Twins

Twins run in families, right? Not necessarily. There is more than one type of twins, and various factors influence each. Generally speaking, identical twins occur at the same rate across the population, regardless of external factors such as age or race. Fraternal twins occur at different rates, depending on various factors. Scientists have found indications that fraternal twins are hereditary, and the age of the mother and number of previous births may also be factors. Some cultural groups have a higher rate of twinning than others.

Identical twins occur when one egg is fertilized and splits into two separate zygotes. A zygote is the cell that is formed when an egg is fertilized. These two entities may remain in one amniotic sac to receive nourishment during development, or they may split into two separate sacs.

The other type of twins is fraternal, which occurs when two separate eggs are fertilized at the same time. This type usually develops in two separate sacs.

Because identical twins begin as a single cell, they receive the same genes; they are genetically identical. Therefore, they will always be the same gender and share many physical characteristics and personality traits. However, approximately 20 percent of our genes manifest themselves differently, which accounts for slight variations that may be observed in identical twins. Scientists have also observed instances in which the right side of one twin will match the left side of the other. These are called mirror twins.

Fraternal twins begin as two individual cells, and therefore, each has a unique set of DNA. The resulting offspring will look no more alike than any other siblings. As such, they may be the same gender or different.

Research on twins continues, due in part to our fascination with identical DNA, as well as the information to be learned regarding the unique DNA code of every human.