Chromosomes are thread-like structures found within the nucleus of nearly every cell, representing condensed packages of an organism’s genetic material, or DNA. For the domestic donkey (Equus asinus), the total number of chromosomes is 62. This figure represents the diploid number (2n), which means it is the total count of chromosomes present in the animal’s somatic, or body, cells.
The Donkey’s Unique Chromosome Count
The 62 chromosomes in the donkey’s somatic cells are organized into 31 distinct pairs. Being a diploid organism, the donkey inherits one full set of 31 chromosomes from each parent. This count of 62 is broken down into autosomes and sex chromosomes.
Thirty pairs, totaling 60 chromosomes, are classified as autosomes, carrying the majority of the donkey’s non-sex-related genetic information. The remaining single pair constitutes the sex chromosomes, determining the animal’s biological sex. Female donkeys possess two X chromosomes (XX), while male donkeys have one X and one Y chromosome (XY).
Comparing Donkey and Horse Chromosome Numbers
The donkey’s close relative, the domestic horse (Equus caballus), has a total of 64 chromosomes, organized into 32 pairs. This difference of two pairs compared to the donkey’s 31 pairs reflects a significant evolutionary distinction.
The difference in chromosome number represents a fundamental restructuring of the genetic material. Scientists believe this variation arose through evolutionary events like chromosome fusion or fission. In the donkey’s lineage, two ancestral chromosomes likely fused end-to-end to form a single, larger chromosome, reducing the total count by two compared to the horse’s ancestor.
The physical structure of the chromosomes also varies. The donkey possesses a greater number of metacentric chromosomes, which have the centromere near the middle, compared to the horse. These structural and numerical differences define them as separate species capable of producing hybrid offspring.
The Genetic Basis of Mules and Hinnies
The difference in chromosome numbers between the horse (64) and the donkey (62) explains the genetics of their hybrid offspring, the mule and the hinny. A mule results when a male donkey (jack) is bred with a female horse (mare). Conversely, a male horse (stallion) bred with a female donkey (jenny) produces a hinny.
In forming the hybrid, each parent contributes half of its chromosomes to the zygote. The horse contributes 32 chromosomes, and the donkey contributes 31 chromosomes. The resulting hybrid, whether a mule or a hinny, therefore has a total of 63 chromosomes in its body cells.
This odd number of 63 chromosomes is the primary reason why mules and hinnies are sterile. Sterility is linked to a failure in meiosis, the cell division required to create functional gametes (sex cells). During meiosis, chromosomes must pair up precisely with their homologous partner.
Since the hybrid has 63 chromosomes, it possesses an odd, unpaired chromosome without a partner to align with. Furthermore, chromosomes that are numerically matched often have structural differences, which prevents proper pairing. This failure to align correctly disrupts the entire meiotic process, leading to a meiotic block and preventing the mule or hinny from having offspring.