On this last day of the year we’re at the end of the alphabet with a special significance. W and Z are the sex chromosomes of birds.
Mammals have sex chromosomes called X and Y which determine the sex of the individual. A mammal embryo is born female if it has two of the same chromosomes: XX. It’s male if it has two different chromosomes: XY.
Birds are similar but very different. Like mammals they have two sex chromosomes but the structure and origin of these chromosomes are so different that they’ve been labelled W and Z. They also combine in the opposite way to determine the sex of the individual. Female birds have two different sex chromosomes: ZW. Male birds have two of the same: ZZ.
In birds, unlike mammals, nearly every cell has its own sexual identity so if an aberration occurs during the first cell division of a bird’s fertilized ovum, the resulting individual can be half-male and half-female, neatly divided down the length of its body. These unusual individuals are called “bilateral gynandromorphs.”
Pictured above are three evening grosbeak specimens from the Smithsonian*. One is male, one is female and the third (at the top of the photo) is a bilateral gynandromorph. It’s right half is dull like the female. Its left half is bright yellow like the male. This sexual difference continues inside its body where its organs are female on the right and male on the left.
Gynandromorphs are rare but have been documented in a variety of bird species. It’s not seen in humans because most of our embryonic cells are sex-neutral. Hormones, not the individual cell, govern our sexual characteristics.
Click here to see more photos of bilateral gynandromorphs.
(photo from Flikr by ap2il, licensed under the Creative Commons License 2.0. Click on the image to see the original where one of the keywords is Smithsonian *hence my assumption on the location of these specimens.)