Explanations and couplings
Why, these three mutations combined are so difficult to predict ?
Simply because we can not speak at the genetic level of different mutations but rather allelic versions of a single gene. The pale back, the masked and the old type mask (OT) are due to the same gene but which has three allelic versions. You can find the phenotype of each one in photo in this article : Illustrated glossary of mutations in zebrafinch.
In order to understand well, let us make the parallel with the man, the color of the eyes for example, whatever the color of our eyes, our color of the iris and coded by the same gene, but this discomfort has many different versions (alleles) that allow us to have the color panel that we know.
Now that we know a little more, let's see how each allele behaves in relation to each other.
A small table to illustrate all this :
Allèle \ allèle Back pâle Masked Masked OT* Back pâle X Dos pâle Back pâle Masked Back pâle X Masked Masked OT* Back pâle Masked X
* OT = old type
In this double-entry chart you can see that it allele dominates the other, so the bird will have the phenotype of the allele that dominates. Be careful, it's not because the allele is dominated that it does not influence. See also the pale back / OT mask : The back is more diluted because of the masked OT allele.
From this result we can draw the first conclusions :
- The pale back may be masked or mask OT.
- The masked can be masked OT but can not be bearer of pale back (pale back dominates the masked).
- The masked OT can not be bearer of pale back, nor masked because these last two dominate it.
Female masked gray old type
To know :
- Each bird has two chromosomes so it has twice the same gene (but not necessarily the same allele).
- The pale back, the masked and the masked OT are mutations linked to the sex, they are thus carried by the sex chromosomes. In birds, the male has twice the same sex chromosome (ZZ) and the female has two different sex chromosomes, one of which bears the genes linked to the phenotype (ZW, the W carries the genes related to the phenotype).
Now let's detail each possible coupling. Let's start with the pale back and the masked below.
Male pale back X masked female
Female\Male ZBp* ZBp ZM ZBpZM* ZBpZM W ZBpW ZBpW
*Bp = Back pâle; *M = masked