However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father's genes can't make a woman release two eggs.
However, since only women ovulate, the connection is only valid on the mother's side of the family. While men can carry the gene and pass it on to their daughters, a family history of twins doesn't make them any more likely to have twins themselves.
To form identical or monozygotic twins, one fertilised egg (ovum) splits and develops into two babies with exactly the same genetic information. To form fraternal or dizygotic twins, two eggs (ova) are fertilised by two sperm and produce two genetically unique children.
But you may be left wondering: Can twins have different fathers? The answer is yes, but only in cases in which they're fraternal, as identical twins form from a single egg/sperm combination and thus cannot have different fathers.
A dad who's one of a twin may inherit the gene, but it won't increase his chances of having twins too, because the gene only affects ovulation. The same dad may pass on the gene to his daughter, who then goes on to get pregnant with twins, making it look like twins have skipped a generation .
Factors that increase the chance of twins include: consuming high amounts of dairy foods, being over the age of 30, and conceiving while breastfeeding. Many fertility drugs including Clomid, Gonal-F, and Follistim also increase the odds of a twin pregnancy.
When both eggs are fertilized, the resulting siblings are fraternal twins. Because this gene can be passed on, the tendency to have fraternal twins can in fact run in families. Identical twins, on the other hand, result from one fertilized egg randomly splitting in two, creating two siblings with identical DNA.
The biological phenomenon of giving birth to twins with two different biological fathers is called heteropaternal superfecundation. In a rare and surprising event, a 19-year-old Brazilian woman gave birth to twins who belong to two different biological fathers.
If two sperm fertilize one egg which is a very rare occurrence in 1% of the case the embryo can't survive. So, two sperm cannot fertilize the same egg.
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.
What is the likelihood of having more than one set of twins? If existing twins are identical the chance of having another set is the same as most women, about 1 in 250. If the twins are non- identical the chances of having twins again are much higher.
"Today, twins can usually be diagnosed as early as six to seven weeks into the pregnancy," he says.
Multiple pregnancy usually happens when more than one egg is fertilized. It also can happen when one egg is fertilized and then splits into 2 or more embryos that grow into 2 or more babies. When one fertilized egg splits into 2, the babies are called identical twins.
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.
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".
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.
They may share the same mother but different fathers (in which case they are known as uterine siblings or maternal half-siblings), or they may have the same father but different mothers (in which case, they are known as agnate siblings or paternal half-siblings.
This means the baby has three genetic parents: the father who supplied the sperm, the mother who supplied both womb and the egg nucleus, and an anonymous donor who supplied healthy mitochondria. Of these, the mitochondrial DNA is by far the smallest contribution.
Despite these jokes, it's generally understood that it takes two to make a baby (for the most part!!) and that half the baby's DNA comes from an egg and half from a sperm. But sometimes, a real “mini me” is born with the majority of DNA coming from only one parent.
Do Twins Skip Generations? Many people believe twins skip a generation, but that's just a myth. The idea that twins skip generations likely comes from the fact that the genetic factors contributing to twins only come from the gestational parent's side.
Monozygotic (identical) twins will have the same blood type, with a few very rare exceptions. Dizygotic (fraternal) twins may have the same blood type, or they may have different types. Therefore, it may be concluded that twins with differing blood types are dizygotic, or fraternal.
Although, twin pregnancies can happen by chance and not always because of genetics. As of 2003, the average of twins that were born was about 16 sets per 1,000 births in the U.S. While the ratio of natural twin pregnancies is estimated to be 1 in 250 pregnancies.
It's all about Dad's genes
A man's X and a woman's X combine to become a girl, and a man's Y combines with a woman's X to become a boy. But if the sperm don't have equal Xs and Ys, or if other genetic factors are at play, it can affect the sex ratio.
OBJECTIVE: By convention, twins are labeled as twin A or twin B at the time of birth, depending on their birth order. However, antenatally twins are labeled as A or B according to their location relative to the birth canal.