Mendel hypothesized that an individual inherits an allele from each parent for each trait. In animals, gametogenesis occurs in the reproductive organs via meiosis. In humans, each sex cell is haploid, which combine to form a diploid zygote with 23 pairs or 46 total chromosomes, of which 44 are autosomes that code for traits not sex-related. The remaining pair of chromosomes, the X and y, determines the sex of the individual and control certain traits found on the individual, called sex-linked traits (see Genetic Variation and Natural Selection).
Gametangia in plants are structures that produce and create an acceptable, usually moist, environment for the site of fertilization and embryo development.
Oogenesis, the production of eggs in animals, involves a unique process. The diploid reproductive cell undergoes meiosis to produce one egg cell or ovum. During both cytokinesis I and II, the division of the egg cell is uneven, creating one large egg cell and three cytoplasm-deprived cells, called polar bodies, which are degraded and recycled by the cell. The remaining large cell develops into a mature egg cell. Conversely, in spermatogenesis, the diploid reproductive cell divides via the normal meiotic route to create haploid spermatids, all of which mature into normal sperm cells.
A sporophyte results from the fusing of two haploid gametes to become a diploid, spore producer; a gametophyte is a haploid, gamete producer that grows from a spore. A pine tree is an example of the most recognizable sporophyte form.
Curiously, most fungi reproduce both asexually and sexually, which is a typical sign of a species using the alternation of generations life cycle. In this life cycle, the sexual pathway involves a plus (+) and a minus (-) mating type meeting and joining gametangia where the haploid gametes combine to form diploid zygotes. Meiosis then occurs in the zygote, reducing it to a haploid stage for the remainder of the life cycle.
Except for our reproductive organs, our bodies are diploid; however, plants vary greatly from animals because their life cycles include both a haploid and a diploid cycle for prolonged periods of time. Accordingly, male and female haploid gametophytes produce gametes by mitosis; diploid sporophytes produce spores by meiosis.
Gametophyte haploid gametes combine in the gametangia to produce a fertilized diploid egg. The diploid offspring divides mitotically and grows into maturity as a sporophyte and then undergoes meiosis to produce haploid spores that grow into haploid gametophytes.
In modern times, the overwhelming majority of plant species, including all seed plants, exhibit a dominant and usually recognizable sporophyte stage in their life cycle. An exception is moss. The green velvety growth that is often seen in dense shade is actually the gametophyte stage of the life cycle. When the male and female gametophyte gametes join in the gametangia, the resulting zygote develops into the sporophyte stage, which remains attached to the gametophyte by growing a stalk with a capsule full of spores on top.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.