There are several different ways in which relationships can be classified. Perhaps the most important is according to whether they are direct or collateral. Exceptions can be seen under the further headings of combined and clonal relationships.

  1. Direct versus Collateral

    Figure 15 Direct Relationships

    In Figure 15, A, C, E and G are all directly related to each other. G is also directly related to F, D and B, E to D and B, and C to B. The genes an individual inherits can only come from preceding direct relatives.

    Collateral relatives are related through common ancestors. They hold a proportion of their genes in common because they have received them from a common source. The important point is that no one receives any genes from collateral relatives.

    Figure 16 Collateral Relationships

    In Figure 16, D and E are collaterals (full sibs). So also are G and H (single first cousins), E and G (aunt-nephew) and D and H (uncle-niece). The common ancestors of all the above collaterals are A and B.

    Combined Relationships

    It is possible, if there has been any inbreeding such as a cousin marriage, for two people to be both directly and collaterally related.

    Figure 17 First Cousin Marriage

    In Figure 17, D is directly related to I (grandmother), D is also collaterally related to I (great aunt). The second relationship results from the fact that D and I have common ancestors in A and B

    Inbreeding can also cause other forms of double relationship such as two different kinds of direct or two different kinds of collateral relationship. Double collateral relationships can also occur without inbreeding. See Enhanced Half Sibs and the section on Double Collateral Relationships.

    Clonal Relationships

    Although identical twins, triplets and quadruplets are siblings, they are not related through common ancestors in the same way as normal sibs and are thus in a separate category (see here).

  2. Unilineal versus Bilineal Relationships

    When two relatives are connected through a single parent (of one or both of them), they are said to be unilineally related. Examples are: grandparent _ grandchild, parent _ child, half sibs, single first cousins and uncle/aunt _ nephew/niece. When they are related through both parents, as full sibs and double first cousins are, their relationship is bilineal. In the diagrams below, the relevant individuals and lines of descent are printed in red.

    Figure 18 Unililineal Relationships

    Figure 19 Bilineal Relationships

    This difference is mainly of genetic significance. The phenotypic correlation (or resemblance) between unilineal relatives for a particular character is largely due to the average quantitative effects of single genes in the population. i.e. The effects of single genes working alone and not interacting with their allelic partners. With bilineal relatives, an additional cause of resemblance is the effect of dominance interaction between pairs of genes. This is because bilineal relatives, such as full sibs and double first cousins, inherit both their genes, at a particular locus, from the same source (their two parents or four grandparents). Unilineal relatives, however, never have more than one shared gene resulting from their relationship. Without the chance of inheriting two genes jointly from a common source, dominance interaction can not be a cause of resemblance. Bilineal versus unilineal relationships also have relevance to the inheritance of harmful genetic abnormalities. Unilineal relationships between two people can allow dominant conditions to be passed between direct relatives or inherited jointly by two collaterals; but if the conditions is recessive, the relationship must be bilineal for both to be affected.

  3. 'Blood' versus 'Non-Blood' Relationships

    Relationship by 'blood' or 'consanguinity' is a true genetic relationship, where individuals pass genes directly from one to the other, or where they are related through recent common ancestors. 'Non-blood' or 'notional' relatives are only connected by marriage, remarriage or adoption. The term 'affinity' is sometimes used to cover relationship by marriage (i.e. step relatives and in-laws) but I prefer to use the more inclusive terms: 'notional', 'nominal' or 'non-blood' relationship, as they also cover adoption. Examples are: the spouse of a nominated person, spouses of collateral relatives and descendants, in-laws, step relatives, adoptive parents, adoptees and adoptee's spouses.

