Most people don’t realize that there are many different types of twins in the world. Not only can you be identical or fraternal twins but you might be one of any number of other type. Below are some of the most common other types of twin.
1. Conjoined Twins are monozygotic multiples that do not fully separate from each other due to the incomplete division of the fertilized ovum. The individuals will be connected at certain points of the body, and may share tissue, organs or limbs.
2. Twins Conceived Separately or Superfetation Twins occurs when an egg is released while a woman is already pregnant, resulting in twins that are conceived at different times.
3. Twins with Different Fathers or Heteropaternal Superfecundation occurs when fraternal (dizygotic) twins which are the product of multiple eggs being released in a single cycle are fertilized by sperm from separate incidences of sexual intercourse. In a case where a woman has sex with different partners, the twins could have different fathers and the appropriate term is heteropaternal superfecundation.
4. “Half Identical” Twins or Polar Body Twins is a theory at this time. There are two types of twins, right? Dizygotic (fraternal) twins result when two eggs are fertilized. Monozygotic (identical) twins come from a single fertilized egg that splits. But what if the egg splits and then each half meets a sperm? That’s the proposed theory for polar body or “half-identical” twins, twins who are very much alike but aren’t a 100% DNA match. Although it seems to be a reasonable theory, there is no definitive test to confirm polar body twinning.
5. Boy/Girl Identical (Monozygotic) Twins occurs when there is a genetic mutation in male twins where one twin loses an Y chromosome and develops as a female. The female twin would be afflicted with Turner’s Syndrome, characterized by short stature and lack of ovarian development. Of course, another explanation for gender differences in identical twins is an identical twin who undergoes a sex change operation.
6. Mirror Image Twins are monozygotic, twins that form from a single fertilized egg. When the split occurs late – more than a week after conception – the twins can develop reverse asymmetric features. This term is not really a type of twin, just an way to describe their physical features. For example, they may be right- and left-handed, have birthmarks on opposite sides of their body, or have hair whorls that swirl in opposite directions. In theory, if the twins faced each other, they would appear to be exact reflections of each other. About 25% of identical twins are mirror image twins.
7. Parasitic Twins is a type of conjoined twins that develops asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin. A variation of parasitic twinning is fetus in fetu, where an abnormally formed mass of cells grows inside the body of its monozygotic twin. It survives during pregnancy, and even occasionally after birth, by tapping directly into the blood supply of the host twin.
8. Semi-Identical Twins is a type of twinning identified as identical on the mother’s side but sharing only half their father’s genes, the rare twins developed when two sperm fertilized a single egg, which then split.
9. Twins with Different Birthdays the most common explanation is a labor and delivery that begins before midnight and ends on the following day. If that day happens at the end of the month, or even the end of a year, the two babies can have birthdays in different months and even different years. Sometimes a pregnancy is prolonged to provide each baby with an optimal chance for survival. If preterm labor forces the delivery of one baby, doctors can successfully control the labor and delay delivery to give the other baby more time in the womb. Twins and higher multiples have been born days and even weeks apart.
10. Twins of Different Races can be caused by heteropaternal superfecundation. There are cases of fraternal (dizygotic) twins with differing racial characteristics. In one case, the differentiation was due to a lab mix-up during an in-vitro procedure. However, there is a case where two bi-racial parents conceived fraternal (dizygotic) twin girl twins, experts explain that the girls inherited different genetic characteristics from their mixed race parents. One is fair-haired and light skinned, while the other has dark hair, eyes and skin.
By Alex Blake
DNA testing is done for a variety of reasons. DNA evidence can link an alleged criminal to a crime scene. DNA paternity and maternity testing can identify a child’s father or mother. DNA relationship testing can determine if two individuals are from the same family. DNA ancestry testing can determine ethnic origins and genealogical roots.
