Twinning occurs when a single pregnancy results in two offspring, a biological phenomenon that arises from two distinct paths of conception. The resulting pairs can range from looking almost indistinguishable to having appearances as different as any two children born years apart. This difference in physical likeness is a direct consequence of the earliest cellular events following fertilization.
Fraternal Twins and Their Separate Origins
The answer to the question of what twins who do not look alike are called is most commonly fraternal twins. Scientifically, these pairs are known as dizygotic twins, a term that literally means “two zygotes.” This type of twinning begins when the mother releases two separate eggs during the same ovulation cycle, a process called hyperovulation.
Each of these eggs is fertilized by a separate sperm cell, creating two completely independent embryos. Because they originate from two distinct fertilization events, dizygotic twins are genetically no more alike than non-twin siblings born at different times. They share, on average, about 50% of their genetic code, which is the same proportion of shared DNA between any two full siblings.
This separate origin is why fraternal twins can be different sexes, having one boy and one girl, and why their physical appearances can vary significantly. Their development proceeds independently, with each twin typically forming its own placenta and amniotic sac. While they share the same womb environment and age, their distinct genetic code dictates individual traits like hair color, facial structure, and height, resulting in their non-identical appearance.
The tendency to release multiple eggs is often an inherited trait in the mother, meaning dizygotic twinning can run in families. Certain factors, such as maternal age over 30 and the use of assisted reproductive techniques, also increase the likelihood of this type of twin pregnancy.
Identical Twins and the Single Split
In contrast to the two-zygote origin, twins who look highly similar are called identical twins, or monozygotic twins. This designation means “one zygote,” referring to their creation from a single fertilized egg. The process begins when one egg is fertilized by one sperm, forming a single zygote.
At a very early stage of development, this fertilized egg spontaneously splits into two separate embryonic masses. The timing of this split is what determines how many supportive structures, like the placenta and amniotic sac, the twins will share. This single-zygote origin is the reason that identical twins share nearly 100% of their nuclear DNA.
The mechanism that causes this single fertilized egg to divide into two is not fully understood, but it is considered a random event that is not influenced by heredity. Because they share virtually the same genetic material, identical twins are almost always the same sex. This shared genetic blueprint provides the context for the striking physical similarity that is characteristic of this type of twinning.
Genetic Implications for Appearance and Shared Traits
The fundamental difference in genetic sharing profoundly impacts the traits of the two types of twins. Dizygotic twins, sharing only half their genes, can possess different blood types and are just as likely as non-twin siblings to have different susceptibilities to inherited conditions. They can easily be of opposite genders, which is the clearest indicator of their non-identical nature.
Even with their near-perfect genetic match, monozygotic twins are not exact copies of one another. Slight physical differences can emerge due to environmental factors, such as variations in nutrition or blood flow within the womb. After birth, individual lifestyle and epigenetic modifications—changes in gene expression not involving the underlying DNA sequence—further contribute to subtle variations in appearance and health outcomes.
A clear example of this individuality is found in physical markers like fingerprints. While identical twins share the same DNA, the precise pattern of their fingerprints is determined by small, random differences in their environment during development, making each twin’s print unique. However, their shared DNA means they generally have the same blood type and display a much higher concordance for traits and diseases with a strong genetic component compared to fraternal twins.