How Genetic Diseases Are Inherited

  • by Amy Young

Modes of Inheritance

Most identified equine genetic diseases have autosomal dominant or recessive modes of inheritance. “Autosomal” means that males and females are equally affected. Here, “unaffected” and “affected” represent the phenotype, or presence of the disease. “Aa”, “aa”, "BB", "Bb", and "bb" represent the genotype, or genetic combination of alleles (one from each parent) that causes the phenotype.

Autosomal Dominant

ONE copy of the gene variant (allele) is needed to have the disease/trait. Each offspring of an unaffected parent x affected parent has a 50% chance of inheriting the disease allele (A) and being affected.

Example of autosomal dominant mode of inheritance

Autosomal Recessive

TWO copies of the gene variant (allele) are needed to have the disease/trait. Each offspring of parents who carry the condition has a 25% chance of inheriting the disease allele and being affected. Carriers are unaffected, but can pass the disease/trait to their offspring.

Example of autosomal recessive mode of inheritance