Twins

Contents:

1.Introduction 2.Identical and fraternal twins 3.Differences between identical and fraternal twins 4.Different ways in which cell division can occur in twins 5.Conjoined twins 6.Types of conjoined twins 7.Bibliography

Introduction

At least everybody knows some one or knows of someone who is a twin, but none of us really ever stop to think about why there are twins and how are they formed What processes does it involve? how many types of twins can we have? What are conjoined twins?etc. Here is an attempt to explain what twinning is, and to explain the various questions related to it. Basically there are two types of twins: 1.

2.

Identical twins: Monozgotic twins, having a very similar genetic make-up, are always the same sex, have the same blood type, and usually look very similar. There are exceptions though. Some identical twins do not have the exact same genetic composition. The rate of identical twinning shows little variation with the mother’s age but identical twins occur approximately once out of 250 births. Fraternal twins:. Dizgotic or fraternal twins are twice as more common than identical twins. These twins arise from multiple ovulation. Normally, just a single ovum is released from an ovaryeach month. When more than a single ovum is released it is called multiple ovulation. If all of the ova are fertilized, genetically dissimilar twins or higher multiples may occur. Dizgotic twins usually do not share anymore similarities than would be expected with any siblings. Fraternal twins often have a different gender, blood type and other characteristics. Studies also show that the rate of fraternal twinning increases with the mother’s age.

Then there can be polar body twins also, which are very rare.

There is one special type of twins called conjoined twins. They have fascinated the people for centuries now. They are also called as Siamese twins. The rarest type of twinning would be the twins by two different fathers. This is called “Heteropaternal Superfecundation. This occurs when an egg is released, even though another egg has already been fertilized. If the second egg is fertilized by another man’s sperm, the fetuses would be no closer genetically than half siblings. There are not very many cases depicting this type of birth. Now we will get an idea of what identical, fraternal and conjoined twins are:

Identical and fraternal twins

Identical - monozygotic (one zygote) -- twins formed when a single fertilized egg splits into two genetically identical parts. The twins share the same DNA set, thus they may share many similar attributes. However, since physical appearance is influenced by environmental factors and not just genetics, identical twins can actually look very different. Identical twins are always same-sex sets. There is also such a thing as “Half-Identical” twins which is not to be confused with “fraternal” twins. Half-Identical twins occur when and unfertilized egg splits and then the two eggs are fertilized by two separate sperm. The fetuses share about half of their genetic code, which they receive from their mother.

Fraternal - or dizygotic (two zygotes) -- twins develop when two separate eggs are fertilized and implant in the uterus. The genetic connection is no more or less the same as siblings born at separate times. They may look alike, or they may not. Scientists have theorized a third, hybrid type called polar body twinning, that occurs when an unfertilized egg splits into two parts and each part is fertilized by a different sperm. The twins would then share one-half of their gene set (from their mother). Because it is the father's DNA that determines the sex, the twins can be either same-sex or male/female.

Definition of monozygotic or identical twins: A type of twins derived from a single (mono) egg (zygote). Monozygotic twins form when a single fertilized egg splits into two embryos. Because the twins share the same DNA set, they tend to have similar features. (see figure 4)

Definitionof dizygotic or Fraternal Twins : Derived from a two (di) eggs (zygote). Dizygotic twins form when two separate eggs are fertilized by separate sperm. (see figure 1)

The differences between the monozygotic, dizygotic and the polar body twins are given in the table below:

Monozygotic Twins Dizygotic Twins

Polar Body Twins Occur when a single Form when a single Form when two eggs unfertilzed egg splits fertilized egg splits are fertilized by two into two and is into two. separate sperm. fertilized by separate sperm. Sometimes referred to Also called identical. Also called fraternal. as hybrid or halfidentical twinning. It is not known what Only one-third of all Two-thirds of twins percentage of twins twins are identical. are fraternal. falls in this third category. Can be hereditary on the mother's side. The Don't "run in tendency to hyper families" except by ovulate (release more coincidence. No than one egg in a Not known. hereditary influence cycle) is a genetic for identical twinning trait that can be has been identified. passed from mother to daughter. May have one shared May have two May have two separate placenta, two separate separate placentas or placentas or two placentas, or two two placentas fused placentas fused into placentas fused into into one. one one. Share 100% of their Share about 50% of Share about 75% of genetic markers. their genetic markers, their genetic markers, same as singleton more than fraternals

