Most backyard flock owners will never experience dwarfism in their flocks, however, the genetic condition is quite interesting. There are several types of dwarfism in poultry, though this article deals specifically with thyrogenous dwarfism, the form with which I have quite a bit of personal experience.
Other types of dwarfism are autosomal and sex-linked recessive, however, the subject of this article, thyrogenous dwarfism, is related to a malfunction of the thyroid gland and is passed on by a semi-lethal gene, meaning that it does not kill a chick outright nor kill a chick before hatch, but it does not allow for the chick afflicted with this genetic abnormality to live until sexual maturity.
This type of dwarfism was first discovered in the early 1900's. The chicks will show a general growth delay, though the condition would not be obvious until the chick reached 3-4 weeks of age and its siblings were passing it in size/height. At that point, you will begin to notice subtle differences. The legs will be unusually short and will not continue to lengthen. Usually, the outer toe will curve backwards, though you may just see a crook in the middle joint of that toe. The skull is unusually high and wide, making it appear "apple-ish". The beak will be short and curved downward, what some refer to as parrot-beak. The tongue is short and appears swollen, though that is difficult to see. The eyes sometimes have a droopy, almost mongoloid-ish appearance, similar to a Down Syndrome child. There can sometimes be feather abnormalities. The comb and wattles stay small, though if the chick lives long enough, they will turn pinkish at some point.
To produce a dwarf chick, generally, one gene from each parent must be provided. If only one parent passes the gene, the chick will be a carrier of the gene and will show no outward signs of it. If both parents pass the gene, they produce a dwarf chick. Each carrier passes the gene to 50% of its progeny. In very rare instances, though this is not commonly known, one parent may carry two dwarf gene copies and, no matter which partner it is paired with, that parent will produce only dwarf chicks. More on that later in the article....
My first encounter with thyrogenous dwarfism was in a line of heritage Delawares, hatched from eggs sent to me by a reputable breeder improving her line and helping to bring the breed back from extinction. They grew up normally, no dwarf produced from those eggs. Customers who hatched from eggs from the birds produced from those eggs suddenly began seeing these unusual chicks from time to time, which precipitated my study of the gene. The breeder had never encountered a dwarf in her breedings, which is entirely possible; most breeders realize that when you make new breeding pairs, you are delving into the unknown, genetically. I liken the dwarf gene to balls bouncing on a roulette wheel-if two balls land in the same slot, BAM! A dwarf chick is produced. Since each carrier parent passes the gene to approximately 50% of its progeny, it could be years and years of breeding before a dwarf chick is produced, if ever.
The dwarf gene has come back to haunt us here at Mountain View Heritage Poultry, though I was positive that the gene was completely out of my flocks. My 3 year old Barred Plymouth Rock rooster is 1/4 Delaware and for years, I saw zero evidence in all his progeny that he was a carrier. This year, one of his sons, penned with two of his daughters, one a half sister and one a full sister, produced not one, but TWO dwarf chicks, one male and one female. The male chick passed away at 7 weeks of age. The pullet chick is still alive at 12 weeks of age, but because they do not live to maturity, we expect her to pass away soon.
Two years ago, the uncle of my 3 year old Barred Rock rooster produced two dwarf sons with one of his sisters. Both parents of the dwarf chicks were progeny of my late Delaware rooster. I knew that they could never be bred again, certainly not with each other; the mating that produced those two male dwarfs was the first and only mating of those birds, who have since passed on (chicks and parents). We did not expect to see a dwarf ever again, but we now have little pullet, Pooh-bear. Her legs are literally one inch long, outer toes have a crook in the middle joint, and her body is becoming very wide and large for her legs to support. The two hens who could be her mother have been relegated to layer status now, her sire, with full disclosure, has gone to a new home to be bred with unrelated hens and/or be relegated to flock guardian only.
I mentioned that one bird may, in rare instances, be a carrier of two dwarf genes. Years ago, a friend who lived in Oregon at the time, hatched two daughters out of my Delaware flock, one very large and one slightly smaller-bodied. She mentioned the dwarf gene to her veterinarian, who coincidentally, and lucky for us, was doing a study on dwarfism in poultry at the time! With her permission, he drew blood from both hens and did genetic testing. The smaller-bodied hen carried NO dwarf gene. The larger bodied hen, named Fattie for her wide girth, carried TWO copies of the gene! He told us that many did not believe it possible, however, this hen should never reproduce because every chick would be a dwarf.
The worst aspect of this type of dwarfism is that you must watch them pass away at a young age. Usually, it's quite sudden, just a seizure and they're gone. What they lack in size, they make up for in heart and courage, we have found. An adult bird who carries a copy of the gene is unaffected by it in any way and you will never know it's there until the bird is paired with another carrier and they both pass the gene to the same chick.
I hope this article was informative! Here are pictures of the dwarf chicks that have hatched here. Note the odd look about the head, eyes and beaks of all.