An excellent subject for discussion is the pairing of two dark factor birds. Before we do this let us restate what we know about colors. The two basic colors are Green and Blue plus their dilutes. Yellow and White have three shades. Light Green, Skvblue, Light Yellow and Skyblue White have no dark factor. Dark Green, Cobalt, Dark Yellow and Cobalt White carry one dark factor whilst Olive Green, Mauve, Olive Yellow and Mauve White carry two dark factors.
The above colors can be modified by the Grey or Violet genes, although the three shades still exist within the modified colors. A practical consideration is that from experience and observation the continual pairing, generation after generation, of two dark factor birds' results in the reduction in size of the resulting program.
All budgies have a level of "dark factor" ranging from no dark factor, one dark factor, or two dark factors. Wild budgies have no dark factor. Dark factor basically darkens the blue in the body feathers. (In budgies totally lacking normally colored feathers, such as albinos and lutinos, the budgie's dark factor will be present but unknown). A green (yellow-based) budgie with no dark factor will be the original very bright green; this variety is called "green" or "light green." One dark factor will result in a darker green; this variety is called "dark green." Two dark factors will result in a deep olive drab green color; this variety is called "olive." A blue (white-based) budgie with no dark factor will be the original bright sky blue; this variety is called "sky blue." One dark factor will result in a slightly darker blue; this variety is called "cobalt." Two dark factors will result in a deep grey bluish color (more grey than blue); this variety is called "mauve." Within each level of dark factor is room for some variation in darkness. One sky blue may look a little darker than another sky blue and one olive budgie may look a little lighter than another olive budgie. But usually there is no mistaking which dark factor category a budgie falls into, and the pictures below can be used as a guide.
Basic Genetics: Dark factor - semi-dominant Normal - recessive
There are only two alleles that determine the darkness of a budgies body color: the normal gene and the dark factor gene. The dark factor gene is semi-dominant to the recessive normal gene. This means that a budgie that has one dark factor and one normal gene looks different from a budgie that has two dark factor genes.
A skyblue has no dark factor, a cobalt has 1 dark factor and a mauve has 2 dark factors. A violet is a blue with 1 dark factor (cobalt) plus the factor for violet.
Table of expectations
The following table shows the theoretical expectations from various pairings.
KEY: Lt=No Dark Factor, D=One Dark Factor, DD=Two Dark Factors
|Lt x Lt||=100% Lt|
|Lt x D||=50% Lt, 50% D|
|D x D||=25% Lt, 50% D, 25% DD|
|Lt x DD||100% D|
|D x DD||=50% D, 50% DD|
|DD x DD||=100% DD|
The offspring in this table can be of either sex irrespective of the dark factor genes carried by either parent. It is also noticeable that in general the progeny carrying two dark factors are smaller than the birds carrying none. Of course there are always exceptions to this type of observation.
Many fanciers may well recall various exceptional birds carrying either one or two dark factors. If the aim is to produce birds that have a single dark factor, I have found it to be more beneficial to pair a single dark factor bird with a bird carrying no dark factor. This will produce a theoretical 50% of the progeny carrying the single dark factor, and normally without reducing the overall size of the bird.
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