The Avuncular Index (from the Latin "avunculus" for "uncle") can be used to help identify the situation where the alleged father is the brother of the true father (uncle to the child).
The Avuncular Index (AI) tests the hypothesis that the tested man is a paternal uncle of the child versus the hypothesis that the tested man is unrelated to the child.
Paternity Index (PI)
Avuncular Index (AI)
= X / Y
= (X + Y) / (2 * Y) =
= ((PI) + 1) / 2
cumulative AI =
= PRODUCT (AI for individual alleles)
The ratio of PI to AI tests the hypothesis that the tested man is the father versus the hypothesis that he is an uncle to the child.
ratio of cumulative PI to cumulative AI =
= (cumulative PI) / (cumulative AI)
Interpretation:
(1) If the man has not been definitely excluded or confirmed, AND
(2) If there are individual alleles with high PIs favoring paternity while the remaining favor nonpaternity, AND
(3) If the cumulative AI > cumulative PI (ratio PI to AI < 1)
THEN the man may be the brother of the true father and uncle to the child.
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