Gamete

It is generally accepted that isogamy is the ancestral state from which anisogamy evolved, although its evolution has left no fossil records.[5] Oogamy also evolved from isogamy through anisogamy.[6][7] There are almost invariably only two gamete types, all analyses showing that intermediate gamete sizes are eliminated due to selection.[8][9] Intermediate sized gametes do not have the same advantages as small or large ones;[10] they do worse than small ones in mobility and numbers, and worse than large ones in supply.[11]

In contrast to a gamete, a diploid somatic cell of an individual contains one copy of the chromosome set from the sperm and one copy of the chromosome set from the egg cell. Consequently, the cells of the offspring have genes potentially capable of expressing characteristics of both the father and the mother, subject to whether they are dominant or recessive. A gamete's chromosomes are not exact duplicates of either of the sets of chromosomes carried in the diploid chromosomes but a mixture of the two.[12]

A human spermatozoon penetrating a human ovum. The spermatozoon is approximately 100,000 times smaller in size than the human ovum.

Humans and most mammals use the XY sex-determination system in which a normal ovum can carry only an X chromosome whereas a sperm may carry either an X or a Y, while a non-normal sperm cell can end up carrying either no sex-defining chromosomes, an XY pair, or an XX pair; thus the male sperm determines the sex of any resulting zygote. If the zygote has two X chromosomes it will develop into a female, if it has an X and a Y chromosome, it will develop into a male.[13]

For birds, the female ovum determines the sex of the offspring, through the ZW sex-determination system.[13]

Artificial gametes, also known as In vitro derived gametes (IVD), stem cell-derived gametes (SCDGs), and In vitro generated gametes (IVG), are gametes derived from stem cells. The use of such artificial gametes would [QUOTE:] "necessarily require IVF techniques".[14] Research shows that artificial gametes may be a reproductive technique for same-sex male couples, although a surrogate mother would still be required for the gestation period.[14] Women who have passed menopause may be able to produce eggs and bear genetically related children with artificial gametes.[14] Robert Sparrow wrote, in the Journal of Medical Ethics, that embryos derived from artificial gametes could be used to derive new gametes and this process could be repeated to create multiple human generations in the laboratory.[15] This technique could be used to create cell lines for medical applications and for studying the heredity of genetic disorders.[15] Additionally, this technique could be used for human enhancement by selectively breeding for a desired genome or by using recombinant DNA technology to create enhancements that have not arisen in nature.[15]

Plants which reproduce sexually also produce gametes. However, since plants have a life cycle involving alternation of diploid and haploid generations some differences exist. Plants use meiosis to produce spores that develop into multicellular haploid gametophytes which produce gametes by mitosis. The sperm are formed in an organ known as the antheridium and the egg cells in a flask-shaped organ called the archegonium. In flowering plants, the female gametophyte is produced inside the ovule within the ovary of the flower. When mature, the haploid gametophyte produces female gametes which are ready for fertilization. The male gametophyte is produced inside a pollen grain within the anther. When a pollen grain lands on a mature stigma of a flower it germinates to form a pollen tube that grows down the style into the ovary of the flower and then into the ovule. The pollen then produces sperm by mitosis and releases them for fertilization.

• Coenogamete