Hello, thanks for the question!

To start off, let's first talk about fraternal twins, another word for this is "dizygotic" which means a fraternal twin happens when two separate eggs are fertilized by two separate sperm. These babies can come out because of different reasons such as hyper-ovulation (it is hereditary). The chance of having a fraternal twin is also more prevalent if you're older than 35. Generally, fraternal twins will have about the same physical likeliness as any siblings, they're pretty easy to tell apart especially when they're both of opposite sex. These are also the most common form of twins.

Now, for identical twins, these happen when a single egg is fertilized by a single sperm and it splits into identical halves - two babies with identical DNA are now formed. Identical twins are always the same sex and blood type. Identical twins are very similar in appearance. These are the ones that can be difficult to tell apart. To break it all down, fraternal twins are made from two different eggs and sperm so they are kind of just like regular siblings that just happen to be fertilized in the same time frame and will generally have the same likeliness as a sibling would. They can be of opposite sex. Identical twins are made from a single egg splitting and so they have the same characteristics such as DNA and blood type. These are hard to tell apart and are the same sex.

Fraternal twins and identical twins differ in how the set of twins arise. Fraternal twins occur when a woman releases two eggs during ovulation instead of just one (which is what is normally released). Each of these eggs can then be fertilized by a different sperm cell from the father. Basically, fraternal twins are as similar genetically as any set of siblings because they each begin as a unique egg and unique sperm cell joining together. This is why you can have fraternal twins of different genders. Identical twins actually form from a single egg and sperm. After the sperm fertilizes the egg, the embryo will divide a couple times. After the embryo has divided into about 4 cells it can split and become two separate two celled embryos. These embryos have the same exact DNA because they started from a single egg and sperm. Therefore, identical twins have identical sets of DNA!

The main cause of fraternal and identical twins comes from conception; either two completely separate eggs can be fertilized (fraternal twins) or one fertilized egg can split into two during maturation (identical twins). In fraternal, or dyzygotic, twins have two different amniotic sacs and placentas to sustain growth, whereas identical, or monozygotic, twins may or may not two separate sets of structures for their growth. Fraternal twins can be different genders because they are two completely different eggs getting fertilized; but even two same gender fraternal twins do not look completely alike. Whereas for identical twins since one egg is splitting into two, the two cells have the same exact DNA make up and chromosomes.

Fraternal twins are created when two eggs are fertilized by different sperm, so it's as if you just had two kids. This is different than identical twins, which are genetically identical; they have the same DNA. Identical twins occur when the fertilized egg splits, creating two identical copies that develop into two babies.

Identical twins are clones: the embryo splits during development and becomes two fetuses, but the genomes between them are, well, identical. The only differences between identical twins arise from differences in environment and experience.

Fraternal twins happen when the mother releases two eggs, which are in turn fertilized by two different sperm. Genetically, they are no different from any other siblings, except that they are born at the same time and thus are the same age.

This is an interesting question. Identical twins are called "monozygotic" -- mono = 1, zygote = the initial cell when two gametes are combined (egg + sperm). So identical twins come from one zygote that splits into two embryos. Fraternal twins, on the other hand, are called "dizygotic." This means they start out from two different zygotes, which means they come from two different eggs, each fertilized by a sperm. Now, each egg and each sperm was generated by the process of meiosis, which begins with genetic recombination in the process called "crossing over." So each egg and each sperm have slightly different genetic information. This is why fraternal twins look slightly different from one another!

The difference in the types of twins come from how many eggs are involved. To create a fetus you need one egg and one sperm. For identical twins, there is only one egg and one sperm. Between the first and eighth day during cellular replication, the cell will fully split, causing two identical copies. In this case, the twins must be the same gender, as their DNA is the same, and they share the same placenta. A fetus gets its gender from the proteins and hormones expressed during development in the placenta. Because identical twins share a placenta, they will receive the same proteins and hormones, and must develop the same gender. In fraternal twins, you have two eggs and two sperm. I think of this version being two individual people that are birthed at almost the same time. Fraternal twins do not need to be the same gender, because each egg-sperm set will create a different set of genes. Also, fraternal twins do not share the same placenta and can have two different protein and hormones expressed during development. There is also another combination which is called "semi-identical twins" where there is one egg and two sperm. This case is much more rare, and is different from both identical and fraternal twins. In this case, the genes from the mother are the same, but the genes from the father are different.

Women in childbearing years usually produce one mature egg a month. Fraternal twins form when she releases two (or more) eggs. Each one is fertilized by a separate sperm. Genetically, fraternal twins are no more alike than siblings (brothers and sisters).

Identical twins start off as one fertilized egg. Then after the embryo has divided (often around the 16-cell stage) the entire embryo splits. Each half goes on to form a separate individual. These individuals will be genetically identical unless there is some mutation as they continue to develop. This does not mean that the twins will be identical in every way. One twin may get more nutrients or less exposure to a certain hormone. In other words, their environment may be a little different even before they are born. After they are born, they will certainly have different experiences. Some identical twins have very similar personalities. Others don’t.

Armadillos generally produce identical quadruplets. I don’t know why. Do you have an idea?