In principles there exist 3 types of information of an animals breeding value, the parents, own performance and offspring. The information from half and full sibs as well as from grandparents are all included in the parent breeding value. This is both a strength and a weakness of the direct updating method as the parents only can be included once. Therefore new information on half and full sibs can not be utilized. Usually it is of little extra value, so in many cases it is an advantage not to bother about this information.
The information to estimate breeding value from the 3 sources has the following forms. The heritability is designated h2 and common environmental variance c2, the estimated breeding value has the symbol A hat. The formulas for combining information from the 3 possible sources can be seen below.
An example on this condition. An index has been made on full sibs offspring and included in the combination index. A new index on more full sib to the first ones cannot be included without violating the rule, as they are related to the old index both through the father and the mother. In contrast half sib offspring can be added when new information comes in, as they are only related through the animal in question.
Do you want to see examples and use an applet for direct updating of breeding values, click here