A little history
As we know today, Gregor Mendel, best known for his pea experiments, has been at the root of genetics. He has shown by his experiments that if he crossed two peas (F1) with different characteristics such as the color of the flower, the size of the leaves ... Then the resulting (F2) kept the characteristics of only one parent. All the flowers of these young peas had the same color and size of leaves.
It's as if they had "lost" one of the properties. When he crossed these F2 peas between them, the F1 characteristics reappeared among the F3 generation. Mendel called these features shown by the F2 : Dominant. And the hidden features in F2 have been called recessive.
Currently we still call it dominant and recessive. However, we know that Mendel discovered "complete dominance". There are indeed other forms of dominance. We already know these forms so we will especially bring some provisions to remember.
We know that genes carry characteristics and that these genes are located on chromosomes. There are genes dealing with the color of the eyes, the color of the legs, the size of the bill ... The chromosomes are located in the cells of the body: They are stored in the nucleus of each cell. In each nucleus of each cell are the genes for the color of the eyes, the
color of the paws, the size of the beak ... However, the functioning of the genes "eye color" is manifested only in the eyes. In the legs, the genes "eye color" do not appear. Each cell "knows" where it is in the body and what genes it needs to activate.
The chromosomes go by pair, all the genes are found in pairs. For the eye color gene, we have two genes. This also applies to the color of the legs, the size of the bill ... These two genes for the color of the eyes can cause a color of the blue eyes. It is also possible for one gene to take care of the blue color and the other for a brown color. (This does not mean that the being will have one blue eye and the other brown, the brown eye is dominant on the blue, so both eyes will be brown).
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
It is in 1960 that appeared in Belgium the first Dos Pâles Bruns.
Let's try to understand how such a combination of colors could be born.
Brown and pale-back sex-related factors are known to be located on different X chromosomes and at different locations (loci): (1) and (2).
They are therefore not normally linked (otherwise all the browns would be pale as well: which is not the case).
How could they be linked on the same chromosome? (3)
When a brown male is paired with a pale gray - backed female (or vice versa), gray males with brown and pale backs are obtained each time. Each male therefore has two different X chromosomes: one carries the genes "brown" "not pale back", the other carries the genes "not brown" and "pale back". Being recessive, none of these genes can express themselves since it is in a single copy; being non-alleles, none can dominate the other; it is therefore a natural gray color that expresses itself.
How will these genes be transmitted by the male to his offspring ? To understand it well, some explanations are necessary.
Chromosomes are very long molecules (2 millionths of a millimeter thick, 5 cm of average length in humans) entangled, in normal times, with each other in the nucleus of the cell. At the time of meiosis (cell division allowing, in males, the formation of spermatozoa from the mother cells of the testes), these chromosomes split into two exactly identical chromatids attached to each other by a centromere.
Each chromatid then spirals. It is only then that the chromosome becomes visible under an optical microscope. The chromosomes cluster together and pair with each other in pairs.
During this phase, two chromatids of the two contiguous chromosomes can cross, break and then be joined together by exchanging more or less important segments. This is the so-called crossing-over phenomenon.
The "brown" gene could thus be linked to the "pale back" gene on the same X chromosome. A gray male with brown backs and pale backs can (but only in this way) produce pale gray, gray, brown backs and pale brown backs. (12.5% of each).
With this spanning, this same male could have also :
• Crossed with a pale-backed female: 12.5% pale gray-backed male with brown.
• Crossed with a brown female: 12.5% brown male with pale back.
By coupling one or the other of these with their sister "pale brown back", it is possible to obtain (in 3rd generation): 25% of pale brown backs and 25% of pale brown backs.
A : Normal
B : Contracted spiral
C : Schematized
We do not know if this is how the mutation actually appeared but the hypothesis seems likely. The brown masked that appeared at the same time in Great Britain may have had the same origin (a crossing between the chromosomes of a gray-backed gray-backed male with brown and brown can give females masked brown).