Editing Twin (section)

==={{anchor|Fraternal|Fraternal twins|Dizygotic|Fraternal (dizygotic) twins}}Dizygotic (fraternal) twins===
[[File:Red-haired Siblings.jpg|thumb|right|Adult fraternal twins]]
[[File:Fraternal twin brothers.jpg|thumb|Fraternal twin brothers as young babies]]
[[File:Mirjam Weichselbraun and Melanie Binder, ROMY 2009.jpg|thumb|Austrian TV host [[Mirjam Weichselbraun]] (right) and her fraternal twin Melanie look very similar but with a significant difference in height.]]
'''Dizygotic''' ('''DZ''') or '''fraternal''' '''twins''' (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case of females, "sororal twins") usually occur when two [[fertilisation|fertilized]] eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different [[Spermatozoon|sperm cells]], fraternal twins result. The two eggs, or ''ova'', form two [[zygote]]s, hence the terms ''dizygotic'' and ''biovular''. Fraternal twins are, essentially, two ordinary [[sibling]]s who happen to develop in the womb together and who are born at the same time, since they arise from two separate eggs fertilized by two separate [[sperm]], just like ordinary siblings. This is the most common type of twin.<ref name="multi">{{cite web |url=https://multiples.about.com/cs/funfacts/a/twinzygosity.htm |title=Identical and Fraternal Twins – Determining Zygosity in Twins |publisher=Multiples.about.com |date=2013-07-16 |access-date=2013-09-16 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304130136/http://multiples.about.com/cs/funfacts/a/twinzygosity.htm |url-status=dead}}</ref>

Dizygotic twins, like any other siblings, will practically always have different sequences on each chromosome, due to [[chromosomal crossover]] during [[meiosis]]. Dizygotic twins share on average 50 percent of each other's genes, the same as siblings that are conceived and born at different times. Like any other [[sibling]]s, dizygotic twins [[family resemblance (anthropology)|may look similar]], particularly as they are the same age. However, dizygotic twins may also look very different from each other (for example, be of opposite sexes).

Studies show that there is a genetic proclivity for dizygotic twinning. However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one [[ovum]]. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.<ref>{{Cite book
|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome
|editor=Louis G. Keith
|author=R. Derom
|author2=J. Orlebeke |author3=A. Eriksson |author4=M. Thiery
 |isbn=978-1-85070-666-3
|publisher=Taylor and Francis
|chapter=The epidemiology of multiple births in Europe
|year=1995
|page=145
}}</ref>

Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.<ref name="Bortolus">{{Cite journal
|title=The epidemiology of multiple births
|journal=Human Reproduction Update
|year=1999
|volume=5
|issue=2
|pages=179–187
|last=Bortolus
|first=Renata
|author2=Fabio Parazzini |author3=Liliane Chatenoud |author4=Guido Benzi |author5=Massimiliano Maria Bianchi |author6=Alberto Marini
 |issn=1355-4786
|pmid=10336022
|doi=10.1093/humupd/5.2.179
|doi-access=free
}}</ref> With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.{{citation needed|date=August 2022}}