Disorders of sexual development - sexual reversal

What is ... ?: 

At fertilization, the sex chromosomes are established as XX (the animal will be female) or XY (male). During development of the embryo, ovaries develop if the chromosomes are XX, and testicles if XY. An abnormality can occur in chromosome differentiation (to produce XXY, XXX, XO). This is rare in dogs and is not thought to be inherited. Gonadal intersex refers to developmental abnormalities in the gonads (ovaries and testicles) despite a normal set of sex chromosomes. In some cases (pseudohermaphrodites), the sex chromosomes in these dogs match the sex of the gonads, but the genitals are abnormal. This is only seen (rarely) in females, and is generally due to excess exposure to male sex hormones during development - ie. environmental factors rather than an inherited condition.
Sexual reversal describes the condition where the chromosomal and gonadal sex differ. Only XX sexual reversal has been seen in dogs. The dog may be a XX true hermaphrodite (female chromosomes in a dog with both ovaries and testicles), or XX male (female chromosomes, with testicles).

How is ... inherited?: 

XX sex reversal is an autosomal recessive trait in the breeds listed below. It has been reported sporadically in several other breeds where the inheritance is not understood.

What does ... mean to your dog & you?: 

 Affected animals are infertile (so that your apparently female dog will not come into heat), and may have chronic irritation in the genital area, which may look abnormal to you.

How is ... diagnosed?: 

Your veterinarian will typically suspect one of these conditions when s/he notices an abnormality such as clitoral enlargement or an abnormal penis during physical examinataion of your dog, or if a breeding dog is being examined for infertility. To diagnose the specific condition requires your veterinarian to evaluate both internal (generally at the time of spay) and external sex organs, and send away a blood or other cell sample from your dog for karyotyping, which identifies the chromosomes.

How is ... treated?: 

 Affected animals should be neutered.

For the veterinarian: 

Dogs with XX sexual reversal may be XX true hermaphrodites or XX males. These dogs can appear in the same family, and result from the same genetic defect. XX true hermaphrodites have both ovarian and testicular tissue. The degree of masculinization of internal and external genitalia depends on the proportion of testicular tissue in the gonads. XX males have a uterus, bilateral (usually undescended) testes, and  a prepuce that is located abnormally.

Breeding advice: 

 Affected animals, their parents (carriers) and siblings (potential carriers) should not be used for breeding.


Meyers-Wallen V. Inherited disorders of the reproductive tract in dogs and cats. In: Bonagura JD, Twedt DC, eds. Kirk's Current Veterinary Therapy XIV. Missouri: Saunders Elsevier, 2009:1034-1039.
Mickelsen WD, Memon MA. 1995. Inherited and congenital disorders of the male and female reproductive systems. In E.J. Ettinger and E.C. Feldman (eds.) Textbook of Veterinary Internal Medicine, p. 1686-1690.  W.B. Saunders Co., Toronto.
Sargan DR.Sexual reversal. In IDID - Inherited diseases in dogs:web-based information for canine inherited disease genetics. 2002-2011.

What breeds are affected by ... ?