The mutual relationship of the mutation factor

and the original genes

Every organism has two DNA chains. When a gene is changed on one of the DNA chains by mutation its function can be compensated by the original gene on the other DNA chain. In this case the changed gene is subordinate to the wild-type. We call this relation recessive. Only when the mutation factor is found on both DNA chains the function is distorted. Compensation is not possible. The alteration comes to expression in the plumage. An example is the fallow Bourke. Breeders try to mate two birds with the same mutation factor to develop in this way a new colour variety. They say that both the hen and the cock have to be split of the factor. A recessive mutation factor can be hiding in a big population till the coupling is made by accident.

The new mutation factor can dominate the original factor. We call this dominant over the wild-type. If such a dominant factor is found on one of the DNA chains he expresses himself immediately. It is all the same which DNA chain the factor is laying. When the dominant factor is laying at both DNA chains the effect on the plumage is much stronger. We speak about one-factor dominance and two-factor dominance. An example is the pied elegant developed by Bucholtz (Germany) in 1978. I saw on a exhibition a an elegant with a light plumage and a little bit pied. I was invited to visit his home. When I saw this birds in his aviary I was surprised to see birds with a lot of pied also. He had one-factor pied elegants but also two-factor dominant pied birds.

It is peculiar that sometimes a mutation factor is expressing himself but sometimes not. An example is the lutino Bourke. The hen with this factor has no eumelanin at all, it is a lutino colour varieties. The cock with the same factor is a wild-type. This has to do with the sex of the bird. Hens are coloured immediately. Cocks not. The solution of this riddle is the fact that the mutation factor lays on the sex-chromosome. The cock has two X chromosomes (X because of the form of the chromosome seen under the electron microscope). The hen has a X and a Y chromosome. This chromosome looks alike the letter Y. When we compare the letters x and y we can say that in the y a part is missing.

In the DNA of the cock the mutation factor is compensated by the original gene on the other chromosome. Only when both chromosomes have the mutation factor there is no compensation and the factor is expressing in the plumage. The mutation factor is laying on the part of the chromosome that is missing in the hen.

There is an other possibility left. The factor is influencing the original factor partly. An example is the red bellied turquoisine. When we mate a normal turquoisine with a red bellied we have offspring with a bit red coloured belly. Red bellied turquoisines have more intensive red because of selection during several years. We call this intermediate inheritage. The colour is gradually changing from yellow to red. An example is the red bellied turquoisine. When we mate a red bellied turquoisine with a wild-type we get a wildtype with some red . By years of selection the red bellied turquoisine became more red in the belly and breast. Red fronted turquoisines are the result. This kind of inheritance we name: intermediate. The colour changed from yellow to red.

The mutation factors of the Bourke can be classified in recessive, dominant, sex-linked and intermediate.


Recessive inheritance of the mutation factor in relation to the wild-type

gametes k k
K Kk Kk
K Kk Kk

Here the K is standing for the not mutated, original gene and k the mutated gene. The mutation factor is passed on the offspring but does not comes to expression. K is dominating k.

In fact the influence of this mutation factor on one chromosome is compensated by the gene at the other chromosome. An example of this kind of inheritance is the pastel mutation factor in the Bourke.
The recessive mutation factor, par example the pastel factor, is recessive in respect of the wild-type.


When we mate the youngsters, who have the genetic predisposition Kk, we look to the following table:

gametes K k
K KK Kk
k Kk kk

Here we see that 1/4 of the offspring have the double mutation factor (kk). When this factor k is laying at both chromosomes, he comes to expression. In a big population of birds, the factor can be hiding for a long time before he comes to expression. In a limited population the factor can come out more early, because of the limited choice of a partner.


