Quadruplets are defined as four offspring resulting from a single pregnancy. The possibility of all four being genetically identical is rooted in the earliest stages of human development. This possibility depends specifically on how a fertilized egg, or zygote, divides.
Understanding Zygosity in Multiples
The genetic relationship between multiples is determined by their zygosity, which refers to the number of fertilized eggs involved in the pregnancy. Multiples fall into two main categories: monozygotic and polyzygotic. Monozygotic, or identical, multiples develop from a single zygote that splits early in development, meaning they share the same genetic code.
In contrast, polyzygotic multiples, often called fraternal or non-identical, result from multiple separate eggs fertilized by multiple separate sperm. For example, fraternal twins are dizygotic, coming from two different fertilized eggs. The degree of genetic similarity between polyzygotic siblings is comparable to any other siblings, about 50 percent.
The potential for identicality relies on the initial zygote’s ability to divide successfully. For identical twins, the single fertilized egg splits once, resulting in two embryos with identical DNA. This splitting event dictates whether the multiples are identical.
The Mechanism of Identical Quadruplets
The direct answer to the question is yes, quadruplets can be genetically identical, but this is an extremely unusual event. For four babies to be monozygotic, a single fertilized egg must undergo a series of three distinct and successful splitting events early in its development. This process must occur before the cells lose their ability to form an entire, independent organism.
The extreme rarity of this phenomenon is reflected in its statistical probability, which is often cited as approximately one in 64 million pregnancies. The timing of the splitting events determines the number of placentas and amniotic sacs, but the requirement remains the same: one zygote must divide into four separate, viable embryos. Because they originate from a single fertilized egg, identical quadruplets are always the same sex and share virtually 100 percent of their DNA.
The mechanism behind why a single zygote spontaneously splits is not fully understood. The requirement for three sequential splitting events makes the existence of four naturally conceived, all-identical quadruplets an exceptional occurrence.
Other Quadruplet Combinations
While a set of four completely identical quadruplets is the rarest scenario, most quadruplet births involve a combination of identical and non-identical siblings. The most common form of quadruplets, particularly with the use of fertility treatments, is the quad-zygotic or all-fraternal set. This occurs when four separate eggs are released and fertilized by four separate sperm, resulting in four genetically distinct individuals.
Many quadruplet pregnancies involve mixed combinations of zygosity. For instance, a set might consist of two identical twins and two fraternal siblings, which would result from three fertilized eggs, with one of them splitting. Other possibilities include two sets of identical twins, which would be the result of two fertilized eggs that each split once.
Mixed-zygosity sets are more common due to assisted reproductive technologies (ART), which increase the likelihood of multiple eggs being fertilized.