March 2, 1922] 



NATURE 



275 



The Mechanism of Heredity.^ 



By Prof. T. H. Morgan, Columbia University, New York City, U.S.A. 



II. 



Linkage and Crossing-over. 



MENDEL'S second law has been found to be re- 

 stricted in its application. - Two pairs of 

 characters do not always assort independently. This 

 fact was first observed by Bateson and Punnett in 

 1905, and called gametic coupling — not that gametes 

 (ripe germ-cells) are coupled, but that when certain 

 uenes enter together from one parent they tend to hold 

 together, as though coupled, in later generations. 

 A specific case will serve to illustrate this kind of 

 inheritance. 



If a sweet pea with genes for purple flowers and 

 long pollen grains is crossed to a pea of another strain 

 with red flowers and round pollen, the expectation for 

 the two pairs of genes would be in Fg 9:3:3:1. 

 Instead of this ratio there was found approximately 

 177 : 15 : 15 : 49. Pprple long and red round have 

 ome out in the second generation in unexpected ratios, 

 or, in other words, the results are explicable only on 

 the hypothesis that the genes that went in together 

 have shown a tendency to stay together instead of 

 freely assorting. 



This coupling is often spoken of to-day as linkage, 

 because it applies not only to two genes, but to any 

 number of them. A few further cases may be given ; 

 in one the characters, as in the pea, are not sex-linked, 

 and in the other they are. There is a strain of Droso- 

 phila melanogaster that is black. It gives with the 

 wild fly in the second generation a 3 : i Mendelian ratio. 

 There is another strain that has vestigial wings. It, 

 too, gives with the wild fly a 3 : i Mendelian ratio. It 

 is easily possible to make a strain that is pure both 

 for black {bb) and for vestigial {w). If a black vestigial 

 male (pv) is mated to a wild female (B V) (grey long 

 wings) all of the offspring are grey long (Fig. 11), If 

 one of the F^ sons is mated to a black vestigial female 

 of pure stock, only two kinds of offspring are obtained ; 

 half of them are black vestigial, and half are grey long. 

 In other words, the two recessive characters that went 

 in together (black vestigial) have come out together. 

 These characters are completely linked in the male. 

 It may be said, in exactly the same sense, that the 

 other two characters, the dominant ones, namely, 

 irrey long (which went in together from the other side), 

 re also linked. Now if the genes for black and for 

 . cstigial are carried in the same chromosome, then 

 their partners or allelomorphs (grey long) lie in the 

 other chromosome of the same pair, and if these 

 f^hromosomes remain intact the result is what is 

 xpected to take place. 



Linkage is also excellently illustrated in the case 

 ! sex-linked characters. As has been shown, white- 

 ' ye versus red-eye colour of Drosophila gives a Mendelian 

 ratio. Another sex-linked character, yellow colour, 

 also gives the same result. If a strain is made up 

 that has white eyes and yellow colour, and if a female 

 of this strain is mated to a wild-type fly (red eyes. 



NO. 2731 



' Continued from p. 244. 



VOL. 109] 



grey colour), all the sons will be white-yellow, and all 

 the daughters red-grey (Fig. 12). If these are inbred, 

 the great majority of the offspring (98*5 per cent) are 

 yellow-white and grey-red (half and half). In other 

 words, these characters are linked, but only in 98*5 

 per cent, of the cases. The remaining i'5 per cent, is 

 composed of two kinds of individuals, red-yellow and 

 white-grey. It may be said, therefore, in this case, 

 that the white eye of the yellow type has crossed over 

 to the grey type, and in exchange the red eye of the 

 grey type has crossed over to the yellow type. 



The four kinds of offspring obtained in this cross 

 can be accounted for, if once in a hundred times an 

 interchange has taken place between the two X- 



chromosomes of the Fj female, in such a way that the 

 part containing the gene for white eye is interchanged 

 for a corresponding part of the other chromosome 

 with the gene for red eye. 



Another example of crossing-over may be given, 

 one involving the same characters, black and vestigial, 

 which were used to illustrate complete linkage. It is 

 possible to use the same combinations of characters 

 to illustrate both absolute linkage and crossing-over, 

 because in the male of Drosophila there is no crossing- 

 over, but in the female crossing-over occurs. There- 

 fore, in the first case above, in which this combination 

 was utilised, an Fj male was back-crossed, while in 

 the present case an F^ female will be employed. If, 

 as shown in Fig. 13, a black vestigial fly be crossed 

 to a wild-type fly (long wings, grey), the Fj female 

 will be wild-type. If she is back-crossed to a black 



