Question

why does continuous inbreeding may lead to sterility?

Answer 1

~Inbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck. In general, the higher the genetic variation within a breeding population, the less likely it is to suffer from inbreeding depression.
Inbreeding depression seems to be present in most groups of organisms, but varies across mating systems.
~ Crosses between populations may restore heterozygosity, resulting in increased performance (heterosis), but may also lead to the disruption of coadapted gene complexes and to decreased performance (outbreeding depression)
~adaptive plasticity of offspring from selfing and from interpopulation crosses in response to nutrient addition was reduced. Outbreeding depression was also observed in response to stress
~Continuous inbreeding may drastically reduce the fitness of plants, but effects may be environment-dependent. When assessing the genetic effects of fragmentation and interpopulation crosses, the possible effects on the mean performance of offspring and on its adaptive plasticity should be considered.