Dominant and recessive

Here I take the classical example used to explain genetics in budgerigars: the blue gene bl. There exist at least four different alleles (forms) of this gene. In this example we only use the two most frequent ones:

• the not mutated wild allele: bl⁺ (the superscript '+' always indicates the wild allele)
• the mutated 'blue' allele: bl

Since genes come in pairs there are 3 possible combinations of these two alleles:

bl+ ---: normal, lightgreen bl+

bl+   bl  --- = ---: normal/blue (normal split blue), lightgreen/blue bl    bl+

bl  ---: blauw, hemelsblauw bl

A lightgreen/blue bird is in appearance (phenotypicaly) the same as a normal lightgreen. The bl⁺ allele is said to be dominant with respect to the bl allele. Equivalently you can say that the bl allele is recessive with respect to the bl⁺ allele. A recessive allele has to be present twice in order to appear visually. This form of dominance is called complete dominance. There are two other forms of dominance: partial dominance and co-dominance:

• The spangle gene is an example of partial dominance (or incomplete dominance): the heterozygous phenotype is a kind of intermediate between the dominant and recessive phenotype.
• An example of co-dominance is the gene that determines our human blood type:

  Allele combination	Bloodtype
  A A	A
  B B	B
  A B	AB

  Here the heterozygote has the properties of both alleles. (Note: the human bloodtype is not only determined by the alleles A and B, but also by the allele O).

The following mutations are dominant to the wild type:

• Cl Aisley clearbody
• Cr Crested [not nicley 100% dominant and a crest is partially lethal]
• D Darkfactor [partial dominance]
   D⁺/D⁺ = lichtgreen
   D/D⁺ = darkgreen
   D/D = olivegreen
• Pd (Australian) pied [a double factor bird has less markings]
• Pi (Dutch) pied [a double factor bird has less markings]
• Sp Spangle [a single factor bird has the typical spangle markings, a double factor bird is entirely yellow without markings]
• V Violet [there is a lot to say about this mutation]

The following mutation are recessive to the wild type. Some are mutations of the same gene and are set in the order of dominance:

• a^bz > a bronze fallow > non sex linked ino
• b browning (sepia)
• bf blackface
• bl blue [there are different forms of this gene that result in different forms of blue or yellowface]
   bl^aq Australian yellowface (aqua)
 bl^tq yellowface mutation II (turqoise)
 bl¹ first blue allele
 bl² second blue allele - also produces a blue bird but the combinatie of 1 and 2 gives yellowface type I (yellow mask, blue body)

• c^gw > c^cw > c greywing > clearwing (geelvleugel/witvleugel) > dilute [c^gw/c^cw = full body colour greywing]
• fd faded
• ag Australian recessive grey
• g (English) recessive grey
• pl Schottish fallow
• pf English fallow
• s (Danish) recessive pied
• sa Saddleback