Crested Duck (domestic breed)

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Crested Duck can mean:-

A domestic breed descended from the mallard: described below.
A wild species Lophonetta specularioides: see Crested Duck.

The crested duck has its appearance because it is heterozygous for a genetic mutation causing a deformity of the skull. As a result, when a pair of crested ducks breed, the young sort out in the usual 1-2-1 ratio:-

25% are homozygous for the normal allele of this gene and so have no crest and if bred together their offspring will never have a crest.
50% are heterozygous for this gene and hatch with a crest of varying sizes.
25% are homozygous for the crested allele of this gene and die from exposed brain without hatching, as it is lethal in homozygous form.

If a crested duck and a non-crested duck breed together, the resulting ducklings would be expected to be 50% crested and 50% non-crested.