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5 Genetic Disorders That Should Have Landed These Animals In The Freak Show

Not all animals are cute and cuddly...some are down right weird...but even they deserve some love once in a while.

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1. Double-Muscling

The term double-muscling is quite literal, and accurately describes the phenotype of a myostatin knockout. Myostatin knockouts physically produce twice the amount of muscle as a normal animal! Myostatin is a negative regulator of muscle growth which means that animals that don't have myostatin, have increased muscle growth not only in muscle fiber number (2x) but also in size. On top of the already increasing muscle mass, myostatin knockouts have significantly less adipose tissue which accentuates their impressive physique. Double-muscling is an autosomal recessive mutation meaning an animal needs both copies to be The Hulk, but having only one copy can give a lesser "I work out" phenotype.


Mcpherron, A. C., and S.-J. Lee. "Double Muscling in Cattle Due to Mutations in the Myostatin Gene." Proceedings of the National Academy of Sciences 94.23 (1997): 12457-2461. Web. 14 Nov. 2016.

ANSC 452 Notes: "Genetic Mutations in Livestock Affecting Growth and Development"

2. Hypotrichosis

Hypotrichosis is a genetic disorder characterized by varying degrees of "hairlessness". There are believed to be 6 kinds of genetic hypotrichosis based on the degree of hairlessness: lethal hairless, semi-hairlessness, hypotrichosis with anodontia, viable hypotrichosis, hypotrichosis with missing incisors, and streaked hairlessness. The majority of calves born with hypotrichosis succumb to the illness due to a lack of teeth or environmental stress. Viable hypotrichosis (pictured above), however, is survivable with proper management and is caused by an autosomal recessive gene in the homozygous state. It is described as having normal teeth and patches of fine hair limited to the inner ears, eyelids, tail switch, and dewlaps. While the number of hair follicles are normal, the majority of them developed abnormally leading to the patterned hair patches. This lack of normal hair can be attributed to an absence of papillae, and a lack of differentiation in the epithelial cells lining the hair follicle.


HUTT, F. B., and L. Z. SAUNDERS. "VIABLE GENETIC HYPOTRICHOSIS IN GUERNSEY CATTLE." Journal of Heredity 44.3 (1953): 97-103. Journal of Heredity. Web. 14 Nov. 2016.

HUTT, F. B. "A NOTE ON SIX KINDS OF GENETIC HYPOTRICHOSIS IN CATTLE." Journal of Heredity (n.d.): 186-87. Web. 14 Nov. 2016.

3. Dwarfism

Dwarfism is a result of abnormal cartilage formation or chondrodysplasia. There are varying degrees of dwarfism with some of the more well-known types being the snorter dwarf, the bulldog dwarf, and the long-headed dwarf. Dwarfism is controlled by various mechanisms in various breeds of cattle; however, the bulldog dwarf is most commonly associated with Dexter cattle and the ACAN gene mutation. Dwarfism, in general, is caused by early fusion of the growth plate; however, the exact mechanism for the bulldog dwarf is unknown. The bulldog dwarf (pictured above) is the result of inheriting one copy of the mutated gene (Rr-heterozygous) and is characterized by reduced limb length, while the homozygous fetus (rr-two mutated copies) is generally aborted during the third trimester of gestation. This mode of inheritance is referred to as autosomal recessive and the severity of the phenotype of the heterozygote is affected by incomplete penetrance. FUN FACT: The heterozygous mutant is actually the highly sought after "miniature cattle" phenotype.


Cavanagh, J.A.L., Tammen, I., Windsor, P.A. et al. Mamm Genome (2007) 18: 808. doi:10.1007/s00335-007-9066-9

Arsdall, Dan Van. "Dwarfism (Condrodysplasia)." Calfology. N.p., n.d. Web. 14 Nov. 2016.

4. Developmental Duplications/Polymelia

Polymelia (pictured above) is just one of several forms of a genetic disorder referred to as Developmental Duplications (DD). In general, developmental duplications are deformities in the physical appearance of the animal from having an extra limb to having a conjoined twin, but in some cases it can be embryonically lethal. DD is an autosomal recessive mutation that is beginning to appear more frequently in Angus or Angus derived calves. Dr. Jonathan Beever, from the University of Illinois, was able to trace the mutation back to a single amino acid substitution. It is believed that this gene is important in cell to cell signaling during neurulation. In polymelia, for instance, there is a failure to communicate between the lateral mesoderm and the apical ectodermal ridge (AER). This intercellular communication is necessary for the primordial limb buds to regress like they are supposed to do. Consequently, this lack of signaling leads to the persistence of the limb buds and the formation of extra limbs.


Denholm, Laurence, Dr, Lisa Martin, and Andrew Denman. "POLYMELIA (SUPERNUMERARY LIMBS) IN ANGUS CALVES." Flock and Herd. N.p., Dec. 2011. Web. 16 Nov. 2016.

Denholm, Laurence, Dr, and Lisa Martin. "DEVELOPMENTAL DUPLICATIONS (DD) IN ANGUS CALVES." Flock and Herd. N.p., 4 Dec. 2013. Web. 16 Nov. 2016.

5. Syndactyly

Genetic syndactyly that specifically affects the distal limb of cattle is also referred to as mulefoot. Syndactyly is when the two digits of the hoof fail to divide or become fused. This genetic disorder is an autosomal recessive trait and is expressed with incomplete penetrance. This mutation affects the LRP4 gene which has been proven to play a significant role in digit differentiation in multiple mammalian species. The LRP4 gene is believed to be a negative regulator of the Wnt-beta-catenin pathway. Unfortunately, there is still a lot of research to be done on this pathway but the general idea is that a reduction in Wnt affects the production of other hormones such as SHH, FGF8, and BMP. The concentrations of these hormones and others such as RA, IHH, and the expression of Hox genes all play a role in cell apoptosis and the formation of a complete limb bud.


A. Duchesne, M. Gautier, S. Chadi, C. Grohs, S. Floriot, Y. Gallard, G. Caste, A. Ducos, A. Eggen, Identification of a doublet missense substitution in the bovine LRP4 gene as a candidate causal mutation for syndactyly in Holstein cattle, Genomics, Volume 88, Issue 5, November 2006, Pages 610-621, ISSN 0888-7543

Drogemuller, Cord, and Tosso Leeb. "Congenital Syndactyly in Cattle: Four Novel Mutations in the Low Density Lipoprotein Receptor-related Protein 4 Gene (LRP4)." BioMed Central. N.p., 23 Feb. 2007. Web. 16 Nov. 2016.

ANSC 452 Notes: "Limb Development"

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