In one pregnancy if two offspring’s are produced they are called as twins. Twins are of two types; Identical twin and Fraternal twin. Identical twins are the ones who are similar in both genotype and phenotype. They resemble each other. The identical twins are born from the same zygote that has split and formed two embryos. However fraternal twins are born when two ovum are fertilized by two separate sperm. They do not resemble each other. In case of fraternal twins they can be either Male -male twins, Male – Female twins or Female – Female twins. However in case of identical twins as they develop from same zygote it can be either Male – Male twins or Female – Female twins.
Twins are born when the ovum is fertilized by the sperm and the zygote thus formed splits into two to form two embryos or when two different ova are fertilized by two different sperms. They are called as identical and fraternal twins respectively. The identical twins as they develop from the same zygote are similar in their genotype and have perfect resemblances to each other.They are also of the same sex. The fraternal twins do not resemble each other both genotypically and phenotypically like any other siblings. They are siblings of the same age. They can be either all male twins, all female twins or male -female twins.
Identical twins are 8% of all the twins born. Monozygotic or identical twins are born when the single egg used to fertilize the egg to form zygote splits into two embryos. There are three ways in which the baby can be carried in the womb in this case. The baby may have one placenta and one amniotic sac, One placenta and two amniotic sacs or two placentas and two amniotic sacs. The identical twins have the same chromosomes, sex and they have perfect resemblance to each other.
Two eggs are fertilized by two different sperms which are implanted in the uterine walls at the same time and hence are also called as dizygotic or biovular twins. They are different from each other in their chromosomal make up like any siblings and can resemble or look different from each other as any other siblings. They just happen to be of the same age. In this case the baby can be carried in just one way. They have separate placenta and amniotic fluid. They can be either Male- Male twins, Male – Female twins or Female- Female twins. Fraternal twins generally occur in women of older age with chances doubling in case of women above 35 years of age.
Difference between Twins and Identical Twins
1. Fraternal twins are born as a result of fertilization of two ova by two different sperms whereas in case of identical twins one ovum is fertilized by one sperm to form a zygote which splits into two to form two embryos.
2. Identical twins are always of the same sex whereas fraternal twins can be of the same sex or a mix of male and female twins.
3. Identical twins are similar in their chromosomal make up whereas the fraternal twins differ in their chromosomal make up.
4. Identical twins have perfect resemblance to each other; however fraternal twins do not resemble each other strongly. They are like any other siblings who happen to be of the same age.
The twins whether identical or fraternal are two separate individuals and their environment and experience can shape them to two different people though they may look alike.