Two kinds of twins

Fraternal twins & identical twins

About twins

We usually describe twins according to:

• how many eggs they develop from – 1 or 2
• whether they share a placenta in their mother’s womb.

The main types of twins are fraternal twins and identical twins.

Most twins are born healthy, but a twin pregnancy can mean a higher chance of health complications for you and your babies. If you're pregnant with twins, you and your babies will need extra check-ups.

Fraternal twins

All pregnancies start when a sperm fertilises an egg. This fertilised egg is called a zygote.

Sometimes a woman’s ovaries release 2 eggs, and a separate sperm fertilises each egg. This forms twins. These twins are called fraternal twins, dizygotic twins (meaning two zygotes) or non-identical twins.

During pregnancy, the developing babies get oxygen and food from their mother through the placentas and umbilical cords. Fraternal twins have separate placentas and umbilical cords. The technical name for this is dichorionic.

Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different. For example, they might have different hair or eye colour. Occasionally they look quite similar.

Fraternal twins happen in about 70% of twin births in Australia.

Very rarely, fraternal twins share a placenta. These types of twins are called chimaeric twins.

Identical twins

Sometimes a fertilised egg splits within a few days of conception to produce genetically identical twins. Because these twins come from one zygote, they’re also known as monozygotic. Identical twins are the same sex.

There are 3 types of identical twins.

About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas.

The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. The technical name for this is monochorionic.

In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac, called the amnion, in addition to sharing a placenta. The technical name for this is monoamniotic twins. They’re often called MoMo twins.

Identical twins happen in around 1 in 250 pregnancies in Australia.

Although identical twins have the same genes, they don’t always look the same. This is because children’s health and development are shaped not only by genes but also by experiences in the womb and after birth. For example, a twin who gets less blood from a shared placenta might weigh less at birth.

Sharing a placenta or inner sac: why it’s important to know

Sharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins.

Women who are pregnant with twins sharing a placenta need to be checked more often than women who are pregnant with twins with separate placentas. Frequent checks help to pick up any complications early.

Twins sharing an inner sac (monoamniotic) are also at a higher risk of complications during pregnancy because of the chance that their umbilical cords might tangle and cut off their blood supply. These twins are checked even more closely. Medical professionals often recommend that these twins are born at 32-34 weeks. This is earlier than other types of twins, who are generally born at 37-38 weeks.

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, medical staff will look at the placentas to check what type of twins they are.

Fraternal or identical: why it’s important to know

Same-sex twins with separate placentas can be fraternal or identical. For health reasons, it’s good to know whether your twins are 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. The test doesn’t hurt and involves collecting a sample of cheek cells by rubbing the inside of your babies’ cheeks with a soft applicator (like a cotton bud). The cost of this test starts at $199 (for both twins) in Australia.

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.

Other reasons why twins and parents of twins might want to know whether they’re fraternal or identical include:

• trying to work out the chance of having more twins in future pregnancies (only fraternal twins run in families)
• making sure they have the right information about their genetic make-up
• being able to answer questions from family, friends and others
• being involved in twin research
• just being curious!

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. It’s not known why some twins are like this.

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 5000 births) and quadruplets (less than 1 in 100 000 births) can develop as a result of combinations of fraternal and identical twinning. There are no reliable figures for quintuplets (5 babies) and sextuplets (6 babies).

More about twins

In Australia, twins happen in 1 in every 80 births. This means that 1 in 40 Australians is a twin.

The birth rate of identical twins is the same around the world and doesn’t vary with the mother’s age.

In contrast, the birth rate of fraternal twins varies widely across countries and can be influenced by the mother’s age. Women aged over 35 years are the most likely to have fraternal twins, because their ovaries are more likely to release more than one egg at a time.

More twins were born from the 1990s to the mid-2000s. This is because it was common to transfer more than one embryo during in-vitro fertilisation (IVF) treatments. The rate of twins from IVF is now lower, because it’s now usual practice to transfer only a single embryo.

Twins - identical and fraternal

Multiple births are more common than they were in the past, due to the advancing average age of mothers and the associated rise in assisted reproductive techniques, in particular the use of fertility drugs. Twins account for over 90 per cent of multiple births. There are two types of twins – identical (monozygotic) and fraternal (dizygotic).

To form identical twins, one fertilised egg (ovum) splits and develops two babies with exactly the same genetic information. This differs from fraternal twins, where two eggs (ova) are fertilised by two sperm and produce two genetically unique children, who are no more alike than individual siblings born at different times. Twins are more or less equally likely to be female or male. Contrary to popular belief, the incidence of twins doesn’t skip generations.