How DNA testing is done depends on the what kind of results are desired and on what types of samples are available. DNA fingerprinting (or profiling as it’s also known) is the process of analyzing and comparing two different DNA samples. Only identical twins have the exact same DNA sequence, everyone else’s DNA is unique. This makes DNA the perfect way to link individuals to each other or to locations where they have been.
The entire DNA chain is incredibly long, much to long to examine all of it in one test. Human DNA is made up of about 3.3 billion pairs. The differences between DNA samples occur only in small segments of the DNA–the rest of the DNA is pretty much the same. DNA testing focuses on those segments that are known to differ from person to person.
As DNA testing has evolved over time, the testing methods have become more precise and are able to work with much smaller DNA samples. Early DNA testing was done using dime-size drops of blood. Today’s tests can extract DNA from a drinking container. The DNA is extracted from whatever sample is provided (some times there is not enough DNA to provide for testing). DNA must be isolated and purified before it can be compared. In essence, it has to be “unlocked” from the cell in which it exists. The cell walls are usually dissolved with a detergent. Proteins in the cell are digested by enzymes. After this process, the DNA is purified, concentrated, and then tested.
DNA testing is done most often today by using a process called “short tandem repeats,” or STR. Human DNA has several regions of repeated sequences. These regions are found in the same place on the DNA chain, but the repeated sequences are different for each individual. The “short” tandem repeats (repeated sequences of two to five base pairs in length) have been proven to provide excellent DNA profiling results. STR is highly accurate–the chance of misidentification being one in several billion.
By Kevin Camilleri
DNA paternity testing of related potential fathers is fairly common, although in these types of paternity cases it is more difficult to prove paternity beyond any shadow of a doubt. It is always best to included both potential father in this type of test.
While positive DNA tests can be accepted as positive evidence of paternity in the most cases due the statistical improbability of potential fathers having the same DNA profiles, if the possible fathers are related they are more likely to have similar DNA profiles. It isn’t uncommon for brothers to be involved in a parentage dispute, or even for the mother to have a DNA test carried out privately to determine which is the father of her child.
The Legal Aspects
Where DNA paternity testing is to be used in evidence, courts will generally insist on a legal paternity test where the collection of the sampling is carried out by an authorized professional and the identity of the subject legally determined though an ID check and photo documentation. Home DNA testing, or DNA tests on samples presented by the subject, are not admissible as evidence. The probability of paternity accepted as definitive proof varies from state to state, although most states require a probability of 99%.
Such probability is determined largely by the DNA, and the manner the test is preformed. A good DNA test looks at 16 genetic markers, it would run all tests twice independent of each other, and should guarantee resutls of 99.9+%. It also depends on whether the sample of the Mother is included in the testing or not, as this will help to obtain a much higher level of probability.
Common DNA between Family Members
DNA tests on related potential fathers are no different from those carried out on any other subject: it is the results that could potentially create problems. Take the case of first cousins: they share 12.5% of their DNA and so it will be more difficult to prove parentage. This sharing of DNA increases to 25% for uncles, nephews and half-brothers, and to 50% for sons, fathers and brothers of the person being tested.
DNA paternity testing on different members of the same family is not an uncommon situation, and can occur for a number of reasons, including:
a) Pregnancies arising from consensual and non-consensual relationships with related men.
b) Two family members each claiming parentage, whether or not the mother is claiming one or neither to be the father.
c) A brother desiring confirmation that he is the parent.
d) The mother, or potential mother, having a private test to assure herself as to who is the father.
e) A grown child confirming parentage, particularly where an inheritance could be involved.
There are others, but these are common reasons for legal and home DNA tests being carried out where potential fathers are related. Standard DNA paternity testing statistics are based upon unrelated members of the public, and unless the laboratory carrying out the DNA testing has been informed of the relationship, the test results could be misinterpreted. Most reputable DNA testing companies will recommend that you preform a paternity test with each of the potential fathers to verify which would be a closer match to the child.
