294 



PRINCIPLES OF GENERAL PHYSIOLOGY 



both characters, but, when it forms germ cells, the qualities are separated again, 

 so that each germ is either black or white, not a mixture of the two, and an 

 equal number of black and white cells are formed. When one of the characters 

 is dominant, that is, when it is such as to overpower the manifestation of the 

 other, which is recessive (in our case, let us call blackness, dominant, and white- 

 ness, recessive), the result will be as in the diagram of Fig. 79 (Bateson, 1906). 

 The gametes are represented by the single letters and rectangles, the zygotes 

 by each pair of these. To show that blackness is dominant, in the zygotes the 

 black rectangles are placed on the top of the white ones. The possible com- 

 binations are as shown and, since black is dominant, individuals composed 

 of black and white will appear to be black and indistinguishable from those 

 composed of all black. In this way, as can easily be seen, there will be three 

 black to one white, or three dominants to one recessive. In reality, two of 



the three dominants are impure. 

 If one of these, DR, is crossed 

 with RR, an equal number of 

 blacks and whites will result 

 (see the second diagram of Fig. 

 79) ; while, if DR is crossed 

 with DD, all blacks will appear 

 (third diagram). We see also 



I R I I " I I R that when, in fertilisation, a 



white, recessive germ meets 

 another white germ, the result 

 is pure white, and blackness is 

 thrown out from that family 

 altogether. Similarly for the 

 pure black germs. 



It follows that, when we 

 know that a particular char- 

 acter is dominant and another is 

 recessive, we can say with con- 

 fidence that when a recessive 

 D individual has once made its 

 | D appearance, all its descendants 

 will be recessive, assuming, of 

 course, that it is not crossed 



Batio ID 



oil D 



Fig. 2. 



Fig. 3. 



FKJ. 79. BATESON'S DIAGRAM OF SEGREGATION OF 

 DOMINANT AND RECESSIVE CHARACTERS. 



(Bateson, 1906, p. 159.) 



with a dominant individual. 

 For example, the original 

 Chinese primrose has a palmate 

 leaf. About 1860 a vai'iety 

 appeared with a pinnate leaf. 

 The first is dominant, the 

 second recessive, so that when- 

 ever the latter appears it continues to breed true. 



Naturally, the matter is not always quite so simple as this, even when we 

 have found out which character is the dominant. There are interactions, in 

 some cases, between the factors, and the manner in which, say, a colour is 

 produced has to be taken into account. Colour is not necessarily a single 

 character, and the factors which produce it are separately transmitted, giving 

 rise to the variety of colours met with, especially in flowers. The complex 

 inter-relationships, nevertheless, are doubtless capable of resolution. 



Another interesting case is when the same character, as that of bearing horns 

 in sheep, may be dominant in the one sex, recessive in the other. The case of 

 horned and hornless sheep was investigated by T. B. Wood (1905). 



This very short account may serve to indicate the kind of problems that may 

 be attacked from the Mendelian point of view. 



