196 



Chromosome Maps 



haploid chromosome number is 24. It is impossible to set up 

 mating experiments in human beings; therefore, the pedigree 

 culture method cannot be used. A technique based on statistical 

 studies of natural families has recently been worked out, but 



cm 



Fig. 59. A translocation of a piece of the X chromosome to chromo- 

 some IV in Drosophila melanogaster. (a) metaphase and (b) sahvary 

 gland chromosomes. For discussion, see text. (Redrawn from Griffen and 

 Stone in University of Texas Publication 4032). 



since it is very new in comparison with the methods used in 

 studying linkage in other forms of life, much less progress has 

 been made in our studies of human beings. 



Incomplete Sex Linkage in Man 



In Chapter 7 it was pointed out that if a gene is located in a 

 part of the X chromosome which is not homologous with any 

 part of the Y chromosome, that gene follows a pattern of trans- 

 mission known as sex linkage. It was also shown that if a gene 

 is in a part of the Y chromosome which is not homologous with 

 any part of the X chromosome, the gene will always be trans- 

 mitted directly from father to son. If the gene, however, is 

 located in that chromosomal segment which is present in both 

 the X and Y chromosomes it is said to be incompletely sex linked. 

 So far, nine such genes are known in man. 



If a certain dominant gene, 0, is found in the X chromosome 

 of a man while the homologous segment of the Y chromosome 

 bears the allele, o, and if that man is mated w4th a homozygous 

 recessive woman, four types of offspring will be produced. The 



