Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in 2, creating identical twins who share the same genes. Identical twins are always the same sex, so if your twins are identical, you'll have 2 girls or 2 boys.
In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.
It is estimated that 99.9% of identical twins have the same gender. But, semi-identical twins are so rare that they make up only about 0.1% of the twin populations.
They come from the same fertilized egg and share the same genetic blueprint. To a standard DNA test, they are indistinguishable. But any forensics expert will tell you that there is at least one surefire way to tell them apart: identical twins do not have matching fingerprints.
Identical (i.e., monozygotic, or MZ) twins share 100 percent of their genes, whereas fraternal (i.e., dizygotic, or DZ) twins generally share only 50 percent of their genes.
Semi-identical twins are rare, and doctors say they've identified the second case ever | CNN. You've probably heard of identical and fraternal twins, but a report released this week says there's a third kind -- sesquizygous twins or "semi-identical." Researchers say they share anywhere from 50 to 100% of their genomes.
And because the death rate in the womb is higher for twins than for singleton births, female twins are more common than male twins.
Fraternal twins happen because some people release two eggs (or more) at a time. That's called superovulation. If two sperm fertilize two eggs, voilà: fraternal twins. Fraternal twins are also called dizygotic twins (as in, coming from two zygotes).
The majority of triplets are what is known as trizygotic. This is because they are formed when three separate, individual eggs are fertilised by three separate sperm. These babies share the same characteristics as any other siblings with the same biological parents. They can be the same or opposite genders.
Although it may seem to be almost impossible, conceiving with identical triplets, while quite rare, is actually possible. There is no one consistent statistic quoted when it comes to the odds of having identical triplets.
The Dutch Twins are Kit and Kat, 5 years old and not yet big enough to be called by their real names, Christopher and Katrina. They live in a typical Dutch household, around the turn of the last century. The book follows their day-to-day adventures and accidental mishaps.
The longest interval between the birth of twins is 90 days, in the case of Molly and Benjamin West, dizygotic (fraternal) twins born in Baltimore, Maryland, USA to parents Lesa and David West (all USA) on 1 January and 30 March 1996.
Funny Riddle Two children are born on the same day from the same mother but they are not twins. How is that possible? They are triplets!
Identical twins are more likely than non-identical twins (or other siblings) to both be right-handed or left-handed, but many twins have opposite hand preferences.
Around one in three sets of twins is identical. This occurs because the fertilised egg divides in two while it is still a tiny collection of cells. The self-contained halves then develop into two babies, with exactly the same genetic information.
Mirror twins, or mirror image twins, is a term used to describe a characteristic of some twins whose features appear asymmetrically—that is, on opposite sides. When these twins are facing each other, it is as if they are looking in a mirror.
Roughly two-thirds of identical twins from between 4 and 8 days after conception. Identical twins that likely split from days 9-13 after conception: These twins share one amniotic sac, one chorion and one placenta. Only 1-2% of all twins split this late in development.
If one egg is fertilised by two sperm, it results in three sets of chromosomes, rather than the standard two - one from the mother and two from the father. And, according to researchers, three sets of chromosomes are "typically incompatible with life and embryos do not usually survive".
The DNA of monozygotic twins tends not to be 100% identical, and epigenetic and environmental differences further widen the gap between twin pairs. It's not nature or nurture; it's a complex interaction between our genes, our environment, and our epigenetic markers that shape who we are and what illnesses befall us.
The Dionne quintuplets (born 28 May 1934, near Corbeil, Ontario, Canada) were the first quintuplets known to survive infancy. The five girls (Yvonne, Annette, Cecile, Emilie and Marie) were also the only set of identical quintuplets known to live into adulthood.
Occasionally, two sperm are known to fertilize a single egg; this 'double fertilization' is thought to happen in about 1% of human conceptions. An embryo created this way doesn't usually survive, but a few cases are known to have made it — these children are chimaeras of cells with X and Y chromosomes.