Two children who look so alike they are confused for one another, the same dimples, sometimes the same gestures: the identical twins seem to tell a story written twice. At the origin of this closeness, there is a single ovum fertilized by a spermatozoon. The fertilized egg cell, called a zygote, then divides into two embryos: this is the embryonic division that gives rise to monozygotic twins.
Their initial genetic heritage is almost identical, but this does not mean that every cell or every detail of their life will be. Small mutations can occur during development, while the environment, experience, and epigenetic differences influence how certain genes are expressed. These nuances help explain why two children from the same embryo can share a strong physical resemblance while developing fingerprints, tastes, and temperaments unique to them. Genetics explains a common starting point; it never reduces two people to identical copies.
Monozygotic twins: essential reference points on DNA
- A common origin: they come from a single ovum fertilized by a spermatozoon, then split into two embryos.
- An almost identical genetic heritage: their inherited genetic variants are generally the same, although small differences can appear.
- The same sex in the vast majority of cases: rare situations linked to sex chromosomes may make exceptions.
- Distinct individuals: fingerprints, experiences, and gene expression contribute to their differences.
- A pregnancy to monitor: follow-up depends notably on whether or not the placenta and membranes are shared.
How are monozygotic twins formed?
During fertilization, a spermatozoon meets an ovum and forms a zygote. In monozygotic twins, this early embryo splits into two cell groups that continue their development separately.
The timing of this split influences how the two babies develop in the uterus: they may each have their own placenta or share one placenta depending on the stage at which the division occurs. That is why ultrasound and medical follow-up also serve to specify the characteristics of the pregnancy beyond the expected resemblance.
Do identical twins really have the same DNA?
Initially, identical twins inherit the same genetic variants coming from the ovum and spermatozoon. It is often said they have “the same DNA”: this phrase describes their very close genetic proximity, but does not mean that each DNA molecule in every cell is perfectly identical throughout their lives.
Mutations can occur after the split of the embryos or during development. Small differences can also concern mitochondrial DNA, present in the mitochondria of cells and mainly transmitted by the mother. Thus, standard forensic genetics generally does not distinguish monozygotic twins, although more advanced methods can sometimes detect rare variations.
There is therefore no single percentage that perfectly describes all cells and all stages of life. Simplified figures like “100%” refer to their common genetic origin, while a value such as “89%” is not a reliable general measure for monozygotic twins.
Monozygotic and dizygotic twins: what genetic difference?
Dizygotic twins, often called fraternal twins, come from two ova fertilized by two different spermatozoa during the same cycle. Their average genetic proximity is comparable to that of brothers and sisters born at different times; they can be of different sexes.
| Type of twins | Origin | Shared genetic heritage |
|---|---|---|
| Monozygotic | One fertilized ovum, then embryonic division | Very close, with possible acquired differences |
| Dizygotic | Two ova fertilized by two spermatozoa | On average, about half the inherited genetic variants, like siblings |
Physical resemblance and epigenetic differences in identical twins
Two identical twins can have similar facial features, eye color, or voice. Yet, they are not perfect copies: fingerprints, for example, develop under the influence of particular conditions in the uterus and are different, although their patterns may resemble each other.
Epigenetic differences refer to mechanisms that modulate gene activity without changing the DNA sequence. Diet, sleep, stress, and life experiences can contribute to these variations in expression. Within a family, two twin sisters can thus have the same biological starting point while revealing preferences and ways of being that are uniquely theirs.
Imagine Léa and Nina, two twins often confused by those around them. One spontaneously chooses construction games, the other invents stories with figurines: their closeness does not erase their personal drives nor their need to be recognized each by their own name. Resemblance brings closer, but it does not replace identity.
Sex, blood group, and peculiarities of true twins
Monozygotic twins are generally of the same sex, because they come from the same zygote and in principle have the same sex chromosomes. Very rare genetic or chromosomal variations can, however, lead to atypical situations; these require individualized medical explanation rather than a general rule.
Blood group is usually identical in monozygotic twins, but having the same blood group is not sufficient to prove they are identical: dizygotic twins can also share the same blood group. The phenomenon called “mirror twins” sometimes describes particularities arranged inversely, such as a different dominant hand or a birthmark on the other side. This is not a distinct category of twins, but a way to describe certain asymmetries.
Are fingerprints the same?
No. Even when genes are almost identical, fingerprints form under the effect of multiple factors during pregnancy. The ridges can show similarities, but each fingerprint is unique to the person: identical twins therefore do not share perfectly identical fingerprints.
Heredity and conception: what genetics allows us to say
The birth of monozygotic twins is generally considered a random event, and not a trait directly transmitted in a family. Dizygotic twins, however, can be more frequent in some families, notably linked to a maternal tendency to release multiple ova during a cycle.
In rare cases, two ova can be fertilized by spermatozoa from two different men during closely spaced intercourse around ovulation. This phenomenon, called heteropaternal superfecundation, concerns dizygotic twins: it does not apply to monozygotic twins originating from a single fertilized ovum.
A family tree can therefore provide clues about dizygotic pregnancies, but it does not allow predicting with certainty the arrival of identical twins. For any question related to twin pregnancy, the medical team specifies the type of pregnancy and adapts the follow-up to the situation.
Frequently Asked Questions about monozygotic twins and their DNA
Do monozygotic twins have exactly the same DNA?
They start from an almost identical genetic heritage because they come from the same zygote. Small differences may appear with mutations, development, and time; gene expression may also vary.
Do identical twin sisters always have the same sex?
They generally have the same sex, since their sex chromosomes are normally identical. Very rare chromosomal variations can, however, lead to a different situation and must be evaluated by health professionals.
Do monozygotic twins have the same fingerprints?
No. Their fingerprints can show similar patterns but are not identical. Their formation also depends on particular conditions encountered during development in the uterus.
Are monozygotic twins hereditary?
Their occurrence is generally considered random and is not directly attributed to a familial gene. Familial tendency may more often concern dizygotic twins, linked to the release of multiple ova.
