380 



The Journal of Heredity 



marry. ^ But an approach to these 

 ratios we do find constantly, and that 

 law furnishes an important guide, not 

 only in recognizing heredity itself in a 

 given family history, as distinguished 

 from an infection, but in also indicating 

 when the defect is dominant or recessive. 

 In other words, this relation between 

 genetics and ophthalmology throws 

 light on the differential diagnosis (the 

 next step in our study), and is very 

 decidedly practical in connection with 

 eugenics. 



With this reassurance as to the reason 

 for continuing, let us pass to the exami- 

 nation of family histories to determine 

 whether a given defect is in reality 

 hereditary, or whether it is due to infec- 

 tion from syphilis or other causes. 



It would necessitate a rather long 

 digression to show just how this differen- 

 tial diagnosis is made. Suffice it to say 

 that our criterion is the technical defini- 

 tion of heredity given at the outset. 

 The fact is that a very considerable 

 proportion of defects which we all sup- 

 posed formerly were hereditary are 

 probably not hereditary, but the effect 

 of some infection more or less obscure. 

 An excellent example of this is a family 

 history of aniridia reported by one of 

 the oldest and best known members'^ 

 of this section. The author and all 

 other ophthalmologists accepted this as 

 an undoubted example of heredity, but 

 competent geneticists now demonstrate 

 that to be highly improbable. 



The term "hereditary " we should also 

 define more exactly as including two 

 groups — one in which the recurrence of 

 the defect is only "possible," especially 

 when that defect is "recessive," the 

 other in which its recurrence is "proba- 

 ble," especially when that defect is 

 "dominant." Evidently, therefore, we 

 have as many conditions to deal with as 

 there are combinations, in pairs, of these 

 four factors. 



Still another factor enters into the 

 problem — the personal equation of the 

 parties to a marriage contract. For 

 when two persons contemplate matri- 



mony, it can usually be taken for granted 

 that their judgment has, for the time, 

 taken wings. Professor McCready, in 

 his lectures at the College of Physicians 

 and Surgeons, used to say his case 

 records showed that for various reasons 

 he had advised one or both parties 

 against marriage some sixty-eight times ; 

 and his records also showed that sixty- 

 eight times the couples went almost 

 straight to the altar. 



But temporary obsessions by those 

 who ask an opinion is no excuse for 

 inability to advise intelligently, or, if 

 necessary, to restrain persons from a 

 fatal mistake. Therefore the other four 

 factors in our problem demand all the 

 more careful study. While the differ- 

 ence between a simple deformity and 

 blindness is evident, it is not easy to- 

 say whether the recurrence of a given 

 defect is "possible" or "probable." 

 Our decision, then, must be based on 

 three groups of data: first, whether the 

 defect is inherited as a dominant or a 

 recessive; second, whether it is a sex- 

 linked characteristic, and, third, whether 

 both families show the defect. 



A few illustrations will show how 

 these four cardinal conditions in our 

 problem may serve as the basis for an 

 opinion by an ophthalmologist when he 

 is called on to give advice in the case of 

 a proposed marriage. Thus: 



(a) When we have to do only with a 

 deformity, and its reappearance is 

 doubtful, no special objection need be 

 raised. 



(b) When we have to do only with a 

 deformity, and its reappearance is 

 probable, the parties to the marriage 

 should both be warned of the probability 

 of the reappearance of that defect in 

 their offspring. 



(c) When we have to do with blind- 

 ness, and its reappearance is doubtful, 

 a stricter attitude toward the union can 

 be assumed. 



(d) When we have to do with blind- 

 ness, and its reappearance is probable, 

 the question assumes its most serious 

 aspect. If the family history shows 



Ed. 



*Risley, S. D.: Jiereditary Aniridia, The Journal Amer. Med. Assn., April 17,1915, p. 1310. 

 ^ Matings between brothers and sisters are not essential to the formation of Mendelian ratios. 



