THE BEHAVIOUR OF INDIVIDUAL CHROMOSOMES 



8l 



be included in the same germ cell. This phenomenon, which gives rise 

 to germ cells containing one sort of chromosome represented twice 

 over, is known as non-disjunction, and the name has now been used 

 to cover other processes which lead to the same result. 



I. Non-disjunction of X 



Non-disjunction of the X leads to a reversal of the usual criss-cross 

 sex-linked inheritance. In a type with female homogamety, Drosophila 

 for example,^ the non-disjunctional XX eggs with the extra chromosome 



Fig. 35. Secondary Non-disjunction. — Start with a female, derived from non- 

 disjunction, having the constitution XXy, and bearing the factor w for white 

 eyes in both X chromosomes. The proportions of the different types of pairing 

 are inferred from the observed ratios of offspring. 



(After Bridges.) 



vvvvy? 



when fertiUzed by X sperm give super-females (p. 221) most of which 

 fail to survive, and, with Y sperm, sexually normal females which show 

 the recessive characters of their mothers and are thus exceptions to the 

 general rule of criss-cross sex-Hnked inheritance. Similarly, the eggs 

 lacking an X chromosome (O eggs) give with X sperm exceptional males 

 showing their father's characters and with Y sperm inviable zygotes 

 which fail to develop. 



The non-disjoining XX pair probably lags behind the other chromo- 

 somes at anaphase and often fails to be included in either daughter 

 nucleus, so that more O eggs than XX are formed and exceptional 

 males are found more frequently than exceptional females. 



The discovery of exceptional males and females of this kind was 



made by Bridges, who proposed the explanation depending on the 



occurrence of non-disjunction. This was confirmed by the prediction 



of the types to be expected from the breeding of the exceptional females 



^ Bridges 1914, 1916. 



