Short free response question
1. Female bees are diploid, and male bees are haploid. The haploid males
produce sperm and can successfully mate with diploid females. Fertilized eggs develop into females and unfertilized eggs develop into males. Propose a mechanism for sperm production in male bees, and indicate how it would differ from sperm production in other animals with diploid males.
2. Somatic cells from a horse have 64 chromosomes and those from a donkey have 62 chromosomes. A cross between a female horse and a male donkey produces a mule, which is usually sterile. How many chromosomes do you expect are in the somatic cells of a mule? Propose an explanation for why most mules are sterile.
1. In male bees, the mechanism for sperm production differs from that of other animals with diploid males due to their haploid nature. Haploid males undergo a process called haplodiploidy, where their sperm is produced through a type of cell division called meiosis.
Meiosis is a specialized form of cell division that occurs in diploid organisms to create haploid cells, such as sperm or eggs. In most animals with diploid males, meiosis produces gametes (sperm or eggs) that are also diploid. However, in haplodiploid organisms like bees, males only have one set of chromosomes to start with, so meiosis does not produce haploid cells but rather directly produces sperm.
During meiosis in male bees, a diploid cell undergoes two rounds of cell division. The first round of division separates the pairs of chromosomes, resulting in two haploid cells with half the number of chromosomes. The second round of division separates the chromatids, resulting in four haploid cells. These four cells then develop into mature sperm cells.
In contrast to diploid males in other animals, where meiosis produces diploid gametes that contribute half of the chromosomes to the offspring, haploidy in male bees leads to haploid sperm that can successfully mate with diploid females.
2. In somatic cells, a mule is expected to have a different number of chromosomes compared to its parent species, the horse and the donkey. The horse has 64 chromosomes, while the donkey has 62 chromosomes. When a female horse (64 chromosomes) and a male donkey (62 chromosomes) mate, the resulting offspring, the mule, inherit chromosomes from both parents.
The somatic cells of a mule would typically contain a total of 63 chromosomes. This is because the mule inherits a mix of chromosomes from its parent species, with one set of chromosomes coming from the horse and another set coming from the donkey.
Most mules are sterile due to a phenomenon called hybrid sterility or hybrid infertility. Sterility in mules arises from the difference in the number of chromosomes between the horse and the donkey. The mismatch in chromosomal numbers disrupts proper pairing of chromosomes during meiosis, leading to reproductive difficulties.
During meiosis, chromosomes pair up to exchange genetic material, a process known as recombination. In the case of mules, the mismatch in chromosome numbers prevents proper recombination and results in errors in chromosome segregation. These errors can lead to the production of non-viable gametes, making it difficult or impossible for mules to produce offspring, hence their usual sterility.