How can fraternal twins look so different

These twins are checked even more closely. Medical professionals often recommend that these twins are born earlier than other types of twins. Medical professionals use ultrasound to work out how many placentas twins have. The earlier the ultrasound, the more accurately it can say how many placentas there are. It gets harder to work out later in pregnancy. After the birth, the placentas should be looked at to confirm or determine what type of twins they are.

Same-sex twins with separate placentas can be fraternal or identical. To find out whether twins are identical or fraternal, you can ask for a genetic test after your babies are born. This is called a zygosity test. Identical twins are more likely than fraternal twins to get the same illness. If one of a pair of identical twins is diagnosed with a particular disease or health condition, like high blood pressure, the other twin should be checked often for early symptoms.

Because of their genetic make-up, identical twins will always be compatible for organ transplantation , if they ever need it. Fraternal twins are compatible only sometimes. Some identical twins are mirror twins — for example, their hair parts on opposite sides, they are oppositely handed, or they have birthmarks on opposite sides of their body. In some rare cases, their internal organs can be mirror images of each other. In very rare cases, twins can be born physically joined together in different ways.

These twins, called conjoined twins previously called Siamese twins , can happen if the fertilised egg splits quite late after fertilisation. Triplets 1 in births and quadruplets less than 1 in births can develop as a result of combinations of fraternal and identical twinning. Read on for more details about twins. The National Library of Medicine says that monozygotic, or identical, twins are conceived from one fertilized egg.

This egg separates into two embryos after it has begun to divide. These two embryos develop into two babies. Genetic materials called chromosomes in both babies are completely identical. This is because both babies come from the same egg and sperm. For this reason, both children are assigned the same sex at birth and share the same genetic characteristics, such as eye and hair color. Not all twins are identical, of course.

More often than not, twins are born with unique physical characteristics. Nonidentical twins are generally known as fraternal twins. A different sperm will fertilize each egg. Since fraternal twins are the result of different eggs and different sperm, they share the same percentage of chromosomes as any other siblings.

Traditionally, the science around twins has taught that identical and fraternal are the only two types. But a third type might exist, called polar body or half-identical twins. Though this has never been confirmed, a research review from suggested that a third twin type would explain why some fraternal twins look so similar.

After the ovaries release an egg , the egg can split into two halves, the smaller of which is called a polar body. This polar body contains all the chromosomes necessary to join with a sperm to create a baby. Meanwhile, the larger half of the original egg could also be fertilized by a separate sperm. The result? Polar twins. Polar twins share the same chromosomes from their birthing parent, but they get different chromosomes from their non-birthing parent. For this reason, they may or may not be assigned the same sex at birth, and they may look very similar but not exactly identical.

In a basic twin pregnancy, two embryos go their separate ways and develop into twin babies, whether identical or fraternal. Some unique sets of twins follow a different path. Mirror twins are exactly what they sound like! These twins are actual mirror images of each other. This means that:. What causes this phenomenon? In a typical identical twin pregnancy, an egg splits during its first week after fertilization. Conjoined twins are a rare twin type in which the two siblings are physically connected.

Typically, conjoined twins are joined together at the chest or abdomen, but this varies. Some conjoined twins are connected to a greater extent than others.

DNA is large and very little of it has to do with, for example, skin and eye color. By chance, the DNA differences for these girls happened to be in the smallish part of their genome that deals with looks. These two girls let us see what goes on in every identical twin. And in every one of us. Identical twins do indeed start with identical DNA—they are the result of the same sperm from dad and the same egg from mom.

The original fertilized egg divides one or more times before the resulting clump of cells splits into two. Each clump of cells goes on to become one of the identical twins. In this process of becoming a brand new baby with trillions of cells, the cells in each clump divide over and over again. DNA differences or mutations can happen any time a cell divides.

This is because a cell needs to copy its DNA before it can divide. Every now and then it makes a mistake. All of the new cells that come from the one with the mistake will have that same mistake. One consequence is that if it happens early, the baby will have more cells carrying that mistake.

Think about it like one of those medieval monks patiently copying manuscripts in a monastery somewhere in medieval Europe. Imagine he makes a mistake during copying and the original manuscript is destroyed. Now every new manuscript contains his mistake. For the English twins, one may have developed a mutation in a gene that affects skin and eye color.

Most likely it would be in the lighter child as it is easier to break something than to fix it.