Heterozygous and homozygous twins

Heterozygous and homozygous twins: what’s the difference? Heterozygous twins look alike like two brothers, while homozygotes are identical

Heterozygous and homozygous twins: what’s the difference?

Heterozygous and homozygous twins

A twin pregnancy is always a particular event: heterozygous twins do not look alike more than two normal siblings, because they are formed when two eggs mature at the same time and are fertilized by two spermatozoa, one independently of the other, on the contrary, monozygotic twins are identical. 

It is estimated that the birth of twins has an incidence of 1/70 compared to single pregnancies, but in recent years we have witnessed an increase in the frequency of twin pregnancies, caused by the increase in the use of drugs to stimulate fertility and assisted fertilization techniques. 

The incidence of mono-ovular pregnancies is about 1/300 pregnancies, while that of bi-ovular twins can vary between <1/100 and 1/500 pregnancies.

Heterozygous twins

Today it is fairly certain that heterozygous twins can form even from a single egg, which splits shortly after conception . Each part is then fertilized by a different sperm. The inheritance of the mother is in this case identical for the two children, while that of the father is different.

Monozygotic twins

Monozygotic twins are practically alike and can be mistaken for one another. In fact, they have the same genetic heritage and the same sex. Monozygotic twins are born from a single egg fertilized by a single sperm, but then this egg divides into two halves and a child grows in each being. 100% of their chromosomes are identical and therefore they will have the same DNA and the same facial features, from the color of the eyes to that of the hair.

If the egg divides shortly after fertilization, each twin has its own placenta and amniotic cavity. If, on the other hand, the division occurs four to seven days after fertilization, the twins share the placenta. Sometimes the so-called ” fetal-fetal transfusion syndrome ” occurs : one twin steals nutrients from the other. Therefore it is important to have the twins’ growth checked regularly with an ultrasound. Consider that twins with only one placenta and one amniotic cavity are rare.

The genetic heritage is not always identical

Adult monozygotic twin pairs have been found to have a different genetic makeup . This astonishing discovery was initially made by a team of researchers who examined 19 pairs of twins. Each of these pairs showed the so-called “Copy Number Variations “: sometimes a twin was missing a part of the chromosome, other times even more than one was available.

Scientists believe these variations have matured over the course of life as a result of cell divisions. They hope with the help of this discovery to be able to identify the genes responsible for certain diseases: if only one twin gets sick, the causes could possibly be located in the differences between his genetic makeup and that of the twin.

What are the odds of having twins?

In some families, twins are quite frequent, so it is believed that there is a genetic predisposition and that the female line plays a greater role.

In recent years, the number of heterozygous twins has increased: this is due to hormonal treatments to treat infertility, as a result of which more eggs often mature. In the case of artificial insemination in women, several fertilized eggs are often implanted.

Another reason is the increase in the age of mothers . In fact, with age a woman also increases the follicle stimulating hormone (FSH) and often more eggs are available to fertilize.

Homozygous twins: what does it mean in a twin pregnancy

Homozygous twins are a very rare phenomenon. Let’s find out together what is the difference between homozygous or heterozygous twins.

The identical twins are a very rare phenomenon. The heterozygous ones look alike, while the homozygous ones are just identical. In the second case, the two eggs mature simultaneously and are fertilized by two spermatozoa, one independently of the other, while the homozygous twins are practically the same and can be mistaken for each other.

Homozygous twins: what does it mean

 Many twins are not identical, because, unlike monozygotes, which develop from the same fertilized egg, they are born from the fertilization of distinct eggs. So here is the difference between homozygous and heterozygous twins. 

Twins are born from a single birth, but only the first ones share 100% of their DNA and are always of the same sex. It all begins with the fertilization of a single egg, which then, within ten days, divides into two embryos. 

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.

Homozygous twins: how they are formed

Homozygous twins form about 10 days after ovulation . They are divided into monochorionic monovular twins, which means they can share the same placenta, or monochorionic monovular twins, which means they can develop two distinct placentas. 

The twins are of the same sex and are practically identical in appearance, since they have the same genetic heritage, the same eyes, the same hair, the same blood group, and therefore the same somatic characteristics. It is therefore difficult to recognize them, even if, over time, they add different characteristics. 

Sometimes the “fetal-fetal transfusion syndrome” occurs: one twin steals nutrients from the other. This is why it is important to have twins’ growth checked regularly with an ultrasound.

What is the older twin

There is no bigger or older in a pair of twins. From a personal point of view, in the birth certificates in twin births, the order of birth is shown, so there are those who were born as first and who as second. 

If we refer to the size of the newborn, in the scientific literature it has been repeatedly reported that, on average, the first born is also the one with the highest birth weight . 

Finally, if you think about which of the twins was conceived first, there is no way to answer. Monozygotic twins are conceived at the same time and therefore there is no twin conceived before the other. Nor is it true that those born earlier were conceived earlier.

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