From one egg...
Are well-known that twins are two types. Dizygotic (two-egg or raznoyaytsevy, not identical) twins develop from two or several at the same time impregnated ova. Monogerminal (one-egg, identical) twins - from one impregnated ovum, at an early stage of development split on two (three, four...) parts. On average it comes in three - four pregnancies from one thousand. The reasons of such splitting still are definitely not established. One-egg twins are genetically identical. Raznoyaytsevy twins from the point of view of the geneticist are ordinary brothers and sisters.
Not only inquisitive people, the interested “a twin phenomenon“, but often even parents of twins quite often ask how the type of the been born twins is defined: mono - or dizygotic, genetically identical or carriers of different genotypes. The question is not so simple as it seems at first sight. Monogerminal twins, naturally, always are same-gender. Heterosexual twins, certainly, always raznoyaytsevy.
And if twins same-sex? Similarity - a thing subjective. Mother very much early begins to distinguish one-egg twins and sincerely assures all that they “absolutely different“, the stranger is frequent also two-egg seem “as like as two peas“...
to continue our almost detective narration, we will remind that the germ developing in a uterus is surrounded with amniony - the internal germinal cover forming the cavity filled with liquid which protects a fruit from mechanical damages and provides the water environment for its development; (b) horiony - the external germinal cover covered outside with fibers which grow into a mucous membrane of a uterus, forming placenta .
At dizygotic twins both germinal covers always different though happens that placentae grow together in one (if it happened so that two oosperms were implanted into a uterus wall very close from each other).here the case with one-egg twins is not so simple
A - here everything depends on at what stage of development of the impregnated ovum there was its splitting. Seldom or never (1% of all monogerminal twins) splitting happens late enough when the amniotic bubble and horion was already formed. Then twins develop in the general amniotic cover and with the general placenta (monokhorionichesky and monoamniotic type). In one amniotic cover and with the general horiony and a placenta only monogerminal twins can develop.
If splitting of a zygote (the impregnated ovum) happens later when the hollow ball was formed of the sharing cages, then twins divide among themselves horion and a placenta, and amniotic covers at them individual. It is the most frequent option - it meets approximately in two thirds of cases of development of monogerminal twins (monokhorionichesky and diamniotic type). In different amniotic covers, but in one horion only monogerminal twins can develop too.
After fertilization any ovum irrespective of whether she is fated “to generate“ twins or the only fruit begins to share actively. The sections which are formed in the course of such crushing of egg are called blastomer . Blastomera do not grow, and only decrease twice at each subsequent division. And so, splitting can happen already at a stage of two (several) blastomer and go on an “individualistic“ way. We understand the following as “individualism“: from these blastomer identical germs (they are “children“ of one ovum) develop, but everyone has own horion and an amniotic cover (dikhorionichesky diamniotic type). About a third of all monogerminal twins develop thus. The placenta at the same time most often one, but happens that “individualism“ comes so far that even two placentae (or are formed a little if fruits more than two). So both one-egg, and raznoyaytsevy twins can “divide“ a uterus.we Will sum up of
the results. Existence of one horion at one or two (several) amniotic covers is indisputable demonstrates that we deal with one-egg twins. And here if horion more than one, only special analyses - from simple definition of a blood type and a Rhesus factor - a factor to more difficult, but quite available genetic tests - a tipirovaniye of anti-genes of a gistosovmestimost of HLA, the analysis of DNA can give the exact answer. For “the house use“ when determining like twins, as a rule, nevertheless rely on “accuracy“ of external similarity.
“And why, as a matter of fact, in general to define zigotnost type?“ - you ask and, in principle, you will be right. What, in effect, to parents (and especially all other) a difference, from one or from several eggs developed their beloved children? Any. That, according to some psychologists, the identical genetic code leaves a mark on the relations of monogerminal twins among themselves and with surrounding reality what demands the special relation to them and special efforts directed to growing up them two independent persons, but not the uniform indivisible and self-sufficient tandem substantially isolated from the outside world.the Phenomenon of twins long since drew with
close attention of biologists and physicians: whether a joke - two (or even a little) absolutely identical an organism, developing independently from each other. Charles Darwin wrote in the letter to the cousin, the English psychologist and the anthropologist Francis Galton (1822 - 1911):“ Nothing seems to me more curious, than similarity and distinction of twins“. Really, identical heredity and development conditions different (in a varying degree) - ideal material for the analysis of “specific weight“ and interaction of hereditary factors and influence of external environment. On “scientific rails“ the twin method was put just by the cousin of Darwin Francis Galton mentioned above: in 1876 it published the article “History of Twins as Criterion of Influence of Heredity and Environment“.
Galton developed a so-called “method of concordance“, i.e. comparisons of one-egg twins with same-sex raznoyaytsevy, and also the one-egg twins who are brought up together with the one-egg twins for one reason or another growing separately from each other.Similar researches are conducted by
to this day. So, for example, the American psychologist Thomas Buchard in 80 - x years of the past (i.e. the twentieth) centuries investigated about 30 couples of one-egg twins separated at early age. Perhaps, the destiny of brothers Oscar and Jack was the most amazing. Twins in 1932 were born. Parents dispersed and “divided“ children. An Oscar mother took away to Germany where he “got sick“ with Nazism and as soon as the age allowed, entered “Hitler Youth“. Jack spent the childhood and youth in Palestine and on islands of the Caribbean Sea. The father brought up it in traditions of Judaism. Even not a twist of fate, but some spiteful sarcasm: the same genotype gave the nazi - the anti-semite - and the devout Jew. It would seem, “heredity has a rest“. But here in forty years brothers met. It became clear that at them much in common: both carry equally cut moustaches, at both sight, and even points at them almost identical weakened with age. Addictions in clothes are extremely similar too. Brothers love the same food, probably behave in a family...
from this not too cheerful case, in our opinion, can draw conclusions very consolatory: many components of identity are defined by heredity: temperament, many traits of character, taste. But to nobody “on a sort it is written“ to be a villain or the villain. The moral position nevertheless depends first of all on education and personal will of the person.