A long as a bird does not inherit the two different dominant side sprigs genes they will not have side sprigs. [b]My[/b] question is: How is it autosomal dominance if one doesn't cause the condition, but it takes the compliment to be hetero to have the manifestation of the condition? What is this called? Is that right?

Monsterpony Sun, 01/17/2010 - 22:31

It is an epistatic condition, meaning it requires at least two genes to show. The bay color is an example. If the sire is EE aa, then he is not bay, but has the needed black gene. Then say the mare is ee AA, which means she has requisite agouti genes for bay, but not the black. But you breed them together and you end up with all bay offspring. You need both an E and an A to express that specific genotype.

For the side sprig, at least one parent must be able to donate an Sp1 and an Sp2. So the one could be Sp1- and Sp2- and the other have neither. Or one have Sp1- and the other have Sp2-.

horsegen Mon, 01/18/2010 - 11:59

The technical term would be "polygenic", meaning the trait is controlled by more than one gene. Epistasis is actually defined as the effects of one gene masking another, and bay is not a great example of epistasis. A better example of epistasis in horses is dominant white, or gray. If a horse gets either of these genes, they mask whatever the other genes code for. Bay is, in some ways, epistatic (the chestnut color does mask whatever the agouti allele codes for, only because the horse is already producing pheomelanin), but the extension gene does have a phenotype on its own, which is modified by the dominant agouti allele. That's a slightly different phenomenon from true epistasis.

Polygenic is the term to use in this case, because neither of the side sprig genes have a phenotype on their own...they must work together to produce a single phenotype. It's not true epistasis because one gene isn't masking the effects of the other...if both are not present, the phenotype (normal comb) is the same.