Various types of Twins

  

 

    Twins are one of the separate kinds in human birth history. As it is very rare during birth it always marks a separate place in life. Also not every twin’s looks like same. On the before topic of fertilization, we saw what fertilization is, and how a normal baby is formed through fertilization. In a nutshell, fertilization is the process, where a sperm meets the egg, and brings a new life. But it needs to be considered, that how a sperm fertilizes the egg and how a baby is born. A baby can be born as single if only one sperm fertilizes the ovum and that ovum makes its journey without dividing. But there are several possibilities in which fertilization can occur. Those possibilities may result in various types of twins, which are,

• Identical twins
• Non-identical twins
• Semi identical twins
• Mirror image twins
• Mixed chromosome twins
• Heteropaternal superfecundation
• Superfetation
• Turner twins
• Polar body twins
• Parasitic twins
• Biracial twins
• Vanishing twins.

    Even though there are more types of twins, most types will share a similar pattern. Except for the identical twins, all other types of twins are very rare types. Let’s see about each of these types of twins with a clear diagrammatic representation.

Identical twins

    Identical twins are the types of twins in which the two babies born, will be identical to each other. To produce identical twins, one sperm fertilizes a single egg, and that egg splits (divides) into two on the way through the fallopian tube. This results in two babies, who share the same genetic code; which means both will look like the same. This happens when a blastocyst has two inner cell masses corresponding to two babies. Thus the two cells separate, and each grows into a separate baby. Such twins are called identical twins or monozygotic twins. But sometimes monozygotic twins can also be born differently, due to chromosome defects.

 

Formation of Identical Twins

 

Non-identical twins

    In this type of twin also, two babies are formed. But unlike identical twins, non-identical twins do not look the same, which means their genetic code will not be identical. This type of twin occurs, when two sperm fertilize two different eggs. If the eggs are fertilized, then non-identical twins (Dizygotic twins) are born, as each sperm and egg will have a different genetic code. But the point is, a woman could ovulate only one ovum in a month and if that ovum is fertilized then it can develop into a baby. Till the baby comes out, no more ovulation occurs. So there is no chance of bringing dizygotic twins at the same time. But sometimes a woman can ovulate two ova at the same ovulation. If that happens, both ova have the chance to be get fertilized and develop into dizygotic twins at the same time. But delivering two ova at single ovulation is somewhat a rare case.

Formation of Non-Identical Twins

 

Semi-identical twins

    There is also another possibility where two sperm fertilize a single egg. It results in semi-identical twins or sesquizygotic twins. Actually, this case is very rare and an egg fertilized by this method would not survive at maximum. But above all, nature is the leader! There is a record of existence in the US in 2007 of semi-identical twins, who are born by two sperm and one egg. Unlike normal babies, who will have only two sets of chromosomes, generally, from their father and mother; sesquizygotic twins will have three sets of chromosomes. In the three sets of chromosomes, two sets will be from the father and one will be from the mother. This means, the semi identical twins will be the same on their mother's side, such that if the mother has the dominant gene in hair color and eye color for example. But the children will be different on their father’s side, as there are two sets of chromosomes.

Formation of Semi-Identical Twins

Mirror Image twins

    Mirror images are also one of the types of rare events, in which the twins formed will be mirrored to each other. Let’s consider a mirror opposite to us. If we show an object to it, it just reflects back to us, which we perceive it as in a reversed form. This is the same situation in mirror image twins. If one has right-handed, the other one will have left-handed. If one has a birthmark on one side of the body, the other will have a birthmark on the other side of the body. If one uses the right leg to kick the ball often, the other will use the left leg to kick the ball. This strange twin happens due to the splitting of the egg. Normal twins occur when the egg splits in half after two days of fertilization. During this time, the zygote will not start to form the right and left side of the body. But in 9 to 12 days of fertilization, the zygote will be determined to form the right and left side of the body. So in such a time if the egg splits, the right side will be the primary in one body, and the left side will be the primary in another body. But it can be concluded, that even though the twins have different characteristics, they look similar in appearance like normal identical twins.

