Half-sider budgerigar

A domestic half-sider budgerigar

A half-sider budgerigar is an unusual congenital condition that causes a budgerigar to display one color on one side of its body and a different color on the other.

This is not a simple genetic mutation, as can be observed in other color and pattern variations in this species. It is a rare example of a somatic mutation in an early stage embryo, tipically in the first mitotic division. In that moment, the heterozigotic zigote carrying a green/blue genotype losses the green allele function in one of the two original cells. This lost will lead to express only the blue allele in half the cells of the entire body.The so-acquired condition is known as mosaicism, in oposition to chimaerism, which is the result of two embryo merging into one. tetragametic chimera theory, which states that chimerae originate when two fertilized embryos merge during a very early stage of development — between the 2-cell and the 64-cell stage is not accepted anymore, since avian eggs contain only one embryo surrounded by york -the vitelo-, a condition that prevents embryo from merging, even if two of them are contained in the same egg, as occurs in two yorked eggs.

Each half of the mosaic individual has almost identical DNA, except for the mutated gene, with genetically distinct cells for the trait involved.

The half-sider's coloring is usually divided bilaterally down the center, although, it can differ depending on which stage the mutation happened during development. Later mutations in development will result in a budgerigar that has a splotchier distribution of the different cell populations.

Halfsider budgerigars may be much more frequent than initially supposed, but only in those cases the mutation determines an externally perceptible feature will they be spoted.

It is also possible for the half-sider to be male on one side and female on the other. When this happens, it is due to loss of the Z chromosome in an ZW female embryo, as sex determination in birds is controlled by a pair of gonosomes (sexual chromosomes) similar to that operating in mammals. In birds, however, the WW chromosome set determines a male, while the ZW leads to female development. Loss of the Z chromosome or some mutations such as deletions in key regions of this, could prevent the correct development of female gonads and reproductive ducts, leading to a male reproductive system in one half of the bird.

Breeding a half-sider is unlikely to produce more half-siders, even when breeding two half-siders together, as the genetic makeup of the half that contributed the cells that make up the reproductive system is that which would then be perpetuated, assuming that the bird is even fertile in the first place. The chance of producing another half-sider would be the same as for any other budgerigar pairing.