A duckling may emerge from the egg without webbed feet if it inherits a recessive gene for this trait. In most duck species, webbed feet are dominant, so a duckling needs only one copy of the webbed-feet gene to develop them. However, if both parents carry the recessive gene for webless feet and each passes it on, the duckling will inherit two copies and hatch without webs.
Genetic mutations can also produce webless feet. A spontaneous change in DNA—or one triggered by environmental factors such as radiation or chemicals—can disrupt normal foot development.
Environmental conditions during incubation may play a role as well. Since webbed feet are an adaptation for swimming, a duckling raised in an excessively dry environment might not develop proper webbing.
The frequency of webless feet in a population depends on how common the recessive gene is. When the gene is rare, webless ducklings are rare; when it is more widespread, the trait appears more often.