Mating between a male donkey (2n=62) and a female horse (2n=64) produces sterile mules. Recently, however, a very rare event occurred—a female mule gave birth to an offspring by mating with a horse.

Determine how many chromosomes are in the mule karyotype, and explain why mules are generally sterile.

Key Concepts

Chromosome Number and Karyotype

A karyotype is a complete set of chromosomes in an organism, organized by size and shape. In this case, the male donkey has 62 chromosomes and the female horse has 64, resulting in mules typically having 63 chromosomes (2n=63). This odd number of chromosomes complicates meiosis, leading to infertility.

Meiosis and Fertility

Meiosis is the process of cell division that produces gametes (sperm and eggs) with half the chromosome number. For mules, the presence of an odd number of chromosomes (63) disrupts the pairing of homologous chromosomes during meiosis, preventing the formation of viable gametes and resulting in sterility.

Hybrid Vigor and Sterility

Hybrid vigor refers to the enhanced biological qualities that hybrid offspring may exhibit. However, in the case of mules, while they may display hybrid vigor, their sterility arises from the genetic incompatibility between the horse and donkey chromosomes, which prevents successful reproduction in most cases.