The Romans had an expression for something that was not likely to happen, "Cum mula peperit". This translates into "When a mule foals." A mule is the result of mating between a horse and a donkey. For almost 3000 years, people have valued mules for their combination of the horse's size and strength, and the donkey's stamina and hardiness. But why did the Romans use the expression "When a mule foals" to describe an unlikely event?

The genes that control the development of donkeys and horses from embryo to adulthood are so similar that mating these two species leads to the normal development of a mule. However, the resulting adult mule cannot produce eggs or sperm.

Egg cells and sperm cells are called gametes. Sexual reproduction requires the production of eggs and sperm by a cell division process called meiosis. The key feature of meiosis is the reduction of the number of chromosomes from two of each homologous chromosome, (2n), to one of each homologous chromosome, (1n), per gamete. Homologous chromosomes have genes for the same traits encoded on them, one inherited from the mother and one from the father. They line up in pairs during a critical period in meiosis.

If we look at the gametes from a horse and a donkey, we see that a horse gamete contains 32 chromosomes and a donkey gamete contains 31 chromosomes. The resulting mule will have 63 chromosomes that are non–homologous, so they are unable to pair up during meiosis. Therefore, mules cannot make viable eggs or sperm, so they cannot have foals.

In most eukaryotic organisms, the life cycle alternates between a diploid (2n) stage and a haploid (1n) stage. The alternation of these two stages, utilizing meiosis to generate the 1n stage contributes to the genetic variation among individuals. Depending on the organism, one stage may be the dominant, or most evident stage. For instance, in the case of humans, we are all multicellular diploid organisms. In contrast, most fungi and some algae have a dominant haploid multicellular stage. We'll learn more about life cycles in later chapters.

Some organisms can "skip" stages of the life cycle such as fertilization. These organisms can reproduce via asexual reproduction, which involves a single parent that produces genetically identical offspring. Unlike asexual reproduction, sexual reproduction creates offspring with unique gene combinations. That’s why you resemble your siblings but don’t look exactly alike, even though you have the same parents. Your parents’ chromosomes were rearranged during a type of cell division called meiosis, which leads to the formation of genetically unique sperm and egg cells. The genetic variation created by meiosis during sexual reproduction results in individuals with differences that may be selected for by the environment. In this way, meiosis contributes to the genetic variation that enables species to change, or evolve, to meet the demands of a changing environment.

In this presentation, we’ll learn about asexual and sexual reproduction. Then we’ll take a closer look at the process of MEIOSIS, and compare and contrast this to mitosis. We'll next examine the roles that meiosis and sexual reproduction play to provide GENETIC VARIATION. We'll also explore other means of reproduction and their adaptive advantages and disadvantages. After the presentation, we’ll watch a DEMONSTRATION where we’ll use meiosis data from the fungi Sordaria fimicola to measure distances between genes.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education