Creeper chickens are recognized by their distinctively short legs, a trait that sets them apart from other chicken varieties. This unusual physical characteristic is a direct result of a specific genetic condition.
Distinctive Physical Traits
The most noticeable feature of creeper chickens is their legs, which are considerably shorter than those of typical chickens, causing their bodies to be carried only a few centimeters from the ground. These legs often appear thick and bent, leading to a disproportionate body structure. This skeletal shortening is a form of dwarfism, known as chondrodystrophy.
Their unusual leg structure results in a characteristic waddling gait, giving them the appearance of “creeping” when they move. Despite these physical adaptations, creeper chickens are generally healthy and active, engaging in typical chicken behaviors. Several breeds, including the Chabo and Jitokku from Japan, the Courte-pattes of France, and the Scots Dumpy, exhibit these short-legged characteristics.
The Genetic Explanation
The short-legged trait in creeper chickens is caused by a specific genetic instruction called an allele, denoted as “Cp”. This Cp allele, a variant form of a gene, is responsible for the reduced leg length.
This Cp allele is considered a dominant lethal allele. Only one copy of the allele is needed for the short-leg trait to be observable in the chicken. However, the allele becomes lethal when two copies are inherited, resulting in the death of the embryo.
Creeper chickens always possess one copy of the Cp allele and one copy of the normal leg allele, making them heterozygous (Cp/+). This heterozygous genotype allows them to survive and display the short-legged phenotype, which is their expressed physical characteristic.
Embryos that inherit two copies of the Cp allele (Cp/Cp), making them homozygous dominant, do not survive to hatching. This explains why adult chickens with two dominant creeper alleles are never observed. The expression of short legs in heterozygous individuals, rather than fully normal or fully lethal, suggests a form of incomplete dominance in the phenotypic expression of leg length.
Breeding Outcomes and Survival
When creeper chickens are bred, the genetic ratios of their offspring demonstrate the influence of the lethal Cp allele. If a creeper chicken (heterozygous Cp/+) is bred with a normal-legged chicken (homozygous for the normal allele), approximately 50% of the offspring will be creepers (Cp/+) and 50% will be normal-legged. This outcome is consistent with a single dominant factor being inherited.
A different outcome occurs when two creeper chickens (both heterozygous Cp/+) are bred together. In this scenario, statistically, 25% of the embryos will inherit two copies of the dominant creeper allele (Cp/Cp). These homozygous dominant embryos typically die early in embryonic development, often within the first week of incubation, or well before hatching.
The remaining 75% of the offspring will survive, with a ratio of approximately 50% creeper chickens (Cp/+) and 25% normal-legged chickens (homozygous for the normal allele). This results in a phenotypic ratio among hatched chicks of about two-thirds creepers to one-third normal-legged chickens. The high mortality rate of homozygous dominant embryos underscores the lethal nature of the Cp allele when present in two copies.
Implications for Breeding
Understanding the genetics of creeper chickens has practical implications for breeding practices. Because homozygous dominant creeper embryos do not survive, adult chickens never possess two copies of the Cp allele. This means that all short-legged creeper chickens are heterozygous for the trait.
Breeders aiming to maintain the creeper trait often choose to breed creeper chickens with normal-legged chickens. This breeding strategy avoids the 25% embryonic mortality associated with breeding two creeper chickens together, as no homozygous lethal embryos are produced. While breeding two creepers together does yield the desired short-legged offspring, it also results in a significant number of unhatched eggs due to the lethal allele.
Ethical breeders manage breeding programs to minimize embryonic loss. This ensures the well-being of the birds while perpetuating this unique genetic characteristic.