Factors that increase the odds of having twins

Some women are more likely than others to give birth to twins. The factors that increase the odds include:

• Advancing age of the mother – women in their 30s and 40s have higher levels of the sex hormone oestrogen than younger women, which means that their ovaries are stimulated to produce more than one egg at a time.
• Number of previous pregnancies – the greater the number of pregnancies a woman has already had, the higher her odds of conceiving twins.
• Heredity – a woman is more likely to conceive fraternal twins if she is a fraternal twin, has already had fraternal twins, or has siblings who are fraternal twins.
• Race – Black African women have the highest incidence of twins, while Asian women have the lowest.
• Assisted reproductive techniques – many procedures rely on stimulating the ovaries with fertility drugs to produce eggs and, often, several eggs are released per ovulation.

Fertilisation

Hormones secreted by the ovaries, and a small gland in the brain called the pituitary, control the menstrual cycle. The average cycle is around 28 days. After a menstrual period, rising levels of the hormone oestrogen help to thicken the lining of the womb (the endometrium) and release an egg from one of the ovaries (ovulation).

If the egg is fertilised on its journey down the fallopian tube, it lodges in the thickened womb lining, starts dividing and evolves into an embryo.

Identical or ‘monozygotic’ twins

Around one in three sets of twins is identical. This occurs because the fertilised egg divides in two while it is still a tiny collection of cells. The self-contained halves then develop into two babies, with exactly the same genetic information.

Twins conceived from one egg and one sperm are called identical or ‘monozygotic’ (one-cell) twins. The biological mechanisms that prompt the single fertilised egg to split in two remain a mystery.

Approximately one quarter of identical twins are mirror images of each other, which means the right side of one child matches the left side of their twin.

Fraternal or ‘dizygotic’ twins

Around two in three sets of twins are fraternal. Two separate eggs (ova) are fertilised by two separate sperm, resulting in fraternal or ‘dizygotic’ (two-cell) twins. These babies will be no more alike than siblings born at separate times. The babies can be either the same sex or different sexes, with the odds roughly equal for each.

The proposed ‘third-twin type’

Some researchers believe there may be a third type of twin, although medical opinion is still divided. It is proposed that the egg splits in two, and each half is then fertilised by a different sperm. This theory is an attempt to explain why some fraternal twins look identical.

The normal length of gestation for a single baby is around 40 weeks. However, gestation for twins, either identical or fraternal, is usually around 38 weeks. This shorter time is due to the increased demands on the mother’s body, and the inability of the babies to receive all the nutrients they need in utero.

Since twins are usually premature, they are more likely to have lower birth weights. Prematurity is associated with increased risk of a number of disorders, including jaundice.

Giving birth to twins

Childbirth can give rise to complications when just one baby is present, so twins present extra potential for difficulties. It is advised that women carrying twins give birth in hospital, rather than at home. The babies can be delivered vaginally, but caesarean section delivery may be considered a better alternative in some circumstances.

Zygosity testing

It is difficult to tell if twins are identical or fraternal at birth. Some identical twins may be born with individual sets of membranes, which may lead to the mistaken assumption that the babies are fraternal.

One way to tell the difference is to have the twins DNA-tested. Identical twins share the same genetic information, while fraternal twins share around half. The test can be done with a sample of cheek cells, collected painlessly. Other tests include blood group examinations.

Where to get help

• Your doctor
• Paediatrician
• Maternity hospital

Things to remember

• Twins account for over 90 per cent of multiple births.
• To form identical or monozygotic twins, one fertilised egg (ovum) splits and develops into two babies with exactly the same genetic information.
• To form fraternal or dizygotic twins, two eggs (ova) are fertilised by two sperm and produce two genetically unique children.

Independent examination of twins. Price. Examples.

Twins are children of one mother, developing during one pregnancy and born as a result of one birth almost simultaneously. There are two main types of twins: homozygous twins (or identical) and heterozygous twins (twins, "twins").

Single-zygote twins are formed from one egg fertilized by one sperm. The fertilized egg divides, and each daughter cell gives rise to a separate organism. Identical twins are always of the same sex and are genetically nearly identical. nine0003

Twins develop when two eggs are fertilized by two sperm. Naturally, two-zygotic twins have different genotypes. They are no more similar to each other than brothers and sisters, and have about 50% identical genes. Fraternal twins can be of the same or different sexes and will have significant differences in DNA structure.

Both identical and fraternal twins are not only twins, but also triplets, quadruplets, and so on up to 9children.

