SEX-CHROMOSOMES AND INHERITANCE 179 



somes, including two Z's, The egg that lacks a Z (8), fer- 

 tilized by a sperm (8 + 1), makes a female with 17 chro- 

 mosomes, including one Z. 



This scheme gives consistent results for sex-linked 

 inheritance in birds. Since the daughter gets her single 

 Z-chromosome from her father, she will show any sex- 

 linked characters carried by his Z-chromosome. If the 

 father carries a sex-linked dominant gene his sons and his 

 daughters will be alike. It should be noticed that while 

 Guyer's scheme gives the same results so far as sex-link- 



n 



BlackcJ* Barred 



Barredc? Black 



15 



zV tz\ ^ z V zV 



Barredd Black d DarredQ Black o 



Fig. 79. — Scheme showing the transmission of the sex-linked characters B = barred, and 



b =black in the cross shown in Fig. 78. 



age is concerned, as the one described by Seller for some 

 moths, the machinery in the male is different in the two 

 cases, while that in the female is presumably the same. In 

 both the female is heterozygous for Z ; in the moth the 

 male is homozygous (ZZ), but in the bird the two Z'5 

 described by Guyer both go to one pole at one of the 

 maturation divisions, and reduce at the other — a proce- 

 dure not known in any other animal. 



In the reciprocal cross (Fig. 78) a black cock is bred 

 to a barred hen. The sons are barred — like their mother — 

 the daughters are black — like their father, criss-cross 

 inheritance. When the barred Fj cock and the black hen 



