Twins can be either monozygotic (identical) or dizygotic (fraternal). Identical twins develop from one zygote that splits into two embryos, while fraternal twins develop from two separate eggs fertilized by two separate sperm. There are three types of identical twinning depending on when the zygote splits. Dizygotic twins always have two placentas but the placentas can fuse. Factors like maternal age, height and weight can influence the likelihood of dizygotic twins. The causes of monozygotic twinning are still unclear but may involve lack of oxygen or disturbances in developmental clocks. There are different kinds of rare twins including mirror image twins, half-identical twins

1. Twins
Dr.Ga.BAKAVATHIAPPAN, M.Sc., M.Sc (Yoga)., M.Phil., B.Ed., PhD.,
Associate Professor, HOD of Zoology,
Saiva Bhanu Kshatriya College
Aruppukottai -626 101.
Virudhunagar District.
Tamilnadu

2. Twins
• Twins are
two offspring produced by the
same pregnancy.
• Twins can be
either monozygotic ('identical'),
meaning that they develop
from one zygote, which splits
and forms two embryos,
• or dizygotic ('non-identical' or
'fraternal'), meaning that each
twin develops from a separate
egg and each egg is fertilized by
its own sperm cell

8. There are three types of identical twins.
About one-third of identical twins split soon after fertilisation and form
completely separate twins. Like fraternal twins, these twins have separate placentas.
The other two-thirds split after they attach to the wall of the womb.
As a result, they share a placenta. The technical name for this is monochorionic.
In a very small number of identical twins, splitting might happen even later.
In this case, both twins share an inner sac, called the amnion,
in addition to sharing a placenta.
The technical name for this is monoamniotic twins.
They’re often called MoMo twins.

13. • All most all dizygotic twins have two placentas,
two chorions and two amnions, i.e., be
diamniotic and di-chorionic.
• However these two may fuse and look like one.
The highest dizygotic twining rate is seen in the
black populations (Africans) and lowest in Asian
populations.
• But dizygotic twining rate is closely related with
maternal age, parity, height, weight and also on
gonadotropin levels.
• Tall and heavy women are more likely to give
birth to dizygotic twins than are short and thin
women

14. • The major cause for the monozygotic twining in
human is still unknown, however several mechanisms
have been proposed, which are:
• (1) Lack of O2 prior to implantation which caused
developmental arrest and splitting in the zygote
• (2) Delayed implantation
• (3) Disturbances in the developmental clocks
• (4) Delayed fertilization
• (5) Rupture of zona pellucida
• (6) Congenital anomaly or an abnormality in
development
• (7) Discordance in the expression of genetic
information like X-inactivation’s, imprinting,
uniparental disomy, changes in the chromosome
number and also mitochondrial mutations.

15. Different Kinds Of Twins
1) Mirror Image twins
These twins will often have “mirror images” of their features, such as hair whorls that run
clockwise in one and counter clockwise in the other.
2) Half-Identical twins
These are the type of twins who share half their genes in common (from the mother) and
the other half different (from the two sperms).
3) Mixed Chromosome twins
This occurs if two separate sperms fertilize two separate eggs which then fuse, producing
individuals with different sets of chromosomes.
4) Superfetation
Superfetation is the occurrence of another pregnancy during an ongoing pregnancy.
5) Superfecundation
Twins can have different fathers.
6) Parasitic Twins
Parasitic twins refer to a type of conjoined twins that develop asymmetrically, with a
smaller, less formed twin dependent on the stronger, larger twin.
7) Twins With Different Birthdays
The most common explanation for twins with different birthdays is a labor and delivery
that begins before midnight on one day and ends after the clock changes to the next day.
8) Conjoined twins
Conjoined twins are monozygotic twins whose bodies are joined together during
pregnancy and fails to separate completely.
Most conjoined twins are now evaluated for surgery to attempt to separate them into
separate functional bodies.