Haldane has described a method of inheritance which he has called incomplete sex linkage, in contrast to the mode of inheritance exhibited by hemophilia, for example, which is said to be fully sex linked. Xeroderma pigmentosum is one of the diseases said by Haldane to show incomplete sex linkage. An examination of the mode of transmission of traits dependent on (1) a recessive gene, (2) a sex-linked recessive gene, (3) a gene carried in the Y chromosome and (4) an incompletely sex-linked recessive gene will make the matter clear. TRAITS DEPENDENT ON RECESSIVE GENES The mode of inheritance of any trait dependent on a recessive gene not carried in the sex chromosomes is presented in chart 1. Both parents are normal but carry latent a gene for the trait in question. The pair of chromosomes carrying this gene is represented in the chart, all other chromosomes being ignored. The chromosomal