Heredity

HEREDITY may be defined as "the resemblance between an organism and its ancestors, in so far as this resemblance is not due to similarity of environment." Thus rickets is not a hereditary disease, yet a rickety father is more likely to have rickety children than the average man ; because rickets are due to bad food and lighting, and bad environments are to some extent inherited, though not in the biological sense. Similarly syphilis is only hereditary in the sense that a parent may infect his or her child. The causative organism here constitutes a sim ilarity in their environments. In plants certain abnormal condi tions, once believed to be hereditary, are only known to be infec tions because they can be transmitted by grafting. It is very much easier to study the inheritance of such characters as eye colour, which in man and most animals is unaffected by the environment, than body weight or psychological traits, which depend upon surroundings to a greater or less extent.

Pure Lines.—Just as by keeping the environment as nearly as possible constant for all the animals or plants in an experiment, we can study the effects of hereditary differences; so in certain cases, we can eliminate hereditary differences within a group of organisms, and thus determine the limits of the power of heredity in determining structure and function. When an organism can be produced asexually, as in the case of many Protozoa, most fruit trees, the potato, the tulip and many other plants, a population so produced is very uniform, and the differences within it are not hereditary, except in rare cases. Such a race is called a clone (from Gr. « X6vos, a mob) . A similar condition can be reached by very prolonged self-fertilization as in beans and wheats, or in-breeding, as in rats and guinea-pigs. Such a population is called a pure line. In a pure line of beans the heaviest may weigh three times the lightest ; but its progeny weigh no more on the average than those of the lightest bean. The differences of weight are therefore wholly due to environment. It is thought by some biologists that such differences are inherited (see LAMARCKISM) but if such a phenomenon occurs it is both rare and slow. More over it has never been demonstrated with a pure line as initial material. Pure lines of men do not exist, but a pair of so-called "identical" human twins, derived from the same fertilized ovum, have the same hereditary make-up, and all differences between them are due to environment.

Segregation.—All the members of a family have the same an cestors, but the differences between them are not wholly, or usually even mainly, due to environment, but to a phenomenon called segregation. These differences are usually inheritable. Thus a pair of cream guinea-pigs will produce yellows, creams and whites, and the yellows and whites will breed true. Segregation is due to an unequal transmission of the material basis of heredity to the offspring by the parents. It is a universal phenomenon except in pure lines. In the case of human stature about half the variance (see BIOMETRY) is due to heredity, half to segregation and very little to environment.

The Physical

Basis.—Before entering on a detailed account of heredity it is necessary to recall certain facts regarding reproduc tion (see REPRODUCTION ; SEX and CYTOLOGY) . A higher animal or plant is formed by the union of two gametes (Gr. yaµErns, a spouse), a large egg-cell and a small spermatozoon or generative nucleus from a pollen-grain. The egg-cell contributes most of the cytoplasm, but the nucleus is equally or almost equally derived from both parents. In hermaphrodite plants and animals each parent contributes a similar set of chromosomes to the nucleus of the zygote (Gr. O ycvros, yoked), as the product of the two gametes is called. In bisexual organisms this is only true of one of the sexes. In most bisexual animals, and in all bisexual plants so far investigated in which the sex is determined by the chromo somes (q.v.), the eggs all contain the same complement of chromo somes, but there are two types of male gamete. One type contains a set of chromosomes like that of the egg, and produces female zygotes. The other has one chromosome less, or else one of its chromosomes differs from one of those in the egg's set. The chromosome characteristic of the female-producing gamete is called the X chromosome, that (if any) characteristic of the male producing gamete the Y chromosome. Thus the females con tain two X chromosomes, the males an X and usually a Y. (Figs. I and 2.) In birds, Lepidoptera (butterflies and moths) and some fish, the spermatozoa are all alike, but the female forms male-producing and female-producing eggs, the former generally containing a Y chromosome, the latter always an X.

The chromosomes of paternal and maternal origin preserve their identity throughout the life of the zygote, but shortly before the formation of the gametes they usually twist together and appear in many cases to exchange parts (fig. I). The gametes each obtain a set of half the zygotic number of chromosomes, and hence are called haploid (Gr. 67rXoi e, single) ; the zygote with its double set is described as diploid (Gr. StirXoi c, double). The special cell-division responsible for the halving of the chromosome number is called the reduction division. Except in rare cases the fact that two chromosomes have come from the same parent does not give them any tendency to pass into the same gamete. They are assorted according to the laws of chance. This is the principal material basis of segregation.