IDENTICAL TWINS

Te Awamutu Courier, Volume 53, Issue 3813, 25 September 1936, Page 9

IDENTICAL TWINS

THE SCIENTIFIC ASPECT. In human beings the frequency of twins varies to some extent with the race, some figures quoted being one in 59 for natives of India, one in 67 for the coloured population of North America, one in 88 for the white population there, one in 71 to one in 83 for Scotch people, one in 129 for Chinese at Hong Kong, and figures varying from one in 92 to one in 301 for Japanese. Human twins are of two kinds, uniovular or binovular. Uniovular twins are developed from one fertilised ovum, in other words, from what in usual circumstances would have given rise to one individual only, whereas binovular twins are derived from separate fertilised cells. In the latter case it is just a fortuitous circumstance that two ova were available of fertilisation at approximately the same time. The uniovular twins are of unusual interest from a scientific point of view. Having arisen from a single cell, their original hereditary make-up is identical, and they start their existence sharing equally all the hereditary characters that they have derived from their respective parents. At a very early stage of development, when the intended single individual was represented by a cluster of only a few cells, some “accident” occurred, leading to a cleavage and a complete separation into two groups of cells. Each of these was able to,.continue its orderly development, ultimately producing two individuals remarkably like each other. Binovular twins are the most common, and here the twins are frequently not of the same sex, and their inherit qualities are not the same It is, in fact, theoretically possible for them to possess not a single heritable character in common. What the uniovular twins inherit is, on the other hand, identical. It is true, that as they develop it may be possible for this or that minor inherited potentiality to drop out or even be slightly modified. Taking them all together, however, they are remarkably alike, in mental and in bodily make-up, and naturally they are always of the same sex. EFFECTS OF ENVIRONMENT. Binovular twins may, of course, inherit much in common, and so may resemble each other closely, just as ordinary brothers and sisters do sometimes. In the uniovular twins we thus see individuals who have inherited the same qualities, and therefore, any difference we see between them in mental or bodily characteristics must be attributed to the effects of the somewhat different environments to which they have been exposed. They inherit the same nature, and any differences seen are due to the differencs in nurture. In other words, in these cases we can study the way in which the surroundings to which an individual is exposed may work upon and alter a common inheritance. The general appearance of identical twins is very similar. By the casual observer they are mistaken for each other, though the parents can readily distinguish between them by the slight differences that have resulted from the differing environment and experiences of the twins. The eyes will necessarily be of the same colour. Each twin will belong to the same blood group, and the fingerprints will present the same pattern. Galton, in his work on finger prints, published as far back as 1892, in writing of sets of twins in general, both uniovular and binovular, states that there cannot be the “slightest doubt as to the strong tendency to resemblance in the finger patterns in twins.” He adds, however, that this remark “must by no means be forced into the sense of meaning that the similarity is so great that the fingerprint of one twin might occasionally be mistaken for that of the other.” In this connection he states that the smaller details, the number of ridges and the minutiae, are practically independent of the pattern. From this it may be inferred that the pattern is inherited and “breeds” more or less true, but that the hereditary qualities in the “smaller details” are subject to slight change during development, or at least that these qualities do not dominate the situation. SAME MERICAL HISTORY. What adds particular interest at present to the subject of identical twins is that a number of cases are now on record in which they have presented the same type of tumor in the same organ and' frequently at the same age. This, of course, does not necessarily mean that the tumor was itself purely an inherited characteristic which had developed in the same way as the liver or any other organ had developed. It may merely mean that certain tissues with a particular hereditary make-up are likely to react in the same way to some externa! - influence or to some environmental change when this has had a chance of acting. It is, however, veiy remarkable, if this be the case, that not,only the same organ should be affected, but that the age at which the tumor appears should frequently be the same. Thus, it is quite clear that the study of identical twins in all their environmental reactions can clearly help us in assessing to what extent nurture in its widest sense van modify our nature,