Twins do not always look alike. There are two kinds of twins: identical and fraternal. The identical share 100% of their DNA and hence resemble each other in terms of looks.
In pregnancy, sometimes a mutation causes the mother to have more than one child. Twins happen in two ways – either one egg will split into two or two eggs will get simultaneously fertilized.
Twins can either be monozygotic(identical) or dizygotic (fraternal). Monozygotic twins are born from one egg, while dizygotic requires two eggs in the uterus.
Pregnancy happens when an egg travels from the woman’s ovary into the uterus. This egg than attaches itself to the walls of the uterus and start lining the egg with blood tissue and other nutrients to prepare itself for a baby. If the egg is fertilized by sperm, the egg will continue to grow into a baby. However, if the fertilization does not happen, the blood lining will break and leave the woman’s body in a cycle known as menstruation.
Sometimes a mutation may occur when the egg splits into two and grows into two individual beings. These beings share a 100% of their DNA and are known as identical twins. They share similar looks and a lot of genetic material.
Another process is when two eggs travel into the uterus by accident and both are fertilized by different sperms, these also grow into individual babies. These are known as fraternal twins and share the same amount of DNA as a person would with a normal sibling.
Since identical twins share 100% of the DNA and have the exact same chromosomes from both the mother and the father, they are identical in looks. However, it is not always necessary that the twins will look exactly alike. Also, they only share looks and differ in other traits. They also grow and mature differently so the twins can differ from each other in terms of height and weight.
Hence, it is not necessary for all identical twins to look exactly alike. However, many identical twins do share many similar traits.