Zygote

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A zygote (from Greek ζυγωτός zygōtos "joined" or "yoked", from ζυγοῦν zygoun "to join" or "to yoke"),[1] or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells—an ovum from a female and a sperm cell from a male—which combine to form the single diploid cell. Such zygotes contain DNA derived from both the mother and the father, and this provides all the genetic information necessary to form a new individual. The term zygote is also used more loosely to refer to the group of cells formed by the first few cell divisions, although this is properly referred to as a morula.

In mammalian reproduction, after fertilization has taken place the zygote travels down the fallopian tube, while dividing to form more cells[2] without the zygote actually increasing in size. This cell division is mitotic, and is known as cleavage.[3] All mammals go through the zygote stage of life. Zygotes eventually develop into an embryo, and then a fetus. A human zygote exists for about four days, and becomes a blastocyst on the fifth day.[4]

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Twins

Twins and other multiple births can be monozygotic (identical) or dizygotic (fraternal). Dizygotic twins arise from one or several—strictly, two—fertilization events. Polyspermic zygotes in mice have been manipulated so as to remove one of the two male pronuclei and made to survive birth.[5]

Conjoined twins, sometimes called "Siamese twins", occur once in every two hundred identical twin pregnancies and are always identical. Actual numbers for conjoined births vary from 1 in 20,000 to 1 in 100,000 pregnancies; 40–60% are stillborn, with many others dying within the first few days after birth. About 70% of conjoined twins are female, the reason for which is unknown.

The first successful separation of conjoined twins was performed in Basle, Switzerland in 1689 on twin girls born joined by a ligament at the sternum (xiphopagus). The first to be successfully separated in modern times are generally believed to be Catherine and Caroline Mouton of Louisiana, born joined at the lower back (pygopagus) and separated in 1953 at eight days of age. Both survived the operation. Separation has been attempted on almost all conjoined twins born since the 1950s, with varying results.

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