If you’ve ever played the board game Scrabble you’re familiar with the custom in which all the lettered tiles are placed in a cloth bag at the outset of the game. Each participant blindly reaches in and takes their allotted amount and continues to do so until there are no more tiles left. The letters can be used singly, but most often are placed in conjunction with others to build words. Genetics is a bit like that. Genes are distributed at random – even to folks that are related – and work together to build a living organism. Single genes might be expressed as a single trait (i.e. eye colour), but occasionally single genes can influence several traits, or conversely several genes come together resulting in only one trait.
Several years ago I watched a documentary about the Russian geneticist Dmitri Belyaev. In the 1950s he began an experiment in guided evolution to replicate how the domestication of dogs might have happened. His team used silver foxes and selected the least aggressive or human averse ones as their breeding stock. After just four generations they found that not only were the offspring becoming more tame they also had higher levels of seratonin and their appearance started to change: shorter, rounded or floppy ears; shortened legs, tail and muzzle and different coat colours. What became clear was that in selecting for just one trait others were connected to the same genes.
The Linkoping University using selectively bred Jungle Fowl to hypothesize how chickens were domesticated conducted a similar study. Like Belyaev, they chose birds with the least fear of people as the initial breeding stock. Within ten generations, the offspring developed smaller brain stems and overall brain size. This change resulted in them being less flighty and more tolerant of potentially stressful stimuli. Domestication also increased the numbers of eggs a hen laid. No longer was mating constrained by a short season, but was spread over a longer period of time.
The founder of modern genetics, Gregor Mendel, worked with pea plants. He observed that seed coat colour was always associated with specific flower colours. That phenomenon is the result of pleiotropy, in which a single gene contributes to multiple traits. The term pleiotropy is derived from the Greek words pleio and tropic, meaning ‘affecting many’.
Genes that affect multiple, apparently unrelated, phenotypes (characteristics) are called pleiotropic genes. This is a different phenomenon than polygenic traits, in which multiple genes result in a single trait (i.e. walnut comb, plumage colour).
In 1936, researchers noted that chickens that expressed the dominant frizzle gene produced curled feathers as well as other observable characteristics: higher body temperatures, metabolic and blood flow rates; greater digestive capacity and higher food consumption. Chickens who carry the gene also have larger internal organs, mature later, lay fewer eggs and may experience fertility issues.
Comb Genetics & Egg Production
The genetics for comb size, egg production and bone allocation are all connected by pleiotropy in chickens.
Chicken combs are one factor in attracting mates for both roosters and hens. Males with larger combs are more likely to be successful attracting mates. In hens it is indicative of greater reproductive potential, through an increase in egg production, which is closely connected to bone formation.
Avian skeletons are unique, in that, bones are constantly being built and resorbed as part of their role in egg production. When a hen lays an egg, calcium is stripped from the hard outer cortical bone and transferred into the soft inner medullary and trabecular bones, which serve as a calcium reserve. Calcium is also mobilized away from the end and into the centre of bones during egg-laying periods, making it is more available. When required, calcium is then drawn from the bone reserves into the developing eggshells.
Both roosters and hens with larger combs have higher bone density and strength, a factor that allows hens to deposit more calcium into eggshells. Hens that carry the two genes involved in bone production lay more eggs and are less likely to go broody – attributes also valued by the commercial egg industry.
There are pleiotropic links between comb color and egg production. In Dongxiang chickens melanin pigmentation, which creates dark combs, is associated with lower egg production.
In addition to the frizzle gene, other traits believed to have pleiotrophic adaptations in different breeds include polydactyly (5 toes), blue eggshells and dwarfism.
Credit: BBC Earth; Ingrid Lobo; PLOS Genetics; Frontiers In Genetics; ThoughtCo. Photos of Frizzled Hens: Bitchin’ Chickens