GRAPHIC REPRESENTATION OF RELATIONSHIPS AND FAMILY TREES

Diagrams are the best way of showing relationships clearly. There are several systems in common use and each is designed for a special purpose. The following diagrams show the most common methods:-

1. The first method is the one used by genealogists to construct family trees. It may be presented in one of four ways:

   Figure 1 Family Tree Diagrams

   a)		b)
   c)		d)

2. The second method is used by geneticists to facilitate the calculation of the coefficients of relationship and inbreeding. I also find it a more logical and less ambiguous method when describing different types of relationship.
   

   Figure 2 Path Diagram

   

   The arrows represent the genetic contribution of each parent to his or her offspring. With the first and second methods the sexes are denoted by the usual sex symbols and , or they may be obvious from the first names.

   
   
3. The last method is used by geneticists studying inherited defects and disease conditions in humans. Females are symbolized by small circles and males by small squares . Symbols of parents are joined by a horizontal marriage line, and their offspring's symbols are placed in a horizontal row below a line to which they are connected. The horizontal line above the symbols for the children is itself connected to the parents' marriage line by a vertical line. The symbol for a single child is directly attached to this vertical line. In a pedigree, sibs are listed from left to right in order of birth. Each individual in a pedigree is designated by a number in order to facilitate reference to him/her. The numbering system may be consecutive from the earliest generation to the most recent, or each generation may be denoted by a Roman numeral and the individuals within a generation by Arabic numerals. Thus, II-4 identifies the fourth individual listed in the second generation of the pedigree.

Figure 3 Genetic History Diagram

A diamond denotes that the sex of the person was unknown to the recorder of the pedigree. A number enclosed in an open symbol indicates the number of sibs of the same sex who are not listed separately. If an individual possesses the trait whose inheritance forms the subject of the pedigree, he or she is said to be 'affected' and is designated by a black symbol. A hollow circle or square signifies absence of the trait ('not affected'). In the above pedigree the marriage of II2 and II3 was childless and II5 and II6 are sibs whose parents are not included in the pedigree. The twins III4 and III5, are identical, as indicated by the short vertical line descending from the sibship line. The twins III6 and III7 are non-identical, as shown by their separate connection with the sibship line. Further examples are shown below:

Figure 4 Sex-linked Colour Blindness (Stern, 1960, see also here )

A
B
C
D

Open circles and squares are sometimes used as an alternative to the symbols and to denote the sexes when unusual names are encountered (see here)

Family Trees

For display purposes there are four basic forms:-

1. It is traditional to begin at the bottom of the page and expand upwards like a tree.
2. The alternative is to start at the top and work downwards.
3. It is also sometimes more convenient to work from left to right.
4. A more unusual form commences at the centre and expands radially. This method probably makes the best use of space in large families but is not as aesthetically pleasing.

The amount of information to include for each person or family is limited by the space available. Dates of birth, death or marriage should have priority. If space is limited, locations where people were born, lived, married and died may have to be restricted to early members of the family and where there was a migration to a different area or country. With occupations, only unusual professions or ranks should be included. In both cases extra information should be kept in a separate record.

If the scope of the family tree is very large, different branches of the family could be presented on separate sheets in such a way that when they are all placed together they make an expanded composite record. Where data have been collected from several researchers who have emphasised their own sides of the family, one has to be ruthless in giving priority to ones own family at the expense of more distant collaterals.

Standard formats which sometimes appear in books on genealogy as prepared charts are never very successful. The problem is that they give equal weighting to all branches of the family. On the one hand, this wastes space because some ancestors can not be traced, and on the other, families vary in size so that not enough space is given to some families and too much to others. For correct spacing each family tree should be specially planned either manually or by computer.

Shown below is how a pedigree should be presented for relationship calculations:

Pedigree Diagram Format for Relationship and Inbreeding Calculations

Most pedigree diagrams used in genealogy follow the format described here with vertical and horizontal lines linking the subjects:

Unfortunately, this type of diagram is unsuitable for determining the number and lengths of paths between related individuals. It also suffers from the disadvantage, that if there has been any inbreeding, some individuals may appear more than once in the pedigree. This is particularly true of Pedigree Certificates issued by the Kennel Club and Cattle Breed Societies, which always show 2 parents, 4 grandparents, 8 great-grandparents and so on. For example, the following pedigree for a Great Dane dog (M), showed that he was the result of a first cousin mating. It was taken from the Kennel Club diagram supplied by the breeder:

Figure 5 Great Dane Pedigree (Letters used to save space)

To avoid these difficulties, it is important that all pedigree diagrams are first converted to the 'path' format (shown below), checking that each individual is only included once, before starting any relationship calculations:

Figure 6 Path Diagram of the Pedigree in Figure 5