Understand, in biological terms, why some twins look physically similar while others look different. Also, understand the difference between identical and fraternal twins.
Using biological concepts, explain why some twins are physically similar while some twins look notably different. (The difference between identical and fraternal twins)
Humans consist of two types of cells: Somatic cells (all body cells except the reproductive cells) and gametes (sperm cells and egg cells). A somatic cell consists of 46 chromosomes, made up of two sets of 23 chromosomes, one set from each parent; whereas, a gamete (or reproductive cell) has half as many chromosomes as somatic cells, or one set of 23 chromosomes. Eukaryotic chromosomes (in this case human chromosomes) are made of chromatin, a complex of DNA and associated protein molecules. Each single chromosome contains one very long, linear DNA molecule and that carries several hundred to few thousand genes, the units that specify an organism’s inherited trait.
When a haploid sperm cell from the father comes in contact with a haploid ovum from the mother, their nuclei fuse together, resulting in fertilization. As a result of this fusion, the ovum becomes a zygote (or a fertilized egg). This zygote, or fertilized egg, consists of two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines, and thus is known as a diploid. About a day after the fertilization, the zygote then divides for the first time. This division of the zygote results in two daughter cells, each consisting of 46 chromosomes which were created by the duplication of the original 46 chromosomes. These daughter cells, consisting of identical chromosomes, and thus identical genes, usually stick together and divide to form a multicellular organism. Nevertheless, on rare occasions, the daughter cells formed by the zygote separate, each forming two distinct embryos. Both these embryos consist of the same genome and the same chromosome profile/arrangement because they were derived from the same eggs, and thus are genetically identical.
Such division of one fertilized egg into two embryos results in monozygotic twins (or identical twins), who have the same DNA and genome, and thus are physically identical. (Nevertheless, although the DNA is the same, the identical twins cannot look exactly the same, because environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes).
The daughter cells of the monozygotic twins (identical twins) split from each other somewhere during the first 13 days. Depending on when the split is made, the twins develop differently. If the split is made within the first four days, then the cells only share the outer layer of the amnion sac and have two different placentas. If the split is made between four to eight days, then the cells share the outer layer of the amnion sac and share a placenta. If the split is made between eight to twelve days, then the cells share the same amnion sac as well as placenta. And, if the cells split around the twelfth day, then the identical twins are conjoined (Resulting in conjoined twins).
In contrast to identical twins, fraternal twins do not look identical because they do not have the same DNA. This is because fraternal twins, unlike identical twins, occur when TWO distinct eggs, with distinct chromosomes arrangement, are fertilized. As a result, dizygotic twins (or fraternal twins) have an extremely small chance of having the exact same chromosome profile.