Storey's Guide to Raising Chickens (66 page)

Paired genes that are identical to each other are called
homozygous
(homo-from the Greek word meaning “same”; -zygous meaning “pair”). A bird with a large number of paired identical genes is also described as homozygous. The more closely birds are related, or inbred, the more homozygous they become and the more predictable their offspring.

When the genes in a pair differ from each other, the pair is called
heterozygous
(hetero- from the Greek word meaning “different”). The same word describes a bird with a large number of paired dissimilar genes. When heterozygous chickens are mated, or an outcross has been made, the genes in their offspring can pair in many different combinations, making the results highly unpredictable.

Each chicken has a combination of dominant and recessive genes. If a dominant gene pairs up with a recessive gene, the dominant gene overshadows or modifies the recessive gene, and the dominant trait prevails.

As long as you work with heterozygous birds, recessive genes can remain hidden to pop up at any time. Old-timers use the word “throwback” to describe a bird displaying traits that have been hidden for several generations. A recessive trait shows up when two genes in a pair control the same recessive trait. Since a homozygous chicken is more likely than a heterozygous chicken to have a large number of matched pairs, the more inbred a bird is, the more likely it is to display recessive traits.

Revealing recessive traits can be a good thing or a bad thing. If the recessive is desirable, you want to encourage it. If it is undesirable, you want to weed it out, which is possible only if you maintain sufficient genetic diversity to prevent the concentration of undesirable recessives in all your breeders.

Not all traits are controlled by dominant or recessive genes but rather by combinations of genes. An example is
rumplessness
, a genetically complex feature of Araucanas determined by an interaction among many different genes. One of the best guides to the genetic complexities of heredity in chickens is
Genetics of the Fowl
by F. B. Hutt.

Rumplessness is a feature of Araucanas determined by the interaction of many different genes.

Among the recessive traits concentrated by inbreeding are
lethal genes
. A chick that acquires the same lethal gene from both parents dies early, often in the embryo stage. More than 50 different lethals have been identified in chickens. They are readily recognized because they are usually accompanied by such quirks as stickiness at hatch, shaking, winglessness, twisted legs, missing beaks, and extra toes.

When you mate two birds carrying the same lethal recessive, 25 percent of their offspring will display the lethal trait. One well-known lethal is the so-called creeper gene carried by short-legged Japanese chickens, once prized as broodies because their short legs keep their bodies close to the ground. Other well-known lethals are carried by Araucana, dark Cornish, New Hampshire, and white Wyandotte. Fortunately, lethal genes are relatively rare.