Formation of Mirror Image Twins

 

Mixed Chromosome twins

    Mixed chromosome twins can be considered as the mixing of genetic information. Sometimes a woman can release two ova per ovulation. If those two ova got fertilized at the same time, then two zygotes can develop into separate babies with different characteristics. If the two zygotes merge together again, then the mother will have a mixed chromosome twin. Anyway, the mother will give birth to only one child. This mixed chromosome twin is also called a chimera.  Such a baby would have both the parts of male and female if the fertilized sperm was X and Y. A chimera can also result in different blood types.

 

Formation of Mixed Chromosome Twins

Heteropaternal superfecundation

    Heteropaternal superfecundation refers to the mating habit of the woman. It refers to two ova of the same women fertilized by the sperm of different fathers. When the woman releases two ova at the same time and has different sexual intercourse, the egg may have the chance to be get fertilized by different sperm. So that the born twins will have the same mother but different father; which leads to the word heteropaternal.

Formation of Heteropaternal Superfecundation

Superfetation

    Superfetation is the process, in which the pregnant woman can again get pregnant by ovulating a second egg after certain days. Usually, a sperm can survive in the woman's uterus for three days. So releasing another egg may also get that fertilized and transform it into a baby. It is the same as the non-identical twins.

Formation of Superfetation

 

Turner twins

    Turner twins are said as normal twins but one of the twins may have a deficiency in sex cells, such that there will be no X chromosome. Such syndrome is called mosaic turner syndrome. This problem mainly occurs when the born twins are girls. A person with turner syndrome will have many health problems like heart problems and fertility problems. This turner syndrome is mainly caused by the absence of an X chromosome, which is named as monosomy. It can also be due to the complications in the cell division called as mosaicism such that, some of the cells will be XY chromosomes and some will be only Y chromosomes.

Formation of Turner Twins

Polar body twins

    Polar body twins are the type of twins, which are formed from the polar body of the primary ovum. This polar body is formed when the primary ovum gets fertilized. Actually, when the ovum gets ovulated from the ovary it will be in the meiotic arrest (metaphase II). If the ovum was fertilized, then the egg will complete its meiotic phase and eject a polar body that also contains 23 chromosomes with a minimum cytoplasm. This polar body later gets died off. But when this polar body too gets fertilized by another sperm, polar body twins are formed. They will be looking much more like non-identical twins.

Formation of Polar body Twins

Parasitic and conjoined twins

    Parasitic twins also refer to normal identical twins. But unlike normal twins where the fertilized egg splits into two, in parasitic twins, the egg does not divide completely, leading to be conjoined. This situation occurs when the zygote splits very later after fertilization. Sometimes in this process, one of the fetuses will not grow properly, leading to a parasitic twin.  This is also referred to as partial molar, which refers to the growth of extra muscle. This can also lead to twin to twin transfusion syndrome, in which one twin gets all the nutrients while the other gets limited nutrients. Also due to the complications of the blood vessels arrangement and the combined placenta, the heart or other parts may not be formed by the twin through the process of twin reversed arterial perfusion (TRAP). Thus the two babies will be formed together without separating. Sometimes this could also be life-threatening.

 

Formation of Parasitic and Conjoined twins

 

Biracial Twins

    Biracial twins refer to the type of twins who can be identified that they belong to different races. For example, that one of the twins will look like white and the other will look like black. This means that their parents had belonged to different races or the combination of both races. It can be simplified, by the consideration of keeping white and black balls separately in two buckets; which denotes the father and mother of different races. Now let’s mix those two buckets into a single bucket by the process of fertilization. Then picking up the white balls in one hand and black balls in the other hand give fraternal twins with different races; meaning it as biracial twins.

Formation of Biracial Twins

Vanishing twin

    A vanishing twin is a type of dying twin in the womb itself. A twin may die in the womb due to not enough growth and improper genetic combination. Such a twin may be reabsorbed by the healthy twin. This is also named as miscarried twin and stillbirths. Both the term, refer to the dying of the baby in the womb. But they are varied depending upon when the baby dies. Miscarried twin usually refers to the loss of pregnancy within 20 weeks of fertilization and stillbirths refer to the loss of pregnancy after 20 weeks of fertilization.

Formation of Vanishing Twin

    There is also another wonder where nature plays with the baby’s genetic identity and genetic appearance. In the above methods, the baby born will be either male or female. But a baby cannot be confirmed based on the genitals. It is confirmed only by the baby, whether the baby feels it as a boy or girl, which we call as genetic identity and sexual orientation. We can see the detailed explanation of gender in our next post.