Types of TWINS

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A twin is one of two offspring produced in the same pregnancy. Twins can either be identical (in scientific usage, "monozygotic"), meaning that they develop from one zygote that splits and forms two embryos, or fraternal ("dizygotic") because they develop

TYPES OF TWINS

A twin is one of two offspring produced in the same pregnancy. Twins can either be identical (in scientific usage, "monozygotic"), meaning that they develop from one zygote that splits and forms two embryos, or fraternal ("dizygotic") because they develop from two separate eggs that are fertilized by two separate sperm.

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In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple. It is theoretically possible for two singletons to be identical if all 23 chromosomes in both gametes from the mother and father were to be exact matches from one birth to the next. While this is statistically improbable (less than one in one billion-billion-billion chance) under natural conditions, a controlled pairing may someday be possible.

A less complex way to engineer genetically identical offspring is through the process of cloning, a procedure that has successfully been accomplished with several species of mammals.

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“The more, the merrier!” is a phrase that resonates beyond hospital walls moments after delivery of the magical biological phenomenon - twins. In approximately every 80 pregnancies, one is positively estimated to produce two offspring. Some women are simply more genetically prone to releasing more than one egg per cycle than the rest. If the woman is in her late thirties, chances of twin pregnancies are higher. In vitro fertilization involves transfer of several eggs to ensure the development of at least one embryo. In effect; the result is quite often- two!

Twins originate from a variety of organic instances. Some newborns may be identical while some aren’t. In some scenarios, twins share the same sex and in others, they don’t. Nonetheless, despite sharing the same birth date and DNA, they are poles apart in terms of personality traits and preferences. This statement can be reinforced by the quote- ‘I may be a twin but I'm one of a kind’. Here are the various kinds of twins present in this world.

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Different Kinds of Twins

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Monozygotic Twins

When a single zygote fuses with a sperm and splits into two separate embryos, the final product of this fertilization is what we scientifically call monozygotic twins. They are consistently identical to the human eye, sharing the same physical features due to matching genetic information. Since they stem from the same zygote containing either a male (XY) or female (XX) sex chromosome, their genders can never differ. However, in a few out of the ordinary cases of genetic mutation, a male fetus loses the Y chromosome and develops as a female with a deterred growth of ovaries. One-third of all twins are reported to be monozygotic.

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Dizygotic twins

This occurrence is set off by the release of more than one egg in an ovulation cycle. When two separate eggs are fertilized by two separate sperms, fraternal twins are nine months away from earth! Also known as dizygotic twins, separate placentas, chorions and amnions confirm their type in the womb. Around two-thirds of all the twins in this world are fraternal.

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Conjoined Twins

Conjoined twins or Siamese twins are essentially monozygotic twins that are joined ‘in-uteri’. This rare and heartbreaking phenomenon is elucidated by unfinished division of the fertilized ovum. The two individuals will be stuck to one another till death brings them apart. Unless they consider surgery, which carries cautioned risks. Connected at certain parts of the body, Conjoined twins usually share the same organs, limbs and tissues.

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Dicephalus (Two Headed) Conjoined Twins Abigail and Brittany Hensel are about to turn sixteen. They’ve shared every single moment of their lives, but it wasn’t a matter of choice. These twin sisters are joined for life.

Polar Body Twins

Sometimes, the egg first splits. Later, each half collides with a sperm. These twins might be extremely alike in several ways including looks but their DNA won’t be 100% compatible.

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Mirror Image Twins

Coming from a single fertilized egg, mirror image twins develop reverse asymmetric features. This can be attributed by a delayed split of the fertilized egg, often surpassing a week. 25% of identical twins tend to be mirror image twins. One will be left-handed and the other, right handed. They may possess matching birthmarks on opposite sides of their bodies and suck opposite thumbs. The name springs from the fact that if face to face, they would be seeing exact mirror reflections of themselves!

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Same Face, Different Birthday

What if a Labor begins at the peak hours of New Year’s Eve and the second twin is still on the way while the first one is already inhaling monoxides? The clock strikes twelve and Oops! Forget their birthdays being one day apart. Picture this- Molly was born in 1983 and her twin sister Polly in 1984! In certain cases, preterm labor forces the delivery of one baby while the other one’s can be controlled.

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Semi-Identical Twins

Identified in a pair of three-year old kids, these rare twins developed when two sperms fertilized one egg and later split. One twin was born a hermaphrodite while the other is anatomically male.

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A mother from the United States gave birth to twins who were found to have different fathers. After undergoing DNA tests, Mia Washington from Dallas, Texas, made everything clear by admitting that she was dating another man when she conceived, thus two eggs were fertilized by the sperm of two men. (Source: http://www.whatsthelatest.net/news/twins-born-fathers/)

Heteropaternal Super fecundation

Very rarely but possibly a woman’s eggs can be fertilized by sperm from differing events of coitus. Suppose a woman engages in sexual intercourse with different partners, the twins could have separate biological fathers! This anomaly is defined as heteropaternal super fecundation.

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Prenatal Diagnosis of Pygopagus Tetrapus Parasitic Twin: Case Report Parasitic Twins (Source: http://www.obgyn.net/ultrasound/articles/pygopagus_tetrapus_parasitic_twin_fig4.htm)

When conjoined twins develop asymmetrically and a tinier and less formed twin depends on the stronger, larger twin to survive, they are known as parasitic twins.

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( Source: Cervical Teratoma.jpg - Some forms of teratomas, Fetus in fetu case)

A variation of this called ‘fetus in fetu’ occurs when an abnormally formed mass of cells swell inside the body of its monozygotic twin. At times, the other twin doesn’t get sufficient nutrition from the placenta.

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Twins of Different Races

Heteropaternal super fecundation speaks of cases wherein fraternal (dizygotic) twins exhibit differing racial characteristics. In one case, the differentiation was propelled by a lab mix-up during an in-vitro procedure. Also, in UK, two bi-racial parents conceived fraternal twin girl twins; one was dark colored and the other white. This is a "one in a million" occurrence, where twins acquire different genetic characteristics from their mixed race.

Reference:

http://en.wikipedia.org/wiki/Twin

http://lifestyle.iloveindia.com/lounge/types-of-twins-10503.html

Images from Google Image

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