Which diseases are autosomal recessive in horses?

• A. HYPP and PSSM
• B. HYPP and MH
• C. HYPP and GBED
• D. GBED and HERDA

Answer: (D)

Autosomal recessive inheritance means an affected horse must inherit two copies of the mutant gene, one from each parent, while one copy usually makes the horse a normal carrier.

Glycogen branching enzyme deficiency (GBED) and Hereditary equine regional dermal asthenia (HERDA) both follow this pattern. A horse must have two mutant alleles to show the disease, and horses with just one mutant allele are typically healthy carriers. That’s why these two are described as autosomal recessive.

In contrast, HYPP and malignant hyperthermia (MH) are inherited dominantly—one copy of the mutant gene can produce the condition—so they are not autosomal recessive. PSSM type 1 is also described as autosomal dominant due to the GYS1 mutation, so it doesn’t fit the recessive pattern.