Dizygotic and monozygotic twins

Human Genetics
Human chromosomes
Determination of sex
Sex linked inheritance
Chromosomal aberrations
Dizygotic and monozygotic twins
Inborn errors in metabolism
Sickle-cell anaemia
Genetic analysis through pedigree charts
Chromosome mapping in humans (including RFLPs, etc.)
Gene transfer in mammalian cells
Chromosome mediated gene transfer
Transformation of cells with free DNA
Use of human genetics in medical science 
Genetic counseling
Amniocentesis and antenatal diagnosis
Gene therapy
Making a choice of baby's sex
DNA fingerprinting in forensic science
Human females usually give single birth but rarely two, and still more rarely multiple births are reported. Cases of upto quintuples (5 babies) are also known. Whenever there are multiple births, these may usually be due to ovulation of two or more than two eggs, each being independently fertilized by separate sperms. In other words, in case of twins, two zygotes are formed. These twins are called dizygotic twins and since two different eggs and two different sperms were involved, the twins will not be identical. Five babies (2 brothers + 3 sisters) born to a mother in Argentina were all different phenotypically and should have resulted from five eggs. On the other hand, identical twins where the two brothers or two sisters completely resemble each other presumably result from a single zygote, which at the first division gives rise to two identical cells independently developing into two embryos.