Are always same sex. Have the same blood type.

siblings. but less than identicals. May be same sex or May be same sex or male/female. male/female. May have the same May have the same blood type or blood type or different. different. Can be attributed to fertility treatments, No one knows what advanced maternal causes this type of age, birth control pills twinning to occur. or other factors that influence twinning. Develop separate sacs Develop separate sacs in utero. in utero.

Not caused by fertility treatments, birth control pills or maternal age. No one knows what causes identical twinning. May be contained in one sac in utero. Can result in conjoined twins or Not conjoined. mirror image twins. May be at risk for Twin-to-Twin Rarely at risk for Transfusion TTTS. Syndrome (TTTS)

Not conjoined.

Rarely at risk for TTTS.

The following pictures and text explain the difference between identical and non-identical twins.

Identical Twins (also known as Uniovular or Monozygotic twins). During the fertilization process, only one ovum is impregnated by only one spermatozoid, but the egg divides in 2 embryos afterwards. They have exactly the same chromosomes, they have a perfect resemblance and they have the same sex. There are 3 different ways that identical twins can develop and be carried: •

•

•

With the first case there is only one placenta that feeds the babies, but there can be two amniotic sacs (as in the diagram at the side). When there is 1 placenta and 2 amniotic sacs then the pregnancy is referred to as having a "mono-chorial" placenta and is "biamniotic". In the second scenario, there is only one amniotic sac. In the case of 1 placenta and 1 amniotic sac then the pregnancy is referred to as having a "mono-chorial" placenta and is "mono-amniotic". The third scenario is where there are two placentae and each embryo has its own amniotic pocket. When there are two placentas and two amniotic

pockets like this, then one speaks of a "bi-chorial" pregnancy that is "biamniotic". (This last case is an identical situation to fraternal twins as shown below).

Non-Identical Twins (also known as Fraternal, Binovular or Dyzygotic twins). When twins are non-identical, then two seperate eggs have been impregnated by 2 separate spermatozoa. They each have a different chromosome make-up, they don't look alike and they can be either the same or different sex. In fact, this is simply two separate pregnancies that just happened to occur at the same time. Each embryo is in its own amniotic sac and has its own placenta. This type of pregnancy is referred to as "bi-chorial" and "bi-amniotic".

The following pictures and text explain the different ways that cellular division can occur in twins.

Scenario 1: Identical twins

Scenario 2: Identical twins

Scenario 3: Identical twins

Monozygotic twin pregnancy

Monozygotic twin pregnancy

Monozygotic twin pregnancy

Bi-chorial and bi-amniotic.

Mono-chorial and biamniotic.

Mono-chorial & monoamniotic.

Scenario 4: Fraternal twins

Dyzygotic twin pregnancy Bi-chorial and bi-amniotic.

What's happening in each scenario? The Monozygotic twins are derived from only one egg and only one fertilization.

Scenario 1: If the separation takes place just after the first cellular division, then in this case both of the twins will have their own placenta and an amniotic sac each. Scenario 2: The separation can also take place a little later in the development of the embryonic cells but before the blastocyte has fully defined the roles of each type of cell. At that moment, twins will be in the same placenta, but they will have 2 amniotic sacs. Scenario 3: In the last of the identical twin scenarios, if the separation takes place at the stage when the amniotic bag is already being formed, then the twins will be in the same placenta, and in the same amniotic sac. Scenario 4: Dyzygotic twins (fraternal twins), as shown in the diagram, are descended from a double ovulation and a double fertilization. The 2 eggs are completely independent. This configuration represents two thirds of all twin pregnancies. Scenario 1 and Scenario 4 clearly have the same configuration at implantation into the uterus wall (bi-chorial and biamniotic) yet they are very different in that when the twins derive from one egg (monozygotic) they will be identical and the twins coming from two separate eggs will be fraternal.

