14 SEX-DETERMINATION 



characterized by the presence of a synaptic mate for the X. 

 The diploid number of chromosomes is the same in both 

 sexes. 



Male Female 



XY X XX 



X Y : X gametes 



XX : XY fertilization 



Females Males 



in equal numbers 



All the mature ova of the female receive a single X. The 



Y is strictly confined to the male line. The son receives his 



Y from his father and his X from his mother. The daughter 

 receives one X from each of her parents. 



The Y commonly, though by no means invariably, is 

 smaller than the X. In the majority of instances the X and 

 the Y are disjoined in the first or heterotypic division, each 

 dividing equationally in the homotypic division. During the 

 growth period of the spermatocytes both the X and the Y 

 typically undergo heteropycnosis and in most instances they 

 unite to form a single bivalent. This then separates into its 

 two components so that half the sperm receive an X, the 

 other half a Y. 



The XY pair differs from the autosomes in that they 

 usually do not take part in the synaptotene stages or in the 

 formation of tetrads. There is a tendency for the X and the 



Y to come together during prophase. In many instances their 

 contact is but slight so that the two chromosomes can still 

 be distinguished. In other cases the union which occurs 

 during the growth stages of the meiotic period is a much 

 more intimate one and the two chromosomes are contained 

 within a single chromosome nucleolus. The joined X and Y 

 may enter into the primary spermatocyte spindle in the form 

 of a tetrad. But the inequality of the X and Y can usually 

 be recognized in these structures and therefore the presence 

 of a tetrad does not necessarily indicate any actual synapsis. 



The Lygaeus type of sex-chromosome sex-determining 

 mechanism is to be encountered in mammals and dioecious 

 plants, is common in diptera and not unusual in bugs and 

 beetles. The size difference between the X and the Y varies 



