62 A SWEDISH FARMER'S LINEAGE 



invisible links from generation to generation, from family to family and from 

 country to country. Below, we shall give examples of this. 



The table shows that there must be a propensity for a certain recessive 

 disease to appear in one or several of the children. If one of the parents is 

 effected by the disease, and the other is a healthy homozygot, all the children 

 will be sound, though heterozygots. If the propensity of the disease is a domi« 

 nant attribute, so in the last mentioned case, all the children will be affected. 

 On the other hand, if both the parents are sound, although heterozygots with 

 propensity for a certain recessive disease, 25 % of the children will be affected 

 This is a verj^ common occurrence with human beings, that is to say, an epilep* 

 tic or a person of weak mind, may have healthy parents or even forefathers for 

 several or many generations, for heterozygotism can be inherited from generation 

 to generation, without the disease appearing. 



In such families in which the propensity of the disease of a recessive nature 

 is inherited, one often finds one or of few cases of the disease here and there 

 in different households, whereas other families or branches are free from the 

 same. The disease appears within households with collateral expansion, though 

 seldom in 2 or more successive generations. Numerous marriages between rela» 

 tions increase the number of such cases apparently, as persons with similar here* 

 ditary propensities are thus often united. 



Moreover it is of great importance to remember, that not a few people, 

 who appear to be quite sound, are often bearers, and transmitters of serious pro* 

 pensities, they are, so called »conductors». 



From the present scientific standpoint, we cannot unfortunately beforehand 

 distinguish between homozygotically and heterozygotically sound persons, which 

 however would be of great importance, because heterozygots often, in future ge« 

 nerations, generate persons affected by the disease. 



In our investigations, we have progressed so far, that we know that the male 

 ancestor of the Lister family was without doubt a heterozygot, the female ancestor 

 was most probably a healthy homozygot. As I have just shown, Mendel's law 

 demands that 50 % of the children should be heterozygots with a propensity for 

 the disease and 50 % healthy without any propensity. Let us now see if this 

 agrees with the true position within this family. The number of the ancestral 

 parents' children was 6, of which three in all probability would be heterozygots, 

 and three healthy homozygots. 



How can we know if this is the case or not, as all are apparantly sound? 

 »By the fruit we know the tree.» On investigation, we find that 3 of the branches 

 of the family had unhealthy offspring, when married with relations, but not the 

 other three. The matter thus stands as we had expected. 



From the heredity table we find further, that 2 unhealthy individuals (females 

 of the Blekinge family) were married, and had offspring, none of whom were 

 diseased. Even this circumstance is quite in agreement with Mendel's law. For, 

 if one of the parents, is a healthy homozygot, (without any propensity) and the 



