White and Recessive White
Genetics Mini-Series Supplement #1dun and smoky (see below) which inhibits the black pigment eumelanin.1 Basically, it removes any black pigment and leaves the feathers white. It doesn’t affect the red pigment, phenomelanin. Birds which are heterozygous, carrying only one copy of dominant white (I/i+), tend to leak some black pigment which shows up as grey or speckles.2 White Leghorns have the dominant white gene.
(Dun (Id) dilutes the eumelanin to brown and two doses (Id/Id) dilute to khaki. As one or two doses produce a different effect, dun is incompletely dominant. Smoky (Is) is a recessive form which dilutes black to blue.3)
Recessive white (c , colorless) is a recessive gene that inhibits the expression of the tyrosinase gene. Tyrosinase is a protein that helps the body to produce pigment. More specifically, “tyrosinase is an essential enzyme in melanin synthesis in pigment cells.”4 With less of this protein, the body is less able to make pigment. Recessive white chickens can’t produce eumelanin or phenomelanin, although there is sometimes some phenomelanin (red) leakage. Interestingly, the little bit of tyrosinase that recessive white birds do make is more concentrated in their retina, causing them not to have the pink eye of albinism.5, 6 They can still have yellow pigment in their legs because the carotenoids are not made by the body but derived from food.
Recessive white is one of three forms of albinism that are recessive c genes. “In chickens, 3 alleles of albinism at the C locus have been reported. These types are red-eye white (cre), recessive white (c), and autosomal albino (ca; Brumbaugh et al., 1983; Smith, 1990). ”7 The dominant form of the c gene is C, or “colored,” which is the norm: The tyrosinase is free to make pigment, resulting in colored plumage.The White Plymouth Rock is colored, or really uncolored, by the recessive white gene. Like any gene in a homozygous state, recessive white will breed true and can be used to make a white line of another chicken breed.
A recessive white (rw) chicken is homozygous recessive, c/c. If you cross the rw chicken to a colored chicken of the breed you want to breed recessive white into, all of the resulting offspring (F1) will be colored but carry one copy of the c gene (C/c).
If bred back together (avoiding actual brother/sister crosses), approximately ¼ of the F1 heterozygotes’ offspring will be homozygous and show recessive white (c/c). One-half will be heterozygous like their parents, appearing colored but carrying colorless, and ¼ will be homozygous dominant with no recessive white gene.
This generation of F2s is still genetically ½ the breed of the original recessive white bird and ½ the breed you are trying to recast in white. You can take the recessive white F2s and breed them back again to a colored, full-blooded bird of your target breed. These F3 will be ¼ of the old rw breed and ¾ of the desired breed, and all will be colored heterozygotes. Breed these together for ¾ desired breed recessive white in 25% of the individuals in the F4 generation.
Alternately, breed the F3 colored heterozygotes back to F2 whites for a 5/8th- desired breed F4 generation to get 50% recessive white offspring in F4.
Continue to breed back to the colored birds of the breed you want and tease out the recessive white individuals until you reach the level of purity you desire. The closer you get, the less impact each progressive generation has on the purity, so you might want to settle for about 75% of the target breed. For the best outcome, begin with a recessive white individual of a closely related breed or a breed that shares most traits with the desired breed. If you want greater than 98% of the target breed, it will take 12 crosses as illustrated in the ridiculously long chart below.