There are several ways to determine whether identical or fraternal twins are born. Let's briefly list them:

• gender of children. If the children born are bisexual, then in any case they are fraternal twins;
• number of egg shells. The earlier the division of the zygote began, the higher the likelihood that each embryo will receive its own living space. Each embryo will have its own chorion and its own fruit sac. This indicates that the twins are fraternal; nine0014
• the presence of a third circulatory system - these are thin connecting capillaries between the blood vessels of both fetuses. This sign is inherent in identical twins;
• comparison of newborn blood types. In identical twins, the blood type and other specific blood factors are absolutely identical to each other.

The disadvantage of all these types of diagnostics (except sex determination), as many years of experience has shown, is the inability to accurately differentiate fraternal from identical twins. For example, in the case of fraternal twins, fusion of the egg membranes sometimes occurs, and a third circulatory system is often found in fraternal twins. To date, the most reliable way to determine fraternal or identical twins is the genetic examination of the twins. nine0003

So, the genetic examination of twins is a test to determine whether the twins are identical or dizygotic.

If the born twins are of different sexes, then it becomes obvious that the twins are fraternal, since there are no identical twins of different sexes. On the other hand, if the twins born are of the same sex (for example, two boys or two girls), then the question arises whether the twins are identical or dizygotic. To determine this accurately, a genetic examination of the twins is carried out. nine0003

To pass such an examination, you need to submit the biological material necessary for the study. As a rule, during the genetic examination of twins, swabs of the buccal epithelium are taken from the twins (scraping with a cotton swab from the inside of the cheek). This is the simplest and absolutely painless procedure, which takes about 1 minute and does not require the presence of a medical professional.

How is it established that the obtained biological material belongs to identical or fraternal twins? In genetic expertise, various systems of genetic markers have been developed to identify the individualizing characteristics of an organism. In particular, four systems are distinguished: 1) autosomal polymorphic loci; 2) polymorphic loci of the Y chromosome; 3) polymorphic loci of the X chromosome; 4) nucleotide profile of mitochondrial DNA (mtDNA). nine0003

Which of these systems are suitable for differentiating identical and fraternal twins?

• mtDNA is passed on from a mother to all her children, therefore, both identical and fraternal twins will always have the same mtDNA. Therefore, this type of genetic marker is not suitable for differentiating twins;
• polymorphic loci of Y chromosomes. The Y chromosome is the sex chromosome that determines the male sex of an organism (Figure 1). As already mentioned, if children of opposite sexes are born, then they are always fraternal twins. But is it possible to identify those born from unizygotic boys by the Y-chromosome? Due to the fact that, firstly, the Y chromosome is transmitted only from father to son, and secondly, in the process of sexual cell division it does not exchange homologous DNA regions (since it does not have a pair), the Y chromosome is transmitted to both children exactly the same. Therefore, it is impossible to differentiate single and dizygotic twins according to the Y chromosome; nine0014
• polymorphic loci of the X chromosome. The X chromosome is the sex chromosome that determines the female sex of an organism. Female cells have two X chromosomes, while male cells have only one. The X chromosome is inherited from both the father and the mother (Figure 1). According to the X chromosome of the father, it is impossible to differentiate single and dizygotic girls, due to the fact that during the process of sexual division of male cells there is no exchange of homologous DNA sections in the X chromosome (since it does not have a pair), and the X chromosome is inherited by children exactly the same. On the other hand, the mother's two X chromosomes recombine with each other through the process of sexual cell division, and, in the case of fraternal twins, the site exchange process can be found in the maternal X chromosomes. In practice, this type of analysis is not used, firstly, because it is necessary to study a large number of genetic markers of the X chromosome, and secondly, there is a simpler method of differentiation based on the analysis of autosomal polymorphic loci; nine0014

Figure 1. Transfer of sex chromosomes from parents to identical and dizygotic children. Different colors depict different loci.

Table 1. Identical and fraternal twins have a number of features that distinguish them both from others and from each other.

• autosomal polymorphic loci. Autosomes are paired chromosomes that are the same in the male and female body. In other words, these are all chromosomes, except for the sex chromosomes (X- and Y-chromosomes). In the process of sexual cell division (meiosis), chromosomes form pairs and exchange homologous regions (recombine). In the process of recombination, chromosomes exchange allelic variants of polymorphic loci, that is, gene variants. It is the recombined chromosomes with "shuffled" alleles that make up the genetic material of germ cells. Moreover, each sex cell has its own unique set of genes ("deck of cards" in this case consists of 25,000 genes). Thus, nature provides material for natural selection and creates even in a large family children who are not similar to each other. Since identical twins are formed from one egg fertilized by one sperm, the genetic material is identical. Fraternal twins are formed from two eggs and two sperm, so their genetic material is very different from each other. nine0014

As mentioned above, when genetic examination of twins is carried out, the only system of genetic markers is autosomal polymorphic DNA loci. When analyzed, all polymorphic loci of identical twins show the same alleles, while alleles of identical twins will be different (Figure 2).

