Twins develop from a single zygote or two separate zygotes. Monozygotic or identical twins are genetically identical and develop from one zygote, while dizygotic or fraternal twins develop from two separate zygotes so they are not genetically identical. There are several types of twins based on chorionicity and amniotic sac development including dichorionic diamniotic, monochorionic diamniotic, monochorionic monoamniotic, and conjoined twins. Complications can arise during twin pregnancies and deliveries.

2.  Twins-two off springs produced by the same
pregnancy.
 The two babies born at the time for a mother.
 Twins are due to multiple pregnancy.
 The early cells of the mammalian embryo can
replace each other and compensate for a
missing cell.
 This regulative ability was first demonstrated in
1952, when Seidel destroyed one cell of a 2-
cell rabbit embryo and the remaining cell
produced an entire embryo.

3.  The regulative capacity of the ICM
blastomeres is also seen in humans.
 Human twins are classified into two
major groups:
Monozygotic twins
Dizygotic twins

4.  Monozygotic twins are otherwise called
as identical twins.
 They are extremely similar in their
characters.
 Develop from single zygote
 During cleavage, the zygote divides into
two blastomeres.
 These blastomeres separate and each
develops into a baby.

6.  The two babies are similar in all
respects.
 Same sex.
 Same type of genes.
 Same type of blood group.
 They are generally opposite handed.
 They show similar whirls of hair on the
head but in a reverse order like mirror
image.

7.  Due to the arrangement of placenta and
membranes it is divided into 4 types
 These types depends on division of
zygote.
Dichorionic diamniotic twins
Monochorionic diamniotic twins
Monochorionic monoamniotic twins
conjoined twins

9.  Where each twin has its own chorionic
and amniotic sacs.
 This type occurs most commonly with
dizygotic twins but may also occur with
monozygotic twin pregnancies.
 This type of pregnancy may have
characteristic findings on ultrasound.

10.  This type of twins are the product of a
single fertilized ovum, resulting in
genetically identical offspring.
 They share a single placenta but have
separate amniotic sacs.
 The occurrence of this type of twins
occurs at a rate of three to four in 1,000
live birth.

11.  They are the monozygotic multiples.
 They share a single amniotic and single
placenta.
 But they have separate umbilical cord.
 This case is very rare and may cause
risk to the babies due to cord
entanglement.

12.  Conjoined twins also known as siamese
twins.
 They are identical twins that have not split
properly after fertilization.
 They share body and may share vital
organs.
 Separation of conjoined twins is usually
performed at a very young stage.
 In some cases the brain or the heart must
be separated in order to provide each twin
with vital organs.

13.  Dizygotic twins are otherwise called as
fraternal twins.
 Fraternal twins are like ordinary brothers
and sisters.
 Develop from two independent zygote.
 So they are called as dizygotic twins.
 Formed by the fertilization of two eggs by
two sperms.
 They may be of same sex or opposite sex.
 Different genotypes.

15.  Dissimilar characters.
 They are called as non-identical twins.
 If both eggs are fertilized by the X-
chromosome female babies are produced.
 If both the eggs are fertilized by the Y-
chromosomes male babies are produced.
 If one egg is fertilized by X carrying sperm
and another by Y carrying sperm male and
female babies are produced.

16.  2 to 3 fold increased than singletons.
 Threatened and spontaneous abortion.
 Severe anemia
 Hypertensive disorders of pregnancy.
 Gestational diabetes.
 Operative delivery
 Increased maternal mortality.

17.  Congenital malformations
 Low birth weight
 Preterm birth
 Monochrionic pregnancy complications
 Perinatal asphyxia
 Fetal death
 Cord accidents
 Increased perinatal mortality.

18.  Monoamniotic twins – 1 in 20
monochorionic twins are monoamniotic
 Associated with high fetal death rate
from cord entanglement, congenital
anomalies, preterm birth or twin-twin
transfusion syndrome
 Diamniotic twins can become
monoamniotic if the dividing membrane
ruptures.