The Case of Identical Twins
Nevertheless, it is possible to test additional DNA markers if any of the above relationships are known, but not in one specific case. This is the case of identical twins. Fraternal twins occur when two eggs are fertilized by two sperm, and are no more alike genetically than were they siblings born years apart. Identical twins, however, are formed when one egg is fertilized by one sperm and then splits into two halves within the first 14 days for no known reason.
These twins share an identical DNA their DNA profiles would be indistinguishable from one another. There is no test that can determine which is the biological father of a child. Unless one is excluded for some reason, either could be the father and that is all that DNA testing can prove. The court would have to disregard DNA results and use other means to determine paternity.
Old-fashioned policing, therefore, still has a part to play where the related potential fathers are identical twins. Apart from this specific case, DNA paternity tests are not entirely foolproof since they rely on probability, but are virtually conclusive where you simply want confirmation of paternity. In such a case, a likelihood of 1 in a 1000 is a virtual certainty and where confirmation is required between a limited number of candidates, a 99.9% DNA genetic testing result is legal confirmation.
A new study failed to provide answers after pursuing a genetic explanation for why one identical twin developed multiple sclerosis while the other stayed healthy. Researchers created complete genetic blueprints for a pair of identical twins, looking for differences that might explain why one developed multiple sclerosis and the other did not. According to researchers there were no traces of a discrepancy in the twins’ DNA. Scientists found no smoking gun when they compared amount of gene activity between the twin with multiple sclerosis and the twin without. The results appear in a study published on April 29 in Nature.
According to Stephen Kingsmore, a geneticist at the National Center for Genome Resources in Santa Fe, N.M., and leader of the new study, “We looked under a lot of rocks and we found no differences that we could replicate.” Kingsmore went on to say that the findings “points to some novel environmental trigger that must be very important to the disease. We don’t know what it is.”
This study was small; it examines only three pairs of identical twins and one type of immune cell known to be involved in multiple sclerosis. A telling difference between sickness and health might be found in other types of cells, says Esteban Ballestar of the Bellvitge Biomedical Research Institute in Barcelona, Spain. Ballestar went on to say, “They are closing a door here, but I think, perhaps, the door should be open.” Multiple sclerosis is a disease where the immune system attacks and damages the myelin sheath that helps speed electrical communication between nerves, this is the equivalent of scraping the coating away from an electrical wire. The damage results in pain and symptoms such as loss of coordination and vision.
In the study, Kingsmore and his colleagues determined the entire genetic makeup of the immune cells called T cells from the female twin who had developed multiple sclerosis at age 30 and from her twin who had remained healthy. It was important that the twins were now old enough that the healthy one is not likely to develop the disease.
Identical twins share the same genetic makeup (it is believed that they have identical DNA), and the researchers confirmed that both women carried variants of genes already known to increase the risk of getting multiple sclerosis. Scientists had thought that maybe the sick twin had developed an additional mutation in her DNA that finally triggered the disease. But the team found no such mutations.
A Texas mother of twins discovers that they have two separate fathers. The mother, Mia Washington, cheated on her partner, James Harrison, with another man. One of her eggs was fertilized by James Harrison’s sperm and the second egg was fertilized by the other man, thereby causing fraternal twins with two separate fathers. (Identical twins occur when a single egg is fertilized by a single sperm and then, when it is a zygote, splits into two which creates a natural clone of it’s self. A DNA test to determine if twins are fraternal or identical is called a Twin Zygosity Test.)
In order for it this to occur, “The infidelity would have had to occur within a 24 to 48 hour period,” says Dr. Joseph Finkelstein, a Manhattan obstetrician and gynecologist. “Once the egg is released and is fertilized, the system shuts down, so although it technically could happen, it would have to happen in two days or less.” The likelihood of having twins without any medical intervention such as the use of fertility drugs is about 1 in 100, Finkelstein says.
“Texas woman gives birth to twins – from different fathers”