Why Do Twins Look Identical

Many people have twin friends, and upon first meeting them, one often marvels at how similar they look. Why do twins resemble each other so closely? What leads to the formation of twins?

Sperm carries the father’s chromosomes and genes, while the egg carries the mother’s chromosomes and genes. The egg’s sex chromosome is always X, whereas the sperm’s sex chromosome can be either X or Y. When the egg and sperm unite during fertilization, the embryo’s sex is determined: XY develops into a male, and XX develops into a female. However, twins do not always look exactly alike, because twins can be divided into two types: identical twins and fraternal twins.

Identical twins occur when a fertilized egg splits during the early stages of development into two separate embryos. Since both embryos originate from the same egg and sperm, their chromosomes and genetic makeup are almost identical. As a result, they are highly similar in appearance and in other traits determined by genes. Sex is fixed at the moment of fertilization, so identical twins almost always share the same sex. In extremely rare cases, however, if a chromosomal abnormality or deletion occurs during the splitting process, the twins may differ in sex, with the one missing a chromosome often experiencing developmental abnormalities or congenital defects.

Fraternal twins develop from two separate fertilized eggs. Normally, a woman releases only one egg per cycle, but if two eggs are released and each is fertilized by a different sperm, two independent embryos form. Because these embryos come from different egg-and-sperm combinations, their genetic makeup differs, so fraternal twins resemble each other far less than identical twins. Their sex may be the same or different. This situation is similar to ordinary siblings, except that they grow in the womb at the same time. The commonly seen boy-girl twins are, in the vast majority of cases, fraternal twins.

In identical twins, if the fertilized egg fails to completely separate during division, conjoined twins may result. The degree and location of connection depend on the stage at which the division occurs, with the chest or abdomen being the most common sites, often accompanied by shared organs. Because the extent of connection and the organs involved vary greatly, many conjoined twins face life-threatening challenges at birth. If vital organs are shared, survival rates are even lower, since such organs cannot sustain two bodies simultaneously. Doctors provide recommendations based on each case. In some situations, surgical separation is possible, but if the twins share indivisible organs such as the heart or brain, separation is nearly impossible. Even when feasible, surgery carries significant risks. There are also cases where families, after careful consideration, choose not to pursue separation, and the twins grow up healthy but must remain physically connected for life. The occurrence of conjoined twins is extremely rare.

The formation of multiple births is similar to that of twins. It can result from a single fertilized egg splitting into three or more embryos, or from a woman releasing three or more eggs in one cycle, each successfully fertilized. In some cases, part of the set may come from a split fertilized egg, while the rest are independently fertilized eggs. In multiple births, the greater the number of fetuses, the higher the likelihood of premature birth. Multiple pregnancies inherently carry certain health risks, but with modern advances in medical technology, survival rates have significantly improved. The occurrence of multiple births is already rare, and the probability decreases further as the number of fetuses increases.