Brown Egg Science

It all started when Mary noticed that brown eggshells are not brown on the inside. I looked into this and found brown eggshell color is a polygenic trait (Many genes have been proposed to account for the wide range of brown eggshell colors). The brown pigment is primarily on the outside of the shell and you can even scrub it off with warm soapy water and a kitchen scrub pad. Eggshells are not brown through and through. This is only surface pigmentation.

Where the brown genes are on the chromosomes and how many there are, is not known. There is a sex-linked gene that inhibits the expression of the brown eggshell genes (it may be that Leghorns have this gene. Leghorns seem to have an ability to suppress brown eggshell color when you breed them to a brown layer.)

We have read conflicting reports on egg coloring and age. Some indicate egg color lightens with hen age. Other reports indicate eggshell darkening with age. It may well be that both are correct depending upon the hen and its particular genetics.