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A pair of leucistic Mallards

I took this photo at Buena Vista Creek, Vista, CA. It is a pair of leucistic mallards. Leucism is a condition in which an animal loses some or all of its pigmentation so that it appears pale, all white, or with white blotches around patches of normal color. Unlike albinos, which lose all of their melanin, leucistic animals usually retain their normal eye color, as in these birds, versus the pink eyes of albinos.

While leucism is the most common form of pigment abnormality in birds, it is still fairly rare. Its frequency varies widely in different studies from none in large flocks of different birds to one report of 8 in a small flock of Eared Grebes (Dudley, 2017).

Leucism is due to a recessive mutation. It occurred in these two Mallards either because the frequency of the mutant gene was unusually high in this flock, or because these are identical twins.

To give a feel for the frequency of the different combination of genes (genotypes) I will term the recessive leucistic gene “c”. I will use quotation marks to distinguish a “c” gene from just the normal letter c. In recessive disorders an animal must have inherited two mutant genes to have the condition, one from each parent, producing in this case an “cc” genotype. Carriers, or heterozygotes, “Cc”, have one normal gene, “C”, at the leucistic locus and one mutant “c” gene. Homozygotes of the normal gene are “CC.” The ratio of these three genotypes, where “c” is the frequency of the “c” allele is given by the Hardy Weinberg equation 1 = c2 + 2Cc + C2. If we assume that the frequency of leucism, “cc”, is 1/20,000 then the frequency of the “c” gene is the square root of 1/20,000 or .007 or .07 percent. The frequency of the “C” allele is 1 - .007 or .993 or 99.3 percent. The frequency of heterozygotes (that are not white because “c” is recessive) = .007 x .993 x 2 = .014 or 1.4 percent or 1 in 71. The frequency of the “CC” homozygote normal = .9932 or 98.6 percent.

Ron Dudley’s blog Feathered Photography July 22, 2017.

Leucistic Mallard

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