Identical twins have been a regular formula used in many blockbuster films of yester years. Although today, the cloning concept is more in vogue, twins still attract lot of attraction, especially when they look almost identical. But it’s not necessary that all the twins should resemble each other. So how is it that some twins are so identical whereas some twins are very different that its hard to think they are siblings? The difference is because there are two types of twins, monozygotic and dizygotic.
Monozygotic twins are also termed as identical twins and they start life as one egg, and are fertilized by one sperm and latter splits into two eggs much earlier in the gestational period. These are very rare and have no link to heredity. But studies reveal that only one of four conceptions of twins is identical. The name monozygotic has evolved from the blend of ‘mono’ which means one and zygote which means fertilized egg.
Dizygotic twins which are also called as fraternal twins start their life as two eggs and are fertilized by two different sperm. These are formed when an additional egg is released by a woman while one egg is already present in the womb. The name dizygotic has evolved from the combination of two words: di which means two and zygotic which means fertilized egg.
The difference between monozygotic and dizygotic twins is because of the split during fertilization. In the monozygotic twins, embryo split occurs much earlier during the pregnancy phase, mostly during the initial 12 days. When an embryo splits before 4 days, the identical twins will look similar to the dizygotic twins as there could be little physical resemblance. Zgosity test is performed by some to confirm the identical status of the babies. When the split happens in between 4 to 8 days, the babies share the same sac which also helps in determining the zygosity. When the split occurs during 8 to 12 days, then the twins resembling in appearance are developed. The conjoined twins are created when the embryo splits after the 12th day. Monozygotic twins are mostly of same gender.
In dizygotic twins, the eggs mostly fertilize at 2 different times during the ovulation, but the time difference is very close. They are the most common twin types, as around two third twin births fall into this category. With the increase in utilization of fertility drugs, fraternal pregnancy has become more common. They may be of same sex or can belong to different gender. They may look very similar or would look totally different. And many times, the fraternal twins have been found to be fathered by sperm of two separate men.
Since monozygotic (MZ) twins are produced from a single fertilized egg, they start their life with the same gene set. In this context, they resemble clones, which have identical set of genes. But during development, they start accumulating genetic and other related differences. Contrary to this, dizygotic twins are genetically are just as close as the two siblings. Although it is a common misconception that sibling genes are 50% similar, it is not true because the fertilization and meiosis has a very random nature due to which the siblings can have few or even many genes of a particular parent as common. Secondly, many human genes have only one common form which is termed as allele. So, two different people can share several alleles, irrespective of their relationship. Owing to this the genes which possess more allele than one gives rise to the basis of variation in human genetics.