Genetics: Sexy Chickens

Background Information:

The ZW sex-determination system is a chromosomal system that determines the sex of offspring in birds, and in some types of fish and reptiles. The letters Z and W are used to distinguish the system from the XY sex determination found in humans and mammals. In th ZW system, the female has a pair of dissimilar ZW chromosomes and the male has two similar ZZ chromosomes.

In animals with the ZW system, the ovum (not the sperm) determines the sex of the offspring. Males are the homogametic sex (ZZ) and females are the heterogametic sex (ZW). The Z chromosome is larger and has more genes, similar to the X chromosome in humans.

1. Suggest a definition for the terms: homogametic and heterogametic.

2. How is ZW sex determination similar to XY sex determination? How is it different?

Backyard Breeding:

It is difficult to determine the sex of very young chickens, but some breeds of chicken are referred to as "autosexing" because when they hatch their sex can easily be determined based on a "barred" feather pattern. The gene for barring is sex linked and dominant.

The barring gene is an absence of coloration in the feather, causing white stripes (bars) on the feathers. As chicks, the down feathers of a barred chicken will be light in color, whereas chicks without the barred gene will be solid.

- Males can have zero, one or two barred genes ( B/B, B/b, b/b), two copies intensifies the barring pattern.
- Females can either one or no barred genes (B/-or b/-)

3. Identify the coloration of the chickens as either solid or barred:

Male Chickens BB = _______________ Bb = _______________ bb = _______________

Female Chickens B = _______________ b = _______________

4. What will be the outcome of a cross between a barred male (BB) and solid female (b-)? Show the punnett square:

How many chicks will have barred feathers? _______

Could you tell the difference between the male and female offspring when they hatch?______

5. What will be the outcome of a cross between a solid male (bb) and barred female (B-)? Show the punnett square:

How many chicks will have solid feathers? ________ What is their sex? ______

How many chicks will have barred feathers? _______ What is their sex? ______

Could you tell the difference between the male and female offspring when they hatch?______

6. Explain how the barring patterns can be used by backyard chicken breeders to AUTOSEX baby chickens.