34 



CANARIES, HYBRIDS, AND BRITISH BIRDS 



feathers ; this did not occur. I have also 

 had experience of dark-eyed females being 

 almost perfectly dominant over jiink-eyed 

 males — i.e., the dark-eyed female is homo- 

 zygous. In the fancier's view, also, it stands 

 to reason that there nnist be a dilTcrence 

 between a pure dark-eyed female, and one 

 bred from a pink-eyed ancestry by a single 

 dark-eyed cross. In like manner it is 

 certain that although the majority of wild 

 males may be homozygous in colour, 

 heterozygous wild males occur {.see account 

 of my Greenfinch family on p. 31). The 

 following table explains the constitution 

 of the pure dark-eyed female, heterozygous 

 in sex, and also of the offspring — all males 

 — obtained from her and a pink-eyed 

 male. 



In the offspring dp,? ^ , dark-eyedness 

 and maleness are dominant over pink- 

 eyedness and femaleness. In tlie oifsjjring 

 d p ,7 :t , dark-eyedness is likewise dominant 

 over pink-eyedness, and tlie gametic con- 

 stitution includes pink-eyed male gametes. 



If we suppose the |)ure dark -eyed female 

 to be homozygous also in sex, d d ? $ , 

 there would result 100 per cent, dark-eyed 

 male progeny, heterozygous in colour and 

 sex (dp,? ? ). 



The above matings with their gametic 

 constitution and the results obtained will 

 be siniplilied by studying 

 what are, alter all. the chief 

 points of itnportatice in the 

 Mendelian theory, viz., the conception of 

 the Unit Character, and the |)rinciple of 

 Gametic Se^tegaiinn . 



R. C. Punnett, in his " Mendelism " 

 (Bowes & I3ov/es, Cambridge), says : — 



Unit 

 Characters 



"The hetcrozygote frequently exhiliils llie 

 form of the pure dominant, from which it can 

 only t)e distinguished by the lest of breeding. 

 Thai the recessive character is likewise carried 

 is sliown by the fact that when heterozygotes 

 are bred inter se, one quarter of the otlsijiing 

 prciiluced are recessive. 



"There are cases, however, in which Ihe 

 lieterozygote does not resemble the dominant, 

 but has a character peculiar to itself. 



" These facts led Mendel to the concejjlion 

 of pairs of unit characters, of which either can 

 l)e carried by any one gamete to the exclusion 

 of the other. A fundamental property of the 

 gamete is that it can bear either one such a pair 

 of characters, though not both. But the 

 hetcrozygote is formed by the union of two 

 dissimilar gametes, and consequently the cells 

 of the individual into which it grows nuist 

 contain l)olh characters. To reconcile these 

 statements it must be supposed that at some 

 cell division in the formation of gametes a 

 primitive germ-cell divides into two dissimilar 

 portions. Instead of the dominant and re- 

 cessive consliluents passing in combination to 

 the two daughter-cells, the whole of the dominant 

 goes into one of these, and the whole of the 

 recessi\e into the other. From this it follows 

 that every gamete contains one only of such 

 a pair of characters, i.e., it is pure for that 

 character. In other words, a simple lietero- 

 zygote produces gametes of tiro l<inds, (nut pro- 

 duces lliem in equal numbers. The characters 

 are said to seijreyate in the 

 Gametic gametes. In this conception lies 



Segre= the simple explanation of the 



gation. facts, that from the inbre<l 



hetcrozygote conu's dominants 

 and recessives in the proportion of 3:1, and 

 that only one dominant in three is pure. Ihe 

 other two being heterozygotes. (See formula 

 below.) 



" A convenient system of notation is to denote 

 the hetcrozygote by the letters D H, thus 

 signifying that it gives oil ecpial numbers of 

 gametes bearing the dominant antl recessive 

 characters. On the same system the i)urc 

 doriiinanl and the pure recessive are represented 

 l)y the terms 1) I) and R H respectively. .So 

 far we have considered only Ihe results olitained 

 by inbreeding the heterozygotes. 



1) U 



I) D -h D R + D R -F R F< 



D RJ 



"The theory of gametic imrity can he fuillKr 

 tested by deducing from it the results which 

 sliould follow from crossing Ihe lieterozygote 

 witli either of the homozygoles, and seeing 

 liow far sucli llieorelical results accord with 

 those obtained by experiment. When the 



