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Polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues. [45] This is known as endopolyploidy. Species whose cells do not have nuclei, that is, prokaryotes, may be polyploid, as seen in the large bacterium Epulopiscium fishelsoni. [46] Hence ploidy is defined with respect to a cell.
For example, most human cells have 2 of each of the 23 homologous monoploid chromosomes, for a total of 46 chromosomes. A human cell with one extra set of the 23 normal chromosomes (functionally triploid) would be considered euploid. Euploid karyotypes would consequentially be a multiple of the haploid number, which in humans is 23. [citation ...
Human reproduction naturally takes place as internal fertilization by sexual intercourse. During this process, the man inserts his erect penis into the woman's vagina and then either partner initiates rhythmic pelvic thrusts until the man achieves orgasm , which leads to ejaculation of semen containing sperm into the vaginal canal.
Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. [6] [7] Sexual reproduction also occurs in some unicellular eukaryotes. [2] [8] Sexual reproduction does not occur in prokaryotes, unicellular organisms without cell nuclei, such as bacteria and archaea.
Polyploidy in lower plants (ferns, horsetails and psilotales) is also common, and some species of ferns have reached levels of polyploidy far in excess of the highest levels known in flowering plants. Polyploidy in animals is much less common, but it has been significant in some groups.
[80] [81] Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis. [78] Polyploids also have more genetic diversity, which allows them to avoid inbreeding depression in small populations.
Polyspermy is very rare in human reproduction. The decline in the numbers of sperm that swim to the oviduct is one of two ways that prevents polyspermy in humans. The other mechanism is the blocking of sperm in the fertilized egg. [4] Only two cases of human polyspermy leading to birth of children have been reported. [5]
Human fertilization is the union of an egg and sperm, occurring primarily in the ampulla of the fallopian tube. [1] The result of this union leads to the production of a fertilized egg called a zygote , initiating embryonic development .