Figure 2. The principle of determining identical and fraternal twins. Different colors and letters show different alleles of the same locus. nine0003

Therefore, the task of genetic examination of twins is to analyze the allelic variants of polymorphic loci and determine whether they are the same (homozygous) or different (heterozygous). To do this, biological material sent for genetic examination is subjected to several stages of analysis:

• extraction of DNA from biological material. The process of extracting DNA from cells is very important and time-consuming. At this stage, it is critical that DNA molecules from the environment do not get into the test tube with biological material (for example, house dust includes dead and exfoliated skin cells and hair particles). Therefore, DNA extraction takes place in especially clean rooms and in special laboratory boxes; nine0014
• , at the next stage of the analysis, the number of studied autosomal polymorphic loci increases with the simultaneous inclusion of a fluorescent label in each copy of the locus. This is done using the polymerase chain reaction (PCR) method. Today, the PCR reaction is used in a variety of molecular biotechnological methods: for example, DNA sequencing, cloning, directed mutagenesis, diagnosis of hereditary diseases, in all of the above methods for analyzing genetic markers, and so on. At 19In 83, this reaction was invented by the American biochemist Kary Mullis, for which he was awarded the Nobel Prize in 1993;
• at the final stage, the resulting mixture of DNA molecules is analyzed. To do this, molecules of different lengths are separated in a porous agarose gel under the action of an electric field. Separated DNA fragments labeled with a fluorescent label are read by a laser, and their length is automatically entered into a computer. Comparing the obtained results with each other, as well as with a standard genetic marker, the expert geneticist determines the degree of their agreement or difference. This point in the analysis of the coincidence or mismatch of DNA allelic profiles is carried out in accordance with Art. 3.4 Guidelines of the Ministry of Health of the Russian Federation No. 98/253 dated January 19, 1999 "The use of individualizing systems based on amplified DNA fragment length polymorphism (AFLP) in forensic medical examination of personality identification and establishing kinship."

The accuracy of the results of the genetic examination of twins, confirming the identity, is 99.99%, refuting - 100%.

Regulatory documents that determine the procedure for conducting a genetic examination of twins in the Russian Federation:

• Federal Law of May 31, 2001 No. 73-FZ "On State Forensic Activities in the Russian Federation";
• Order of the Ministry of Health and Social Development of the Russian Federation No. 346n dated May 12, 2010 "On approval of the Procedure for organizing and conducting forensic medical examinations";
• Guidelines of the Ministry of Health of the Russian Federation No. 98/253 dated January 19, 1999 "The use of individualizing systems based on amplified DNA fragment length polymorphism (AFLP) in forensic medical examination of personality identification and establishing kinship. " nine0014

According to the Decree of the Plenum of the Supreme Court of the Russian Federation of December 21, 2010 N 28 "On judicial expertise in criminal cases" expertise in a criminal case can be carried out either by the state an expert institution, or a non-profit organization established in accordance with the Civil Code of the Russian Federation and the Federal Law "On non-profit organizations", carrying out forensic activities in accordance with the statutes adopted by them. nine0003

Commercial organizations and laboratories, individual entrepreneurs, educational institutions, as well as non-profit organizations for which expert activity is not statutory, do not have the right conduct a criminal investigation. Expertise prepared by these organizations within the framework of criminal process, may be recognized as inadmissible evidence, i.e. evidence obtained from violation of the requirements of the procedural law.

Inadmissible evidence may not be used in the process of proof, including examination or be announced at the court session, and are subject to exclusion from the materials of the criminal case. nine0003

Since ANO "Forensic Expert" is an autonomous non-profit organization , and conducting judicial expertise is its main statutory activity (see section "Documents organization"), then it has the right to conduct examinations, including in criminal cases.

There are 2 types of people according to the sign of the Zodiac. Which one are you? 07 Nov, 2018 04:02 43126 Astrology, Signs of the zodiac

18+

Incredible facts

Did you know that there are two types of zodiac signs?

Each zodiac sign has unique personality traits. This does not mean that all people of a certain sign will have all these features, but most likely most of them will be present.

However, there are people who believe that they do not have the characteristic features of the signs of the Zodiac, and there is a reason for this.

Read also: Were you born at the intersection of the signs of the Zodiac? Here's what it means.

Your zodiac sign, depending on your birth chart, can manifest itself in completely different ways. You can call them the first and second type. At the same time, the first type is characterized by features that can be considered typical for representatives of this zodiacal constellation, while the 2nd type manifests itself a little differently.

So, what type are you? nine0156