Conjoined twins

History of conjoined twins One of the earliest documented cases of conjoined twins are Mary and Eliza Chulkhurst, also known as the Biddenden Maids. Born in 1100, the sisters lived for 34 years in Biddenden, County of Kent, and England. Mary and Eliza, though often depicted as joined at the hip and shoulders, were likely pygopagus twins who were joined at the buttocks and lower backs. Then there were the famous Siamese twins with whose name are the conjoined twins called these days. Chang and Eng Bunker, the famous conjoined twins from Siam who earned their living in the U.S. as a circus attraction in the Barnum and Baily Circus.

What are conjoined twins? The phrase 'Conjoined Twins' is the appropriate given name for twins that are physically conjoined.

How are they formed? Siamese or conjoined twins are the result of a rare embryological accident. The developing embryo begins to split into identical twins but then stops part way leaving the partially separated egg to mature into a fetus. Most conjoined twins are stillborn, and those that survive often die within a few hours. The frequency of the birth of conjoined twins is difficult to estimate, but perhaps 5% of monozygotic (identical twin) twinning fail to separate completely and are conjoined. "According to Dr. Alan Guttmacher, noted physician and monozygotic twin, conjoined pairs are rarities which

occur only once in 50,000 to 80,000 births." There is a great range in the degree of fusion, and depending upon which organs are shared, some twins have undergone separation surgery where both have survived. Points of juncture can be entire torsos, the top or side of the cranium, hips, rear ends, and chests.

Conjoined twins are usually classified by the point at which they are joined (the Greek word pagos , means "that which is fixed.") Hence, the suffix-pagus is used meaning fastened.There have been as many as three dozen separate types identified in the last century. The following basic classifications can be combined to more closely define individual cases. Types: 1. cephalopagus 2. craniopagus 3. craniothoracopagus 4. dicephalus 5. ischopagus 6. omphalopagus 7. parapagus 8. pygopagus 9. thoracopagus

Anterior union of the upper half of the body with two faces on opposite sides of a conjoined head. Extremely rare. The heart is sometimes involved.

Cranial union only, about 2% of all conjoined twins.

Union of head and chest. There is only one brain, and the hearts and gastrointestinal tracts are fused. This type of union is also known as epholothoracopagus.

This refers to one body with two heads.

Anterior union of the lower half of the body, about 6% of all conjoined twins. Heart not involved.

Anterior union of the lower half of the body, about 6% of all conjoined twins. The heart is not involved.

Lateral union of the lower half, extending variable distances upward, about 5% of all conjoined twins. Heart sometimes involved. This type is also known as Diprosopus.

Posterior union of the rump, about 19% of all conjoined twins. These conjunctions always involve the umbilicus. This type of union is also known as Illeopagus.

Anterior union of the upper half of the trunk. The most common form of conjoined twins (about 35%), it always involves sharing the heart.

Parasitic twins: Asymmetrical conjoined twins, one twin being small, less formed, and dependent upon the other.

Fetus in fetu: Situation in which an imperfect fetus is contained completely within the body of its sibling

Specimens of rare twins:

This fetus is a poorly formed blob attached to a complete fetus. There is no heart or brain in the "blob" twin. This is an acardiac twin that consists mainly of lower extremities.

This fetus is a more complete acardiac twin in which actual body regions are present. Nonetheless, it is non-viable.

She has a partially developed acardiac twin in which actual body regions are present. Nonetheless, it is non-viable.

He is a partially developed twin in which the upper body parts are present. Even though he is not a conjoined twin or identical twin, he shows us that it is possible for a partially developed twin to survive if all the vital organs are present.

Recent achievements of science The conjoined twins being successfully separated:

Bibliography Websites

• www.google.com • www.unu.edu • www.healthseva.com • www.otal.com • www.rediff.com

Books

Newspaper • The times of india