THE MENDELIAN METHOD 



in the X, but the two chromosomes never exchange allelomorphs 

 in heterozygous females. The barring gene can be used, therefore, to 

 disthiguish between the X and the Y, and so makes possible the 

 "autosexing" breeds as shown in Fig. ii. 



O Cf 



Fig. II. — The gene for barred feathers (open circle) in poultry expresses itself only 

 partially in females; so suggesting that the Y chromosome carries its non-barred 

 allelomorph (filled circle). Where the basic pigmentation of the birds is brown, 

 rather than black, this ctiect of the Y chromosome is clearly detectable by deeper 

 colouration in newly hatched chicks. The combination of the gene for brown 

 pigmentation and that for barring thus gives a true breeding strain of birds in which 

 the sexes are distuiguishablc at hatching by the deeper intensity of down pigmentation, 

 especially round the head, in females than in males. Such a breed is termed auto- 

 scxine. 



Sometimes, however, as in certain fishes such as Lehistes and in 

 man, the X and the Y chromosomes not only carry the same genes, 

 but also exchange them by crossing-over. These genes, therefore, 

 no longer show complete sex-linkage. Their association with the sex 

 chromosomes is recognizable by the partial, rather than the com- 

 plete, restriction of a particular allelomorph to a particular sex in 



