With all the press the bulldog trait has received, it is common for any stillborn Dexter fetus to be called a bulldog by the breeder; regardless of cause, or presence/absence of deformity. Generally, these events are blessedly rare, so it is difficult for a breeder to compare his loss with other breeders. The fetus is promptly buried, no photos taken, and no DNA samples obtained. Of course, fetal death can be caused by a number of events, including infection, certain plants, and trauma to the fetus from cows fighting, especially in horned cattle kept in confinement.
However, there are two known lethal genetic mutations that appear to be unique to Dexter cattle. They are commonly termed “bulldog” calves and “waterbabies.” Before the advent of genetic testing, many people believed they were variations on the same defect. This confusion delayed the understanding of chondrodysplasia because animals that did not appear affected by chondrodysplasia, (i.e. did not have short legs and a “choppy” gait) could still produce a waterbaby. This led to the false belief that “dwarfism” and “bulldog” calves were not entirely linked. (For more infomation and a list of all the names used to refer to dwarf and non-dwarf Dexters, please see the article Short-legged and Long-legged Dexters.)
First, a little background. Genes come in pairs, called alleles. An animal that has two different alleles for a gene is called heterozygous, one that has two identical alleles is called homozygous. These terms are reused for every gene. So, an animal can be homozygous normal for chondrodysplasia, but be heterozygous for pulmonary hypoplasia.
Dominant means one allele for the trait will change the outward appearance of the animal. This is further defined by the term Co-Dominant, which means one allele for the trait will change the outward appearance a little bit, but two alleles for the trait will change the appearance a lot. (Clinicians often call these individuals affected and severely affected, using the outward appearance to determine how many mutant alleles the individual possesses.)
Recessive means that one allele for the trait will not change the outward appearance of the animal, this is a true “carrier.” In this case, it takes two alleles for the trait to make a change in the animal.
Breeders often maintain that any bad genetic trait discovered in their breed “must be” from some wicked introduction by another breed. But mutations happen. Most go unnoticed. Occasionally, a mutation is actually advantageous. Mutations create variations in animals, and that allows species to survive environmental changes. But some mutations, known as genetic lethals, cause the death of animals that are homozygous for the trait.
Bulldog calves are caused by chondrodysplasia, a form of dwarfism. A rough, literal translation of the term chondrodysplasia is “cartilage in bad form.” Chondrodysplasia is a co-dominant trait, so the heterozygous animal with one mutant allele outwardly shows some signs of the trait. The ability to distinguish these animals from the rest of the herd allowed many breeders to eliminate the occurrence of bulldog calves from their herd; before a genetic test became available.
Several laboratories provide testing for this mutation including UC Davis and Texas A & M. The following is an explanation of the abbreviations breeders will see on their lab certificate after submitting a hair sample for DNA testing.
- A Homozygous Normal (HN) animal has two normal alleles for the chondrodysplasia gene and appears normal. (Figure 1)
- An animal that is Heterozygous for Chondrodysplasia (HC) has one mutant allele and one normal allele. This animal is affected by the mutation and will show some signs of chondrodysplasia such as short legs and a choppy gait. It will be born alive, but has the potential to produce bulldog calves. Some may suffer from difficulties due to tracheal malformations, poor feet, or arthritis. (Figures 2, 3)
- An animal that is Homozygous for Chondrodysplasia will be severely affected,(SA) and typically is spontaneously aborted (miscarried). It will have the appearance of a bulldog calf, including a shortened muzzle, domed forehead, and very, very, very short legs. The “guts hang out” through a pronounced abdominal hernia and the animal has a cleft palate. The spine is also shortened and the rib cage is small which may compress the lungs, causing some edema in calves carried close to full term, but overall the fetus has a very different appearance than a waterbaby. Although calving difficulties do occur, most bulldog fetuses are delivered without assistance. (Figures 4, 5)
It is believed that all chondrodysplastic Dexters in North America share the same mutation. This allele is also common in “Mini” cattle types created from Dexter stock, where affected animals were actually favored over normals.*
Waterbabies are caused by Pulmonary Hypoplasia Anasarca, (PHA). The term pulmonary hypoplasia means the lungs are underdeveloped and anasarca refers to edema (swelling) at birth. PHA is believed to be a recessive trait, meaning animals that are carriers of the mutation cannot be identified unless they produce a waterbaby, or are genetically tested.
- A homozygous normal animal has two normal alleles for the pulmonary hypoplasia gene.
- An animal that is heterozygous for pulmonary hypoplasia is believed to show no outward signs.
- An animal that is homozygous for pulmonary hypoplasia will have little or no lung tissue which causes it to swell up with fluid looking much like a cartoon character that has swallowed a water hose. This tremendous swelling is the most distinguishing characteristic. It does not have the short legs, cleft palate, and other deformities of the bulldog. The dam might have a swollen belly prior to calving due to an excess of amniotic fluid, and may need a cesarean section to remove the calf because it has swollen too large for vaginal delivery. (Figures 6,7)
The existence of PHA in other established breeds such as Maine Anjou does not imply Dexters are related to those breeds. Dexters actually have a unique mutation within the gene.
Chondrodysplasia was once very common in Dexters because breeders actually favored the appearance of the heterozygous animals, and the body type was once popular in show rings. (In some areas of the country, this is still the case.) PHA is rarer, because there was no selection process to create a market for heterozygous animals. Now that there is a test for both, the occurence of both bulldogs and waterbabies can be easy to eliminated from the breed.
You can test your animals for PHA by submitting a hair sample to UC Davis. The link is provided on this website under the Links section.
* The chondrodysplasia test will only identify one mutation, and will not identify other dwarfism mutations within the same gene, or within other genes. Other forms of dwarfism are known to exist in “Mini” cattle due to the introduction of many breeds other than Dexter, so this test is not an end-all diagnostic for dwarfism in these animals.