Homozygous twins: what's the difference with heterozygotes?
In the biological field, twins are defined as individuals born simultaneously from a single birth, but these are rare events, as single pregnancies by far prevail in the human species. Twin births can be divided into two types, depending on whether they are born: homozygous twins, or heterozygous twins. In this article we will look at the main differences, but first watch this video on the things not to do in pregnancy.
The phenomenon of homozygous twins
Homozygous twins are genetically identical as they derive from the same egg cell fertilized by a single sperm, share 100% of their DNA and are always of the same sex. Despite this, over time they can develop different physical and behavioral characteristics, influenced by the different exposure to biological and chemical factors, both during the period before birth inside the belly and after birth.
To be honest, homozygous twins are not so only in their DNA, but in an even deeper way, that is, in the entire network of molecular mechanisms that decide which genes are to be activated. The phenomenon, also called super-similarity, exists only in a small group of genes, many of which unfortunately are linked to the risk of contracting even serious diseases.
Homozygous twins: what are the characteristics
Homozygous twins are practically the same and can be mistaken for each other, as we said they share the same genetic heritage and the same sex. It all begins with the fertilization of a single egg, which then divides into two embryos within ten days. There are also other features to underline.
If the egg divides shortly after fertilization, each twin will have its own placenta and a cavity with amniotic fluid. If, on the other hand, the division occurs four to seven days after fertilization, the twins will share the same placenta.
In the case of twin pregnancies, regardless of whether they are homozygous or heterozygous, the phenomenon of "Fetal-fetal transfusion syndrome": that is, a twin can steal nutrients from the other, putting him in serious danger! From this we deduce how important it is to regularly check the growth of twins through a simple ultrasound.
It must be considered that twins with only one placenta and one amniotic cavity are rare and as we said, the pairs of adult homozygous twins can have a different genetic heritage. A team of researchers examined 19 pairs of twins, and each of these pairs showed the so-called Copy Number Variations, that is, the chromosomes were different.
After the study, the scientists found it appropriate to specify that these variations had matured over the course of life as a result of cell divisions. The hope after this discovery is to be able to identify the genes responsible for some diseases: if only one twin gets sick, in fact, the causes could be located in the differences between his genetic heritage and that of his twin brother.
The risks associated with a homozygous twin pregnancy
From an obstetric point of view, twin pregnancies can be more complicated than a normal single pregnancy, and the main risk is that the two children may be affected by congenital malformations.
In fact, there are some mechanisms that can lead to the development of congenital defects, let's see what they are.
- Incomplete division of the embryo
The same twinship can cause incomplete division of the embryo, with serious consequences on the development of one or both fetuses.
- Fetal compression
The fact that two little bodies have to fit in the uterus can be a problem. Lack of enough space for both twins can lead to fetal compression (the two babies are too tight together) and consequent deformations. Increases the risk of miscarriage. , preclampsia, polyhydramnios, prematurity, low neonatal weight, perinatal death and birth trauma.
- Death in utero
The possible presence of arterial or venous connections at the placental level can determine a differential pressure between the twins, the death of a twin in utero can damage the surviving twin.
Differences between all types of twins: let's recap
- Homozygous or monozygotic twins
Monozygotic twins are also known as identical twins that arise from a single egg cell fertilized by a single sperm. Monovular twins may share the same placenta (monochorionic monovular twins), or they may develop two distinct placentas (monovular bicoronal twins). The twins are of the same sex and strongly similar in appearance, since the individuals, who originate from this random event, have the same genetic heritage, have the same sex, the same eyes, the same hair, the same blood group. , that is, the same somatic characters.
- Heterozygous or dizygotic twins
Dizygotic twins are more common and result from the fertilization of two different egg cells by two different sperms. They are therefore siblings who share the maternal uterus during the same period of pregnancy, but they can be very different from each other, just like common siblings. Furthermore, in very rare cases, the dizygotic twins can have different fathers only in the case in which the mother has had close sexual intercourse with different men.
- Siamese twins
Siamese twins are monozygotic or homozygous twins, who have some organs in common at birth. The event depends on the late division of the embryo, but in general it is a very rare event. The types of conjoined twins change according to the parts in which the twins are joined and the organs they have in common: usually those that do not involve the heart and the navel are distinguished. Apart from the "anomalous" ones, in which one of the two embryos is malformed or internal to the other.
- Semi-identical twins
Bizygotic or semi-identical twins are born from an egg cell fertilized by two spermatozoa. For now in the world the first known case concerns two French twins born the first male, and the second hermaphrodite (had both sexes).
Homozygous twins frequently asked questions
How many homozygous twins can be born?
More than two twins can be born from one birth, if more than two fetuses carry a single pregnancy to term. In this case we speak of multiple births.
Twins to each other can be identical or fraternal twins.
What are the odds of having homozygous twins?
Twin pregnancy is characterized by having more than one fetuses. According to statistics, about 1.6 percent of pregnancies have a twin outcome. But, apart from the statistics, there are several factors that can influence the chances of having multiple or twin pregnancies: one of them is the genetic makeup of the parents.
During a twin birth, which of the twins is bigger?
Although it is often said that the twin born first is the oldest, we can dispel this myth by saying that there is no younger twin and one older by birth. In Italy, the birth order is shown in the birth assistance certificates in twin births, so in a twin birth there will be a first born, a second born, etc.
