Genetics

This section deals with the Mendelian genetics involved in budgerigar varieties. It is only a basic account to allow calculation of off-spring and does not fully explain the genetic processes involved. A more detailed account can be found in a genetics text.

Genes and DNA

DNA is the basic building block for life on earth. Genes are made up from DNA (deoxyribonucleic acid), and it is genes that contain the instructions for cell activity. The genes are located in long sequences called chromosomes.

In reproduction, the chromosomes in each parent divide in two, and the resulting organism receives half of it's genetic make up from each parent. Genes control every aspect in the new organism, and some characteristics are the result of multiple genes working together. In budgerigars, variety is determined with a single gene for each variety, making things somewhat simpler.

On each chromosome, the genes line up in pairs. The corresponding genes are called alleles. Each characteristic is controlled by two alleles that line up at a certain position on a chromosome. When reproduction occurs and the chromosomes split, a single allele is passed on from each parent to the off-spring, so that it will also have a pair of alleles for each spot on the chromosomes.

With determining the variety of budgerigars, a pair of alleles determines whether or not the bird will be a certain variety. With dominant varieties, the bird will show that variety if it has one or both of the alleles for the variety. With a recessive variety, the bird needs both alleles for the variety in order for the variety to manifest in that bird. If only one allele is present for a recessive variety the bird is said to be split for that variety, and can pass the allele on to off-spring.

The genetic makeup of a bird can be represented using letters of the alphabet, with a pair of letters for each variety. If no alleles are present for a variety, then lower case letters are used, if an allele is present then upper case letters are used. For example, if a bird had two alleles for the Grey variety, this could be represented by GG, where a bird with one allele for grey could be Gg or gG, while a bird with no grey in it would be gg.

Dominant Varieties

With the dominant varieties, the bird needs only one allele out of the two to be for that variety. The dominant varieties are Australian Dominant Pied, Dutch Dominant Pied, Continental Clearflight, Grey, Violet and Spangle. Taking the example above with the grey bird, the birds with makeup GG, Gg, and gG would all appear grey, while the bird with gg would appear normal. This example can be applied with all of the dominant varieties, with the exception of spangle. Birds with one allele for the spangle variety will appear spangle, while birds with two alleles appear self-coloured. A bird with make up ss would be normal, Ss or sS would be single-factor spangle, and SS would be double-factor spangle, appearing to be all white or all yellow.

Recessive Varieties

The recessive varieties are Danish Recessive Pieds, Clearwings, Greywings, Dilutes, Saddlebacks and Fallows. In order for the variety to manifest in a bird they need both alleles for the variety. A bird with type CC would appear as a clearwing, while Cc or cC would be called 'split for clearwing' and would appear normal, while cc would be the normal bird. Recessive varieties that are split can pass the variety or the normal onto off-spring, depending on which allele is inherited.

Colours

The colour of the bird is also determined with a pair of alleles. The blue series of birds is the result of a recessive colour gene. In order for a bird to appear blue, it must have two alleles for the colour. A bird with one or no alleles for colour will appear green. Using C for the colour allele, a bird with cc, Cc or cC would appear green, while a bird with CC would be blue.

The shade of colour is controlled with another gene, those birds with no alleles for the colour modification will be light green or sky blue, those with one allele will be cobalt or laurel, and those with two alleles will be mauve or olive.

Sex-Linked Varieties

The chromosomes that are used in determining the sex of an organism are called the X and Y chromosomes. The X chromosome is similar to other chromosomes and carries genetic information, however, the Y chromosome is smaller and almost devoid of information. In budgerigars the male has two X chromosomes and is represented by XX, while the female has one of each chromosome and is represented by XY. The varieties Opaline, Lacewing, Ino and Cinnamon are determined by alleles on the X chromosome. In the male bird, the varieties function like a recessive variety. In the female bird, the Y chromosome has no matching allele for the variety, so the female is determined by the one X chromosome. If an allele is present then the variety will manifest, if it is not present then the bird will be normal.

If O is used for opaline, in the male birds OO will be Opaline, Oo and oO will be split for Opaline and oo will be normal. In the female, OY will be Opaline, and oY will be normal. The male passes on one of his X chromosomes to off-spring, while the female can pass on the X chromosome, making the off-spring male, or the Y chromosome, making the off-spring female.

Examples

It is important to be able to calculate the variety of expected off-spring from a breeding pair. The examples below show how to do this using the theory above.

Using C for the blue colour allele, the mating between a light green bird and a sky blue bird would be represented by cc x CC. The light green bird passes one allele onto the off-spring, in this case the only possible allele is c. The blue bird passes on one allele, in this case C. By drawing a grid with the birds' allele to be passed on each side the off-spring are calculated.

C
c cC

The off-spring in this case will all be of type cC, and will be light green split for sky blue, or light green / sky blue.

Using C for the blue colour allele, and F for the fallow allele, a mating between a light green fallow with a light green / sky blue fallow would be represented by ccFF x cCfF. The combinations of alleles in the first bird is cF. The combinations in the second bird are cF, CF, cf, and cF. These are then put into a grid.

cF CF cf cF
cFccFFcCFFccFfccFF

The off-spring are ccFF, cCFF, ccFf, and ccFF. This translates to 50% light green Fallow, 25% Light Green Fallow/ Sky Blue, 25% Light Green / Fallow.

Using I for the Ino allele(albino is a blue ino, lutino is a green ino) and C for the blue colour allele, calculate the mating between an Albino Hen and a light green / Lutino Cock. This is represented by IYCC x IiCc. The female alleles can be combined to get IC or YC while the male can be combined to get IC, Ic, iC and ic. Putting this in a table gives:

ICIciCic
ICIICCIICcIiCCIiCc
YCIYCCIYCciYCCiYCc

The off-spring are (Albino, Lutino / Albino, Sky Blue / Albino and Light Green / Albino) Males and (Albino, Lutino / Albino, Sky Blue, and Light Green / Sky Blue) Females.

Any mating between two birds can be calculated in this manner, by writing out the birds type in alleles, working out the combinations of alleles available, and drawing up a table with the combinations of the birds alleles on the sides.

Calculating the off-springs type lets you accurately trace the genetic type of your birds.


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