202 



DISCOVERY 



be XX — in other words, the constitution character- 

 istic of a female ; whereas if the " no-X " sperm 

 fertilises an egg, the result will be X — in other words, 

 a male. As the two sorts of sperm will be produced 

 in equal numbers, an equal number of XX and X 

 embryos will be formed, and this will result in an 

 equal number of adult males and females (Fig. i). 



A similar state of affairs is to be found in many other 

 animals, including cattle, horses, and pigs, and probably 

 in man himself. In other cases, the male, instead of 

 possessing simply an unpaired X, has an unequal pair 

 of chromosomes, one being like the two found in the 

 female, and therefore called X, the other unlike, and 

 called Y. Here again there will be two sorts of 

 sperms ; the X-bearing will produce females, the 

 Y-bearing will produce males. 



Let us now turn to the results of breeding experi- 

 ments. Besides ordinary Mendelian inheritance, there 

 has for some time been known another type, known 

 as sex-linked inheritance. An example will make this 

 clear. A mutation producing white instead of red 

 eyes was discovered in the fruit-fly Drosophila. When 

 a white-eyed male is crossed with an ordinary female, 

 all the offspring are red-eyed, showing that white is 

 recessive. In the second generation, there appear 

 75 per cent, red-eyed animals and 25 per cent, white- 

 eyed, as in a normal case of Mendelian inheritance — 

 but the white-eyed individuals are all males. Still 

 more strange, if the cross is made the other way, 

 between a white-eyed female and a red-eyed male, a 

 quite different result is obtained. In the first genera- 

 tion all the daughters are red-eyed like their father, 

 and all the sons white-eyed like their mother ; in the 

 second generation, 50 per cent, are white-eyed, and 

 males and females are equally affected. 



The fruit-fly is one of those animals in which the 

 male possesses one X and one Y chromosome. If we 

 suppose that the Y is inactive — a supposition which is, 

 as a matter of fact, borne out by other evidence — 

 this curious and at first sight very puzzling form of 

 inheritance is exactly what we should have to prophesy 

 mathematically, if the factors for redness and white- 

 ness of eye-colour were situated in the X chromo- 

 some (Fig. 2). That this view is correct, at least for 

 the fruit -fly, has been definitely shown. Certain 

 stocks of the fly were found which gave exception to 

 ordinary sex-linked inheritance. These exceptions 

 were to be expected if, through some accident to the 

 machinery of cell-division, animals were to have been 

 produced which contained a Y in addition to two 

 X's ; and when the stock was examined microscopically, 

 it was actually found that this was the case (Fig. 3). 



Exactly similar sex-linked inheritance is found in 

 most other insects, in mammals, and in man himself. 

 For instance, the human diseases known as hemo- 



philia (habitual bleeding due to the inability of the 

 blood to clot) and night-blindness are transmitted in 

 this way (Fig. 4). It is also found in some plants in 

 which the sexes are borne on separate individuals. 

 Since in Drosophila the association of sex-linked factors 

 with X-chromosomes is certain, and in many other 

 animals in which it occurs in the same way, the male 

 also possesses a single X, or an X and a Y chromosome, 

 we may safely say that sex-linked inheritance implies 

 the existence of sex-chromosomes different in the 

 two sexes, and vice versa. It is a curious fact that 

 in birds and in butterflies and moths, while sex-linked 

 inheritance occurs, it is reversed ; the female more 

 often shows the sex-linked characters, not the male, 

 and the facts can only be explained if we suppose 

 that in these animals it is the male who possesses 

 two X chromosomes, the female but one. This sup- 

 position has been proved to be true in moths, where 

 the microscope reveals that the females have one less 

 chromosome than the males. 



{To be concluded in the September number) 



The Temples of the Later 

 Stone Age at Malta 



By Professor T, Zammit, C.M.G. 



Curator of the Valletta Museum 



The remains of the megalithic buildings ^ at Hagiar 

 Kim and Mnaidra, not far from the villages of Krendi, 

 Tarxien, Hal-Saflieni, and Cordin, close to the grand 

 harbour of Valletta, as well as the Gigantea in the 

 island of Gozo, are marvellous examples of Neolithic 

 art — the art of the Later Stone Age. 



The ruins of the group of temples on the way to 

 Tarxien are of recent discovery, and their excavation 

 has jdelded archjeological material which throws 

 considerable light on the Neolithic culture of the 

 Mediterranean basin. 



The type of these temples corresponds to that of the 

 other Maltese large megalithic buildings which consist 

 of egg-shaped compartments, parallel to each other, 

 and connected by short dolmenic '^ corridors. A wall 



1 The term " megalithic " is applied to these buildings 

 owing to the fact that they are for the most part constructed 

 of huge slabs of stone similar to those of the megalithic monu- 

 ments — stone arches, standing stones and the like found in 

 various parts of the world and dating from prehistoric times. 



2 These corridors, characteristic of the Maltese Neolithic 

 temples, are called " dolmenic " from their structural resem- 

 blance to the dolmen. Dolmens, cell-like structures of two, 

 three, or more slabs of stone standing upright and surmounted 

 by a flat stone or " cap-stone," are of frequent occurrence in 

 Malta. 



