82 AN INTRODUCTION TO MODERN GENETICS 



(secondary non-disjunction). The proportions in which tiie various 

 types should occur could not be predicted theoretically since it depends 

 on the differential affinity of the X and Y as expressed in the relative 

 frequency oi XY and XX pairing. The qualitative prediction of the 

 types of the Fi was however a sufficientiy striking proof of the correct- 

 ness of the theory, which was further confirmed by cytological exami- 

 nation and demonstration of the predicted XX Y and XO individuals. 

 This was the first example of the successful prediction from genetical 

 data that a definite cytological abnormality had occurred and was very 

 important in persuading scientists that the chromosome theory of 

 heredity was justified. 



Primary non-disjunction of X occurs spontaneously in D. tnelano- 

 gaster with a frequency of about i in 2,000 and can be increased by 

 selection, and some environmental agents such as X-rays, ammonia 

 vapour, etc. L. V. Morgan^ obtained a stock showing nearly 100 per 

 cent primary non-disjunction, which turned out to have the two X 

 chromosomes attached to each other near the attachment constriction. 

 This is the famous attached-X stock, much used in work on mutation 

 frequency (p. 382). Apparent non-disjunction of the X and Y some- 

 times occurs in males, both chromosomes being included in the same 

 gamete. This is more probably due to a failure of pairing and chance 

 assortment to the same pole, than to a failure of disjunction occurring 

 after normal pairing. Non-disjunction of sex chromosomes is known in 

 other organisms,^ and has sometimes been invoked to explain abnormal 

 inheritance in man.^ 



2. Non-disjunction of other Chromosomes in Drosophila 



Non-disjunction of the autosomes in Drosophila would give types with 



one too many or one too few of the Ilnd, Ilird, or IVth chromosomes. 



Of these only the types involving the IVth chromosome are viable. 



The type with only one IVth chromosome, called haplo-IV, was 



discovered by chance.* A fly was found which appeared to contain a 



factor with a dominant phenotypic effect (smaller body, hghter colour, 



smaller bristles) which was named Diminished; the factor was lethal 



when homozygous. It showed free recombination with factors in the 



linkage groups of the X, Ilnd and Ilird chromosomes. Crossed with 



flies homozygous for the fourth group recessive factors eyeless, shaven, 



and bent, a few Diminished flies appeared in the Fi and not only 



^ Morgan 1922. ^ E.g. fowl, cf. Crew 1933- 



' Gowen 1933, Haldane 1932c. * Bridges 1921. 



