A variety of structural and functional defects have been described in animals. These defects are usually classified by the body system primarily affected, and many are discussed in this book under the appropriate body system section. Defective newborns have survived a disruptive event during embryonic or fetal development. Defective development may also cause embryonic loss, fetal death, mummification, abortion, stillbirth, or a newborn not capable of living.

Commonly Reported Congenital and Inherited Defects in Cats
Cerebellar hypoplasia, a lack of development of the cerebellum, which controls balance (see Brain, Spinal Cord, and Nerve Disorders of Cats: Cerebellar Disorders) Eye and eyelid defects (see Eye Disorders of Cats: Disorders of the Eyelids in Cats) Heart defects (see Heart and Blood Vessel Disorders of Cats: Congenital and Inherited Disorders of the Cardiovascular System in Cats) Cleft palate (uncommon in cats but occurs more often in Siamese) (see Digestive Disorders of Cats: Congenital and Inherited Disorders of the Digestive System in Cats) Cryptorchidism (apparent absence of one or both testicles) (see Reproductive Disorders of Cats: Cryptorchidism) Polydactyly (more toes than normal; usually occurs on the front paws and rarely causes any problems for the cat) Deafness associated with white fur and blue eyes (see Ear Disorders of Cats: Deafness in Cats)

Susceptibility to environmental agents or genetic abnormalities varies with the stage of development and species, and decreases with fetal age. The fertilized egg is resistant to agents or factors that cause or increase the rate of occurrence of a congenital defect (teratogens), but it is susceptible to genetic mutations and changes in the chromosomes. The embryo is highly susceptible to teratogens, but this susceptibility decreases with age as the critical developmental periods of various organs or organ systems are passed. The fetus becomes increasingly resistant to teratogens except for structures that develop late such as the cerebellum, palate, urinary system, and genitals.

The frequency of individual defects varies with the species, breed, geographic location, season, and other environmental factors. Among domestic animals, cats have the lowest frequency of congenital defects. Some reported congenital and inherited defects in cats include lack of development of the cerebellum (which controls the balance function), eye and eyelid defects, heart defects, cleft palate, failure of one or both testicles to descend into the scrotum (known as cryptorchidism), more toes than normal (polydactyly), and diaphragmatic and umbilical hernias. Most congenital defects have no clearly established cause; others are caused by genetic or environmental factors or interaction between these factors.

Polydactyly (the presence of extra toes) is fairly common in cats.

Genetic Factors

Inherited defects resulting from mutant genes or chromosome abnormalities tend to occur in patterns of inheritance. Such patterns include dominant (in which the defect will occur if either parent supplies an abnormal gene to its offspring), recessive (in which both parents must supply an abnormal gene) or others, such as sex‑linked (in which the gene is associated with the X chromosome and not the Y chromosome). In cats, for example, the appearance of excess toes follows a dominant pattern of inheritance, while diaphragmatic hernia follows a recessive pattern.

Calico and Black and Orange Tortoiseshell Cats
Calico and tortoiseshell cats are usually female. The reason has to do with genetics. Normal cats have 38 pairs of chromosomes. Half of these pairs of chromosomes are from the father; half are from the mother. Female cats receive an X chromosome from both the mother and father. (Male cats get a Y chromosome from the father and an X chromosome from the mother.) The gene that determines the color of a cat's coat is on the X chromosome(s). Calico and tortoiseshell cats receive one X chromosome with the black coat color gene and one X chromosome with the orange coat color gene. The white coat color seen in calicos and tortoiseshells comes from a different gene. Because 2 X chromosomes are required for the calico and tortoiseshell coats, almost all cats showing these coat colorings are female. Thus, calico and tortoise-shell coat colors are considered to be sex-linked traits. A few male cats are born with 2 X chromosomes (only one of which becomes active) and one Y chromosome. The XXY gene defect in male cats is very rare and also causes sterility.

Some common diseases or disorders caused by genetic defects include deficiencies of particular enzymes that lead to the body's inability to perform normal metabolic functions, and chromosome abnormalities that can result in sterility, abnormal growth, increased embryonic mortality, or reduced litter size. Viruses, certain drugs, and radiation are common causes of chromosomal damage.

The complex interaction between genetic and environmental factors is being studied and is slowly becoming better understood.

Environmental Factors

Factors tending to produce abnormalities of formation include toxic plants, viral infections that occur during pregnancy (such as feline panleukemia), drugs, trace elements, nutritional deficiencies, and physical agents such as radiation, abnormally high body temperature, and uterine positioning. These factors may be difficult to identify, often follow seasonal patterns and stress, and may be linked to maternal disease. They do not follow the pattern of family inheritance that is shown by genetic changes.

Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; Dennis W. Macy, MS, DACVIM; Dudley L. McCaw, DVM, DACVIM (Small Animal, Oncology); Barton W. Rohrbach, VMD, MPH, DACVPM; J. Glenn Songer, PhD; Richard A. Squires, BVSc (Hons), PhD, DVR, DACVIM, DECVIM-CA, MRCVS; Bert E. Stromberg, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM; Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Stephen C. Barr, BVSc, MVS, PhD, DACVIM; J. P. Dubey, MVSc, PhD; Paul Ettestad, DVM, MS; Craig E. Greene, DVM, MS; Delores E. Hill, PhD; Johnny D. Hoskins, DVM, PhD