The observation that some pairs of twins are nearly indistinguishable while others appear no more similar than any two siblings reflects human reproductive biology. The process by which multiple births occur determines the degree of genetic sharing, which dictates their physical resemblance. The fundamental answer to why some twins do not look alike lies in the earliest moments of conception and development. Understanding the full spectrum of twin similarity requires exploring the two distinct biological pathways that lead to twin births.
The Primary Difference: Fraternal Twins
The most common reason two individuals born at the same time do not look alike is that they originated from two entirely separate fertilization events. This occurs when a mother releases two eggs during a single cycle, and each egg is fertilized by a different sperm cell. These twins develop simultaneously in the womb, often with their own separate placentas and amniotic sacs, but they are genetically unique.
Due to this separate origin, these twins inherit different combinations of genes from each parent. They share approximately 50% of their DNA, the same genetic relationship as any two siblings born years apart. This means they can differ widely in traits like hair color, eye color, height potential, and facial structure.
It is also possible for these twins to be of different sexes, because the two fertilizing sperm carry different sex chromosomes. When a twin pair consists of a boy and a girl, the fact that they are genetically distinct is immediately confirmed. The lack of strong physical resemblance in these twin pairs is a natural result of having the same amount of genetic overlap as non-twin siblings.
Shared DNA and the Myth of Exact Copies
The expectation of near-perfect likeness arises from the other type of twin, which results from a single fertilized egg splitting into two embryos early in development. Since they originate from one egg and one sperm, they share nearly 100% of their genetic material. This shared genome is why they are always the same sex and frequently exhibit striking physical similarities.
The division process usually occurs within the first few days after conception, leading to two individuals who begin life with the exact same genetic blueprint. This shared blueprint includes the instructions for all physical characteristics, from the shape of the nose to the color of the irises. The high degree of genetic overlap has made these twins invaluable subjects for studying the influence of environment versus genetics in human development.
Despite this genetic identity, it is a misconception that these individuals are absolute genetic or physical duplicates. Even with an identical DNA sequence, differences in appearance and health can and often do emerge due to the dynamic nature of development.
Why Identical Twins Still Develop Differences
One of the most significant factors causing divergence between these genetically identical individuals is a mechanism known as epigenetics. Epigenetics refers to changes in gene expression—how genes are turned “on” or “off”—without altering the underlying DNA sequence. While young twins may have nearly indistinguishable epigenetic patterns, these patterns begin to drift apart as they age.
Environmental factors such as diet, physical activity, chemical exposure, and even chronic stress can influence which genes are activated or silenced. Studies comparing young and older pairs show that the older twins exhibit substantially greater variations in their gene expression profiles. This epigenetic drift means that even with the same genetic code, the two individuals can develop different susceptibilities to disease and subtle physical differences over time.
Variations in the shared environment of the womb also contribute to measurable differences at birth. In approximately two-thirds of cases, these twins share a single placenta, which can lead to an unequal distribution of blood supply and nutrients. Even without severe complications, one twin may receive a slightly better share of resources, resulting in a difference in birth weight and size that can persist.
Subtle genetic differences can also accumulate very early in development through spontaneous mutations that occur during cell division. Researchers have found that these twins can have several differing early developmental mutations not present in the other twin. These minute genetic changes can affect a large proportion of the body’s cells, contributing to minor but noticeable differences in their characteristics.