198 READINGS IN BIOLOGICAL SCIENCfi 



the defect. Yet many women carry the recessive gene and so transmit 

 color bHndness to at least half their sons. 



Four sex-linked characters are known in man at least, and others may 

 be discovered. The most serious is haemophilia, or bleeding, a recessive 

 trait which somehow keeps the blood from clotting and so from sealing 

 wounds. A male bleeder who marries a perfectly normal wife cannot give 

 the defect to his children, nor can his sons pass it on. But his daughters, 

 though healthy throughout their own lives, will give one gene for bleed- 

 ing to half their sons. 



The majority of man's characters are determined by genes in forty-six 

 other chromosomes, most of which are longer than the X, while all are 

 longer than the Y. Since chromosomes are chains of genes or gene capsules, 

 it becomes obvious that the bulk of human heredity depends on sorting, 

 pairing, and dominance that takes place without reference to sex. 



SOME DISEASES ARE HEREDITARY 



Most of US have been taught that disease is not inherited, since germs 

 cannot be transmitted by way of sperms and eggs. The latter statement 

 seems to be true, but the former is not. Mankind suffers from many 

 diseases caused by defects or faulty operation of human tissues and organs, 

 and of these an increasing number are known to be hereditary. 



Bleeding of the kidneys is a dominant character, as are albumin in the 

 urine and a type of diabetes which causes excessive urination, wdth 

 equally excessive thirst. A form of anemia in which red blood cells become 

 sickle-shaped is a dominant character. Pernicious anemia also is inherited 

 in some families, though in others it seems to develop without any help 

 from genes. 



Most familiar of all hereditary ailments are those grouped as allergy. 

 They include hay fever, asthma, hives, eczema, ivy poisoning, indigestion, 

 sick headaches, and other disorders, all caused by abnormal sensitivity 

 to foods, pollens, dust, smoke, medicines — even to heat and cold. One 

 authority says that asthma is a dominant trait, while another thinks it reces- 

 sive. Hay fever has been called a dominant that sometimes skips genera- 

 tions, but it also has been diagnosed as a straight dominant. One author 

 believes that each sort of allergy is the product of multiple dominant genes. 

 This would explain much variation, but the genes also seem to be con- 

 trolled by age and other factors. Thus a child was allergic to milk at 

 birth, but his mother developed sensitivity to wheat at the age of seventy- 

 two. 



MENTAL DEFECTS ARE INHERITED 



Mental qualities are even more problematic than those involved in 

 disease. Thanks to tedious but careful collection of data, we know that at 

 least two kinds of feeble-mindedness are hereditary. Thus one group of 



