Ancestry5_medium.jpg (JPEG Image, 1500x768 pixels) Learn.Genetics™ The Role of Inbreeding in the Extinction of a European Royal Dynasty. The kings of the Spanish Habsburg dynasty (1516–1700) frequently married close relatives in such a way that uncle-niece, first cousins and other consanguineous unions were prevalent in that dynasty. In the historical literature, it has been suggested that inbreeding was a major cause responsible for the extinction of the dynasty when the king Charles II, physically and mentally disabled, died in 1700 and no children were born from his two marriages, but this hypothesis has not been examined from a genetic perspective. In this article, this hypothesis is checked by computing the inbreeding coefficient (F) of the Spanish Habsburg kings from an extended pedigree up to 16 generations in depth and involving more than 3,000 individuals. The inbreeding coefficient of the Spanish Habsburg kings increased strongly along generations from 0.025 for king Philip I, the founder of the dynasty, to 0.254 for Charles II and several members of the dynasty had inbreeding coefficients higher than 0.20.
Outbreeding depression. In biology, outbreeding depression occurs when offspring from crosses between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population.[1] The concept is opposed to inbreeding depression. This phenomenon can occur in two ways. First, intermediate genotypes may be disfavored. For example, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. Gene flow between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population. A second cause of outbreeding depression may be the breakdown of biochemical or physiological compatibilities between genes in the different breeding populations. However, it is critical to understand that reduced inbreeding depression in first generation hybrids can, in some circumstances, be strong enough to more than make up for outbreeding depression.