HEREDITY 



3955 



HEREDITY 



ideas: (l)The inheritance consists, 

 in part at least, of " unit charac- 

 ters " which are inherited as a 

 whole or not at all. which behave 

 as if they were discrete units which 

 can be shuffled about and distri- 

 buted to the offspring in some mea- 

 sure independently of each other. 

 These " unit characters " are be- 

 lieved to be represented by specific 

 constituents or determinants, fac- 

 tors or genes, or architectural ar- 

 rangements of some sort, resident 

 in the germinal material and pro- 

 bably in the chromosomes. It 

 seems probable, however, that 

 several factors may be sometimes 

 involved in one character, or that 

 one factor may sometimes influ- 

 ence more than one character, or 

 that a factor may undergo a kind of 

 disintegration. New facts have led 

 to the invention of numerous acces- 

 sory hypotheses. 



Dominant and Recessive 



(2) When two parents differ in 

 respect of two contrasted unit 

 characters, these do not blend, but 

 one of them appears, more or less 

 in its entirety, in the offspring, and 

 is called dominant, while its ana- 

 logue that drops more or less out of 

 sight for the time being is called 

 recessive. Or the presence of a 

 character may be dominant to the 

 absence of that character, or con- 

 versely. It must be carefully no- 

 ticed, however, that there are nu- 

 merous instances of what is called 

 incomplete dominance, as when 

 the crossing of a black and a white 

 Andalusian fowl yields blue Anda- 

 lusians. Different pairs of factors 

 may interact, and there are many 

 complications which explain how 

 certain distributions of qualities 

 which seem non-Mendelian at first 

 sight may yet come under that 

 interpretation. 



(3) The third idea is that of segre- 

 gation, that in the history of the 

 germ-cells of the hybrids there is a 

 separation of the determiners or 

 factors of the contrasted "unit 

 characters," so that each germ-cell 

 is "pure" as regards the character, 

 either having it or not having it. 



Mendelian inheritance in man is 

 illustrated by such abnormal char- 

 acters as brachydactyly, that is 

 having the fingers all thumbs, with 

 two joints instead of three, or by 

 night-blindness or inability to see 

 by faint light. It is illustrated by 

 such normal characters as brown 

 eyes (dominant to blue), and curly 

 hair (dominant to straight), and in 

 other instances. 



The question arises whether 

 there are characters which do not 

 behave in the Mendelian (alterna- 

 tive) manner, but blend in the off- 

 spring. Thus it is often said that 

 the skin colour of the mulatto is 



intermediate between the white 

 and the black skin-colours of the 

 parents. Castle has shown that 

 when long- eared rabbits are crossed 

 with short-eared ones the offspring 

 have ears of intermediate length 

 and remain so in subsequent 

 generations. He found the same to 

 be true as regards the dimensions 

 of parts of the skeleton, such as the 

 length and breadth of the skull. 



In many hybrid plants, the pro- 

 portions of the leaves, the number 

 of stomata, the thickness of the 

 hairs, and so on, may be approxi- 

 mately intermediate between the 

 conditions seen in the parents. 

 Warren has described a remarkable 

 hybrid between two different 

 genera of cockatoo which does not 

 at first sight suggest Mendelian in- 

 heritance, and cases of blending 

 have been reported in regard to 

 trout and sheep. 



There is some evidence that a 

 feature characteristic of an ances- 

 tral type may lie latent for many 

 generations, and then suddenly 

 find expression in development. 

 This might be the explanation of 

 the appearance of horns in a horn- 

 less race, of some striping on a 

 pony, of a fourth toe on a guinea- 

 pig's foot, of a nectarine producing 

 a peach, and so on. But many of 

 the so-called " reversions " or 

 throw-backs are due to arrested 

 development, or to fresh variation, 

 or to unpropitious modification, or, 

 especially in domesticated animals 

 and cultivated plants, to crossing. 

 Mendelian Experiments 



Mendelian experiments have ex- 

 plained many of the so-called re- 

 versions in the following way. 

 Many domesticated or cultivated 

 stocks have arisen, it appears, by 

 the dropping out of certain factors 

 in the original wild inheritance. 

 Under human guidance there has 

 been a utilisation of the results of a 

 spontaneous " unpacking " of the 

 inheritance of the original type. 

 Thus from the wild rabbit with its 

 subtly coloured pellage numerous 

 colour-varieties of tame rabbit 

 have been established. When these 

 interbreed there are reversions, 

 that is to say there is repacking of 

 the hereditary items which had 

 been analysed apart. 



Modern* studies in heredity ne- 

 cessarily attach much importance 

 to outstanding features which ad- 

 mit of ready recognition and mea- 

 surement. Many of these features 

 are relatively superficial, and in 

 some cases of recent origin. In 

 every species, however, there is a 

 great mass of hereditary character 

 which is no longer in its essentials 

 subject to variation, which is pro- 

 bably passed on en bloc in the 

 lineage of the -germ -cells. 



Thus in every mammal there is a 

 stable inheritance of the essentially 

 mammalian features which are not 

 known to be departed from in any 

 essential way, nor to be departed 

 from in any very remarkable way 

 except on occasions which seem to 

 man's brief span of observation to 

 be very rare. The persistence of 

 this main -mass of inheritance is 

 accounted for by the continuity of 

 the germ-plasm. 



The study of heredity leaves in 

 the mind a convincing impression 

 of the value of good stock. To 

 acquiesce, with open eyes and the 

 possibility of escape, in the taint- 

 ing of a good stock is to do a bad 

 turn to both race and family. The 

 patching-up of what is inherently 

 rotten may be useful to the in- 

 dividual and indirectly to the race, 

 but veneer is not a substitute for 

 sound wood, and some taints persist 

 inexorably for generations. 



Inherited Nature and Nurture 



The study of heredity leaves 

 a fatalistic impression in the mind, 

 for a man cannot choose his par- 

 ents. He can choose his partner, 

 however, and in this there may be 

 progress, if good be mated with 

 good. And as an offset to the 

 inertia of hereditary persistence, 

 must be recognized the reality of 

 those variations which are ever 

 occurring and which are the raw 

 materials of progress. 



Since development is always 

 the result of an interaction of in- 

 herited nature and appropriate 

 nurture, it is of great practical im- 

 portance to secure that this nur- 

 ture is the best procurable. Other- 

 wise the promising variations may 

 fail to unfold. Hereditary char- 

 acters are like seeds requiring sun- 

 shine and rain. Moreover, for the 

 individual it is always possible that 

 alterations of nurture may prevent 

 the actualisation of inherited pre- 

 dispositions to evil. 



While there is no secure war- 

 rant for believing in the trans - 

 missibility of individually acquired 

 modifications as such or in any 

 representative degree, there is not 

 on that account any reason to 

 deprecate the valu of ameliorative 

 nurtural conditions which impress 

 beneficial modifications on succes- 

 sive generations, or the value of the 

 social heritage which has its reposi- 

 tory not in any germ-plasm, but in 

 literature and art, traditions and 

 institutions, and in the framework 

 of society itself. 



Bibliography. Life and Habit, S. 

 Butler, 1878 ; Natural Inheritance, 

 Sir F. Galton, 1889 ; The Germ- 

 Plasm, A. Weismann, 1893 ; The 

 Grammar of Science, K. Pearson, 

 1900 ; The Cell in Development and 



