by Ann T. Bowling PhD
Offspring resemble their parents to varying degrees, but the proportional genetic contribution of each parent is constant: half the genes of an offspring come from he the sire and half from the dam. A relatedness coefficient of 50% or 0.5 is assigned to the parent-offspring relationship.
Full siblings on average share 50% of their genes, based on the likelihood that 25% of the time they will have received the same genes from their dam and 25% of the time they will have received the same genes from their sire. It is possible, with a very small likelihood, that full siblings may have either no genes in common or all genes in common. The random assortment of chromosomes during gamete formation means that we cannot predict the exact proportion of genes that any two full siblings have in common; we can only provide an average value for full siblings as a group. In practical terms, stallion advertisements to the contrary, one cannot assume that a full brother to a proven sire will be an equally successful breeding horse.
Half-siblings on average share 25% of their genes, and first cousins share 12.5%. More complicated relationships can easily be calculated. In the simplest of pedigrees evaluations, breeders may talk of "percentage of blood." Of course blood is not the vehicle of inheritance, but is used in this context to imply genetic traits. These calculations provide the most probable proportion by source for an individual's genes by summing the relatedness coefficients for every occurrence of a particular ancestor in a pedigree. The relatedness decreases by half with each succeeding generation (Figure 1). The sum is not an exact proportion, but a statement of the most likely percentage. It is always possible that the true genetic proportion in common could be larger or smaller than that calculated as a relatedness coefficient.