Biochemistry - Chemical Reactions in Living Cells, Volume 1 - D. Metzler 1980

The Setting
The Evolution of Complex Organisms
Haploid and Diploid Cells

In humans and higher animals, Meiosis produces Gametes—egg Cells and spermatozoa. Their fusion creates a diploid Nucleus once again, from which an adult Organism develops through successive mitoses. The meiotic stage is characteristic of The life cycle of all eukaryotes; however, this process does not always occur at a time analogous to the corresponding point in the human life cycle. For instance, the cells of many protists and Fungi are typically haploid. Following the fusion of two haploid nuclei to form a diploid Cell, meiotic division rapidly ensues, once again giving rise to haploid individuals. The alternation of haploid and diploid phases in the life cycle is common among lower plants and primitive animals. For example, fern spores fall to the ground and germinate into a small, green, moss-like haploid plantlet (prothallium), or gametophyte. It produces motile haploid gametes that fuse into a diploid zygote, which grows into the large, familiar sporophyte form of the fern.

In all likelihood, the dominance of the diploid phase in Higher Plants and animals is due to the ability of the heterozygote to survive even if one or more highly deleterious Mutations arise. For biochemical geneticists, utilizing haploid organisms offers tremendous methodological advantages by making it easy to identify recessive mutations.

¹ In mammals, the presence or absence of the Y chromosome determines the sex of the animal, whereas in other organisms the Y chromosome may be absent altogether. For example, in some insects (Orthoptera) females have a pair of XX Chromosomes and males have XO, whereas in others (Lepidoptera), as well as in A number of vertebrates (such as birds), males carry a pair of XX chromosomes and females carry XY.

² In human females, Messenger RNA synthesis occurs on only one of the two X chromosomes present in the cells. Furthermore, in some cells the maternally derived X chromosome is functionally active, while in others the paternally derived one is. As a result, genes from both parents are expressed throughout the organism as a whole.