  4. Normal versus Enhanced Relationships

    Normal relationship values, as measured by the coefficient of relationship (R), are the standard values between individuals that have a single uncomplicated relationship and where no inbreeding is involved. It could be argued that double first cousins are common enough to be considered standard or normal. However, because even these are comparatively rare, it is probably better to treat all the double relationships described here as unusual and, therefore, enhanced. On the other hand,even with their double connections, see here, it seems reasonable to regard full sibs, single cousins and uncles/nephews as standard relationships. The following table shows standard values for the common types of relationship:

    Table 2 Standard Relationship Values

    Type of Relationship Coefficient of
    Relationship (R)

    Identical twins


    Full sibs, parent _ child


    Half sibs, uncle/aunt _ nephew/niece, grandparent _ grandchild


    Single first cousins, half uncle/half aunt _ half nephew/half niece, great-grandparent _ great-grandchild, great uncle/great aunt _ great nephew/great niece


    Single first cousins once removed, half first cousins, half great uncle/half great aunt _ half great nephew/half great niece


    Second cousins, half first cousins once removed


    Second cousins once removed, half second cousins


    Third cousins, half second cousins once removed


    Enhancement of relationship values can happen in four different ways:

    1. As a result of inbreeding, which increases the number of inheritance pathways and common ancestors. For example, full sibs, whose parents are single first cousins, have a raised multiple relationship of 0.5625 compared with the standard value of 0.5 (see also here, here and here).

    2. Half sibs are normally produced when one person (the common parent) marries twice and has children by each spouse. If the two people he or she marries are themselves related this raises the relationship between the half sibs above 0.25. It also enhances the coefficients of their descendants above the standard values, (see here).

    3. The various exotic types of double or multiple relationships described here and here can also raise R above the standard value without inbreeding.

    4. Finally, if full sibs, which are present in several relationship chains, are replaced by identical twins, this usually doubles the coefficient of relationship (R). To distinguish between the two levels in Table 3, the enhanced relationships are given the title, first degree, (see also here , here and here)

    Table 3 Enhancement through Identical Twins

    Type of Relationship [5]

    Coefficient of Relationship (R)

    Double first cousins (first degree)


    Double first cousins


    Uncle/aunt _ nephew/niece (first degree)


    Uncle/aunt _ nephew/niece


    Single first cousins (first degree)


    Single first cousins


    Great uncle/great aunt _ great nephew/great niece (first degree)


    Great uncle/great aunt _ great nephew/great niece


    Single first cousins once removed (first degree)


    Single first cousins once removed


    Double second cousins (first degree)


    Double second cousins


    Second cousins (first degree)


    Second cousins


    Second cousins once removed (first degree)


    Second cousins once removed


    Third cousins (first degree)


    Third cousins


  5. Half versus Full Relationships

    This classification only applies to collateral relatives. Half collateral relatives are connected through half sibs; e.g. half sibs, half first cousins, half uncle _ half nephew, half second cousins, half third cousins.Full collateral relatives are connected through full sibs; e.g. full sibs, single first cousins, second cousins, third cousins, double first cousins, uncle/aunt _ nephew/niece.

  6. Single versus Double Relationships

    Double relationships can occur between two individuals either because of inbreeding or because they have at least two separate links through other relatives. Inbreeding causes double relationships in both direct and collateral relatives. This classification is not the same as unilineal v. bilineal since some double relationships are unilineal. Although not usually regarded as such, full sibs are the simplest form of double relationship, since they are related through both parents. It follows, therefore, that full cousins and uncles/nephew are also doubly related because of their connections through full sibs; yet they only have a unilineal relationship. Half sibs, half cousins and half uncles etc are the singly-related forms of these relationships.

    The apparent contradiction of single first cousins having a double relationship is due to the hierarchical nature of double first cousins, and is confusing (see also here). However, as a specific relationship in its own right, 'first cousins' can be ether single or double (figure 20).

    Figure 20 Single versus Double Relationships

  7. Regular (Symmetrical) versus Irregular (Asymmetrical) Double Relationships

    Regular double relationships have the same relationship repeated on both sides of the family, whereas irregular forms are related differently on the two sides. R values for regular doubles can always be expressed in terms of (1/2)n, where n is a whole number, but most of those for irregular doubles can not.

    Figure 21 A Regular Double Relationship (Double Half First Cousins)

    Figure 22 An Irregular Double Relationship

    The double relationships between J and K are: left side - first cousins; right side - half first cousins.

  8. Relationships Between versus Within Generations (See also here)

    Relationships between generations cover all direct relationships plus removed cousins (see here) and uncles/aunts - nephews/nieces. Relationships within generations include all types of sibs and contemporary cousins (i.e. first cousins, half first cousins, second cousins, etc.)

[5] The replacement of full sibs by identical twins in the two rare classes of double relationships here will likewise increase their R values and lead to further combinations. (Not shown).