Fraternal twins
Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. When two eggs are independently fertilized by two different sperm cells, fraternal twins result.
Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile. Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different from each other. They may be different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal twins are simply brothers and/or sisters who happen to have the same age.
Studies show that there is a genetic basis for fraternal twinning. However, it is only the female partner that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. Fraternals are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.
Identical twins
Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. Their traits and physical appearances are not exactly the same; although they have identical DNA, environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes. Division of the zygote into two embryos is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about three in every 1000 deliveries worldwide,[8] regardless of ethnic background. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first two days after fertilization) they may develop separate placentas (chorion) and separate sacs (amnion). These are called dichorionic, diamniotic (or "di/di") twins, which occurs 20-30 percent of the time. Most of the time in identical twins the zygote will split after two days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ("mono/di").
In about one percent of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac called; monochorionic, monoamniotic ("mono/mono") twins. Finally, the zygote may split extremely late, resulting in conjoined twins. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Mono/mono twins have an overall in-utero mortality of about 60 percent, principally due to cord entanglement prior to 32 weeks gestation. Many times, monoamniotic twins are delivered at 32 weeks electively for the safety of the babies. In higher order multiples, there can sometimes be a combination of fraternal/identical twins.
Semi-identical
Monozygotic twins can develop differently, due to different genes being activated.[19]. More unusual are "Semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.
This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in western medicine
source: en.wikipedia.com
Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. When two eggs are independently fertilized by two different sperm cells, fraternal twins result.Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile. Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different from each other. They may be different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal twins are simply brothers and/or sisters who happen to have the same age.
Studies show that there is a genetic basis for fraternal twinning. However, it is only the female partner that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. Fraternals are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.
Identical twins
Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. Their traits and physical appearances are not exactly the same; although they have identical DNA, environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes. Division of the zygote into two embryos is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about three in every 1000 deliveries worldwide,[8] regardless of ethnic background. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first two days after fertilization) they may develop separate placentas (chorion) and separate sacs (amnion). These are called dichorionic, diamniotic (or "di/di") twins, which occurs 20-30 percent of the time. Most of the time in identical twins the zygote will split after two days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ("mono/di").In about one percent of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac called; monochorionic, monoamniotic ("mono/mono") twins. Finally, the zygote may split extremely late, resulting in conjoined twins. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Mono/mono twins have an overall in-utero mortality of about 60 percent, principally due to cord entanglement prior to 32 weeks gestation. Many times, monoamniotic twins are delivered at 32 weeks electively for the safety of the babies. In higher order multiples, there can sometimes be a combination of fraternal/identical twins.
Semi-identical
Monozygotic twins can develop differently, due to different genes being activated.[19]. More unusual are "Semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.
This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in western medicine
source: en.wikipedia.com
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