19.  External parasitic twins- grossly defective
fetus or merely fetal parts attached
externally to a relatively normal twins.
 Believed to result from demise of the
defective twin with its surviving tissues
attached to and vascularised by its normal
twin.
 Fetus in early development one embryo
may be enfolded within its twin
 Classically vertebral or axial bones are
found in these fetiform mases supported by
their host by a few large parasitic vessels.

20.  Two amniotic sacs and a common
surrounding chorion.
 Anatomical sharing of the two fetal
circulations through anastomoses of
placental arteries and veins.
 Artery to artery anastomoses are most
common and are identified on the
chorionic surface of the placenta-75%
 Vein to vein and artery to vein -50%

21.  Deep artery to vein connections can extend
from capillary bed of a given villus creating
a common villous compartment or third
circulation.
 Depending on the degree to which they are
hemodynamically balanced severity
occurs.
 With significant pressure or flow gradients
a shunt will develop between fetuses.
 Chorionic feto fetal transfusion results in
several clinical syndromes.

22.  5-17% of monochorionic twin
 Blood is transfused from donor twin to
its recipient sibling- donor is anemic and
growth may be restricted.
 Recipient become polycythemic, with
circulatory overload and may manifest
as hydrops.
 Deoxygenated blood from donor
placental artery pumped into a cotyledon
shared by recipient.

25.  Once oxygen exchange is completed in
the chorionic villus, oxygenated blood
leaves the cotyledon via a placental vein
of the recipient twin.
 Clinically important TTTs is frequently
chorionic, results from significant volume
differences.

27.  It is a condition in which presence of two
or more chromosomal complements
found in the same tissue of an
individual.
 The divergent genotypes are usually
found in all across the genome.
 It occurs in single organism composed
of cells with distinct genotype.
 Chimeras can happen with organ
transplantation.

29.  Natural chimerism
 Organism with mixed sexual
characteristics.
 May occur when two fertilized eggs fuse
together.
 SRY genes gets transferred to the X
chromosome during meiosis.
 Organism developed are known as
hermaphrodite.

30.  Occurs due to fertilization of an oocyte
and its second polar body by two
sperms.
 Also occur when a zygote fuses with a
fertilized polar body.
 The organism developed from the
zygote consist both type of genetic
materials.

32.  It is the presence of
two genetically distinct
and separately derived
populations of cells.
 One population being
at a low concentration
than other.
 The cells genetically
distinct from those of
the host individual.
 Usually occurs to
foetus from mother.

33.  Occurs due to
fertilization of two
separate ova by two
sperm, followed by
aggregation of the
two at the blastocyst
or zygote stages.
 Organism with
intermingled cell
lines.
 Organism developed
possess organs that
have different sets of
chromosomes.

35.  Usually found in Angler fish.
 The male releases an enzyme that digests
the skin of his mouth and the body of the
male
 Formation of single hermaphroditic
individual takes place
 The male develops large testicles and
atrophy of other organs take place.
 Becomes able to have a great number of
offsprings.

36.  Occurs when the germ cells of an
organism are not genetically identical to
its own.
 Mainly found in marmosets
 They carry the reproductive cells of their
fraternal twin siblings
 Occurs because of placental fusion
during development.

39.  It can be done by grafting genetically
different parents, different cultivates or
different species.
 Tissues get partially fused together

40.  These are chimeras in which the layers
differ in their chromosome constitution.
 Occaionally occurs due to loss or gain of
individual chromosome or chromosomal
fragments owing to miss divsion.
 It has various effects on cell size and
growth characteristics.

41.  These chimerism arise by spontaneous or
induced mutation of a nuclear gene to a
dominant and recessive allele.
 One character is affected at a time in leaf,
flower, fruit and other parts.

42.  These arise by spontaneous or induced
mutation of a plastid gene.
 Plastids may sort-out from a mixed egg or
mixed zygote respectively.
 It affects the colour of the plasmids with in
the leaves.

43.  It is the most lethal virus
till date.
 Developed by soviet
union in 1980 as “Project
Chimera”.
 Virus was developed by
combining DNA of
Venezuelan Equine
Encephalitis Virus,
Smallpox Virus and Ebola
Virus.
 Developed as a bio-
weapon.
 The project was
abandoned due to its
significant risks.