278 Heredity and Eugenics 



tions. They are relatively common in any given family, 

 since half or all of an affected fraternity commonly show 

 the trait. If the trait is an undesirable one and it must not 

 be reproduced, then the eugenical advice is for an affected 

 person to abstain from having children. But an unaffected 

 person belonging to this strain may marry an unaffected per- 

 son with impunity, and it is immaterial for the inheritance 

 of this trait whether they be cousins or not. 



We have next to consider the class of abnormalities and 

 weaknesses that are due to the absence of a determiner, to 

 the disappearance of a trait that tends to normahty. A 

 good example of such a defect is seen in albinism, already 

 referred to (Fig. 86). In this large pedigree the number of 

 affected individuals is small, and they are frequently derived 

 from two pigmented persons; cousin marriages are common. 

 The defect may be carried in the germ cells of two normal 

 parents; hence its appearance from such parents. 



Another example is seen in Thomsen's disease, a disease 

 that is characterized by a slow initial contraction of a 

 muscle after stimulus (Fig. 86). Here again two normal 

 consorts have some affected children and cousin marriages 

 are common. 



There is reason for asserting that weakness of the mucous 

 membranes is due to a similar defect (Fig. 88). If both 

 parents are without the determiner for resistance, then of 

 course all offspring will be non-resistant. When one 

 parent is liable to colds and pneumonia and the other has 

 catarrh, the children suffer from tonsilitis, diphtheria, and 

 inflammations of the throat and lungs. But if one parent 

 be non-resistant, and the other, though resistant, have 

 some non-resistant germ cells, at least half of the offspring 

 will be non-resistant. 



