How are loops in a genetic pedigree used to show inbreeding?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

In the construction of a pedigree, horizontal lines represent mating and siblings, while vertical lines connect parents to their offspring. When inbreeding occurs, it is visually represented in the diagram by one or more loops.12

Each loop in the pedigree signifies a path of descent that traces back to a common ancestor shared by both parents. Because these loops indicate that the parents are related by consanguinity, they demonstrate the increased probability that their offspring will inherit identical alleles from that shared forebear. The total coefficient of inbreeding for an individual is calculated by summing the values derived from all such loops that include the individual's parents.1