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.

Verified step by step guidance

1. Step 1: Determine the chromosome number of the mule. A mule is the hybrid offspring of a male donkey (2n=62) and a female horse (2n=64). To calculate the mule's chromosome number, add half the chromosome count from each parent: (62/2) + (64/2). This gives the mule's diploid chromosome number.
2. Step 2: Explain why mules are generally sterile. Mules inherit one set of chromosomes from the donkey and one set from the horse. These two sets of chromosomes are not homologous, meaning they cannot pair properly during meiosis. This failure to form homologous pairs prevents the production of viable gametes, leading to sterility.
3. Step 3: Discuss the rare case of a female mule giving birth. In rare instances, a mule may produce offspring if a viable egg is formed. This could occur if the mule's chromosomes undergo an unusual alignment or if the egg is formed through a process that bypasses normal meiosis.
4. Step 4: Analyze the offspring's chromosome number. If the female mule mates with a horse (2n=64), the offspring's chromosome number would depend on the mule's gamete and the horse's gamete. The mule's gamete would likely contribute half of its chromosome count, and the horse's gamete would contribute half of its chromosome count.
5. Step 5: Summarize the genetic implications. The sterility of mules highlights the importance of homologous chromosome pairing in meiosis. This case also demonstrates how rare genetic events can lead to exceptions in hybrid sterility, providing insights into reproductive barriers and hybridization.

Key Concepts

Chromosome Number and Karyotype

A karyotype is a visual representation of an organism's complete set of chromosomes, organized by size and shape. In this case, the male donkey has 62 chromosomes (2n=62) and the female horse has 64 chromosomes (2n=64). Mules, which are hybrids of these two species, typically have 63 chromosomes (2n=63), resulting from the combination of one set from each parent.

Hybrid Sterility

Hybrid sterility occurs when the offspring of two different species are unable to reproduce. In the case of mules, they are sterile because they have an odd number of chromosomes (63), which complicates meiosis—the process of forming gametes. During meiosis, the chromosomes cannot pair properly, leading to the production of nonviable gametes.

Meiosis and Gamete Formation

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing gametes (sperm and eggs). In sexually reproducing organisms, proper pairing and segregation of homologous chromosomes are crucial for viable gamete formation. In mules, the lack of homologous pairs due to the odd chromosome number disrupts this process, resulting in infertility.