Pedigrees of Negro families / by R. Ruggles Gates.

  • Reginald Ruggles Gates
Date:
1949
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Pedigrees of Negro families / by R. Ruggles Gates. Source: Wellcome Collection.

    17/290 page 3
    woman, II.6. Now generation III shows the offspring of these two brothers. Let us notice first that III.6 and III.7, who marry, are first cousins, the children of brothers. This is indicated by the double line between them. III.6 was the youngest child in her family. Her eldest sister (III.l) had an illegitimate son (IV.1), the illegitimacy being indicated by the broken lines. The other members of this sib- ship (family) are a brother (111.3), who is unaffected, and a pair of twins (III.4, 5), boy and girl, one of whom is syndactylous, the other not. These are evidently fraternal or nonidentical twins, no more alike than ordinary brother and sister but born at the same time. The youngest child, III.6, is the one who married her cousin. Now let us look at the rest of generation III. II.5 and II.6 had five children: an eldest son (III.7), then a normal daughter, then another daughter (III.9) who was affected, and finally a pair of identical (monozygotic) girl twins. This is indicated by the line drawn between them. They might, of course, be of the same sex without being identical. In generation IV we have in one sibship five children (IV.2-6), two of whom are syndactylous, from the cousin marriage, and finally three children (the younger son affected) from the marriage of III.9. Looking at the pedigree as a whole, we see a total of 10 cases of syndactyly (5c? 1 , 5 $) in four generations. Every affected child has one affected parent. There are five different families of children (sibships) in the pedigree, and they contain a total of 9 affected: 12 normal, counting the identical twins (derived from a single fer tilized egg or zygote) as one. Now 9:12 is near a 1:1 ratio (well within the limits of chance), and these relationships fulfill the con ditions necessary for a dominant inheritance. In a dominant pedi gree, one parent and about half the offspring in each family usually are affected. Only half the children are affected because the affected parent had in turn one normal and one affected parent. He or she is therefore heterozygous or hybrid for the condition. In other words, he has a single dose of the gene for syndactyly, so that half his germ cells will carry this gene and half the corresponding normal gene. As a result, half the offspring in each generation from marriages with one normal and one affected parent will be affected. A dominant pedigree thus is easily recognized. Certain other symbols in Fig. 1 can now be explained. The small symbol at the end of the last line represents a miscarriage. A figure over a symbol indicates the number of the individual. For
    digitised image 1
    digitised image 2
    digitised image 3