Dominant inheritance in relation to the wild-type

gametes m m
M Mm Mm
m mm mm

When by mutation during the meiosis and fertilisation a dominant herediting mutation factor appears in one of the youngsters, this factor is expressing immediately. Compensation is not possible. The result is visible in the plumage directly. In the cross-table the dominant factor is represented by the letter M. We say this mutation factor is dominant in respect of the normal wild-type. We name this one-factor dominance. When we mate this young to a normal wild-type this factor one one of the chromosomes will inherits immediately. Mated with a partner who has not the mutated gene gives us 50% youngsters who have this mutation factor. There are no splits.

gametes M m
M MM Mm
m Mm m

When we mate two 1-factor birds (M) there is a possibility of youngsters with a double factor (MM). We name this 2-factor dominant. 25%of these young birds can be 2-factor dominant, 50% can be 1-factor dominant and 25% can be normal wild-type. A dominant factor in a population is seen immediately. This factor is spreading very fast in a population.


Sex-linked inheritance, incomplete dominance in relation to the wild-type

gametes X Y
Xi XiX XiY
X XX XY

Here the mutation factor is laying at the sex chromosome. The form of a sex chromosome, seen under the electron microscope, looks like the letter X. A cock has two series of DNA, two times X. The hen has only one X chromosome. The other chromosome differs somewhat in form. A part from the X is missing. It looks like the letter Y. The hen has one X and one Y chromosome. We So it looks like XY.
The mutation takes place on one of the X chromosomes of the male. If this chromosome goes over to a son, than the mutation factor do not come to expression. The mutation factor of the one X chromosome is compensated by the unaltered gene of the other X chromosome.
When these mutation factor goes over to a daughter, this mutation factor can express himself. The factor is not compensated, because this part of the chromosome is missing in the Y chromosome that comes from the mother.
So the factor comes out in young hens but not in young males. We are breeding hens with a new colour variety and wild type cocks.
In the male the mutation factor has to lay on both of the X chromosomes to come to expression. The dominance is existent ( by the hen) but incomplete (not by the cock). By the cock the mutation factor has to be at both chromosomes. An example of sex-linked inheritance is the opaline colour variety of the Bourke.


Intermediate inheritance, equivalence in relation to the wild-type

gametes R R
Y RY RY
Y RY RY

Here there is no dominance or recessive relationship between the mutation factor and the gene of the wild-type. They are equal. The result of the mutation factor on the colour variety is a mix of the old and new characteristics. An example is the inheritance of the psittacine pigments. The pigments are specified as R (red)) and Y(yellow). Both come to expression. Red and Yellow gives orange. The colour is laying in between both colours. The intermediate factors can be influenced by selection. We saw this in the extension of the red pigment of the red opaline variety
The intermediate inheritance plays a role in the size of the birds also. Big and small gives a medium format. The intermediate inheriting factors can be influenced by selection used by the breeder. Is seems logical that not is spoken about mutation in this case. But it is change. We can speak about modifiers. (Modify=change) This we find only in pigments. Not in structural changes. We don't can underestimate this. In most colour development by birds based upon mutation additional selection is needed. Look for instance to opaline the red rump or the opaline turquoisine.

By the psittacine colours red and yellow also mutations are possible. For instance by a total loss of one of the colour pigments, red or yellow. Here the change is directly and complete.

Remark: In this tables the starting points are the gametes. In the cross-tables of the different colourvarieties in this site the starting point is not the gamete in relation to the wild-type and the altered genes, but also the relation between different mutation factors.


Deviations of the inheritance percentages, the influence of lethal factors

When we find percentages that deviate far from the expected figures there sometimes a lethal factor is playing a role. Youngsters are dying in the egg of short after hedging. This is influencing the percentage negative. When we find strong deviations we should have this in mind.


The mutual relation between mutation factors

In the tables the starting point is the gametes and the relation to the normal gene. In the cross-tables of the different colour varieties the mutual relation between mutation factors is described. The mutation breeder asks himself what the influence of the mutation factors on the plumage is, and how combinations of mutation factors are working.

index menu

Copyright 2004 by Bob Fregeres

E-mail: fregeres@bourkes-parakeet.nl

1/19/04