Contents
Contents 2
Twin Development Diagrams: Monozygotic twins………………………………………………………..3
Twin Development Diagrams: Dizygotic Twins…………………………………………………………..4
Twin information 5
The Minnesota Study of Twins Reared Apart ……………………………………………………………6
Refrences…………………………………………………………………………………………………..7
Twin Development Diagrams: Monozygotic twins
1. One sperm makes its way into the fallopian tube and delves into the egg, therefore it fertilizes the egg and the fertilized egg becomes a zygote. The egg changes so that no other sperm can get in. At the moment of fertilization, the baby's genes and sex are set. If the sperm has a Y chromosome, the baby will be a boy. If the sperm has an X chromosome, the baby will be a girl.
2.
At this stage, after fertilization by the sperm, a ball of cells known as a blastocyst is formed, which then becomes a zygote. It is around this stage (about 2-4 days after conception) that it will split into two, forming two embryos instead of one. This happens at the very earliest stage of development, when the zygote is no more than a bunch of a few cells. These twins are therefore also known as 'dizygotic' as well as 'biovular' twins.
3. Dividing this early in conception means that each baby always has exactly the same genetic information (DNA) as the other, as demonstrated by the colours of the two embryos within the diagram (they are both brown). Monozygotic twins are of the same gender and have the same blood type.
4. Most of the time in identical/monozygotic twins the zygote will split after 2 days, which causes a shared placenta, but two separate amniotic sacs. These twins are called Monochorionic Diamniotic (MCDA) twins, where both babies share one placenta and one outer membrane, but they each have their own separate, inner membrane, which means each embryo has their own amniotic sacs. This occurs in two thirds of monozygotic twins, so is the most common type of monozygotic twins. However, in the case of Monochorionic monoamniotic (MCMA) twins, both babies share a single placenta and one single outer membrane with no inner membrane to separate them, meaning they share an amniotic sac. MCMA twins are very rare and only report for approximately one per cent of monozygotic twins.
Twin Development Diagrams: Dizygotic Twins
1.
In dizygotic twins or 'non-identical twins', the most common type of twins, two eggs are released at the same time from the ovary and each are fertilized in the womb by two different sperm cells. The fertilized eggs are called zygotes.
2. At this stage, after fertilization by the sperm, a ball of cells known as a blastocyst is formed, which then becomes a zygote. It is around this stage that both zygotes split into two, form and form two embryos. This happens at the very earliest stage of development, when the zygote is no more than a cluster of a few cells. These twins are therefore also known as 'fraternal’ twins. These types of twins are basically like ordinary siblings that just happen to be fertilized in the same time frame and will normally have the same likeliness as a sibling would.
3. As shown in the diagram, each zygote is a different colour. As there are two separate zygotes, the genetic makeup (DNA) of dizygotic twins is different, and thus they are not identical. On average, these twins will share half of their genes, just like any other pair of siblings and because two eggs are fertilised by different sperm, the gender of the two embryos may differ. For example, one egg may be fertilised by a sperm with a y chromosome (forms a boy) and the other egg may be fertilised by an egg with an x chromosome (forms a girl).
4. Dizygotic twins develop separately in the uterus. Each zygote develops with its own chorion (or outer sac). The chorion is connected to the placenta, which is the protective membrane that surrounds the developing fetus. The placenta lines the uterine wall, partly encloses the fetus, and is connected to the umbilical cord. The placenta switches nutrients, wastes, and gases between maternal and fetal blood. Each zygote also has its own amniotic sac (inner sac) that encompasses amniotic fluid, and each develops its own placenta. Yet in some cases, if the two zygotes implant in the uterus close together, the two placentas may give the impression of one placenta on an ultrasound.
Twin information
Differences
As monozygotic twins are a result of one sperm fertilising an egg, then the zygote splitting into two, the twins will have the same genetic information (DNA) and will be the same gender depending on whether the sperm has a y chromone or an x chromosome. In the case of dizygotic twins, two sperms fertilise two different eggs, meaning the twins will not have the same genetic information (DNA) and can be of different sexes, as a sperm with a y chromosome can fertilise one egg and a sperm with an x chromosome can fertilise the other egg. Because dizygotic twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures whereas monozygotic twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2. Monozygotic twins are both the same gender whereas dizygotic twins can be both the same gender or different genders.
Similarities
Both monozygotic twins and dizygotic twins share a placenta but each twin, in both cases, has its own amniotic sac. It is very rare for a monozygotic twin to share an amniotic sac (only a third of MZ cases). However, in the case of Monochorionic monoamniotic (MCMA) twins, both babies share a single placenta and one single outer membrane with no inner membrane to separate them, meaning they share an amniotic sac. MCMA twins are very rare and only report for approximately one per cent of monozygotic twins. Both monozygotic twins and dizygotic twins have different fingerprints. In both cases of monozygotic twins (identical) and dizygotic twins (fraternal) the child may develop a different personality to that of his/ her twin because both pairs of twins do experience slightly different environments, even when they’re together, and any early differences between them may be encouraged by family members, or by one another.
The Minnesota Study of Twins Reared Apart
The study initiated in 1979 when Thomas Bouchard started to study twins who were separated at birth and raised in different families. In the study, monozygotic twins are represented by the abbreviation MZA and dizygotic twins by the abbreviation DZA. The youngest participants being 19 and the oldest participants being 68 years old. Two examples of psychophysiological variables that were tested in the study were systolic blood pressure and heart rate. Two examples of anthropometric variables tested in the study included fingerprint ridge court, as well as height and weight. This study was a once in a lifetime opportunity and cannot be repeated again because twins reared apart is very rare.
References:
– Twins – identical and fraternal. (2017). Retrived from https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/twins-identical-and-fraternal
– Types of Twins: Dizygotic, Monozygotic, Dichorionic, & Monochorionic, Mono-Di, Mono-Mono, Di-Di. (2017). Retrieved from https://www.babymed.com/twins/twins-monozygotic-vs-dizygotic-and-monochorionic-vs-dichorionic
– Twins Reared Apart. (2012). Retrieved from https://www.psychologytoday.com/blog/twofold/201206/twins-reared-apart