The genetic circumstances found in outbreeding species are more complicated than in inbreeding species: more than two alleles may be present at each locus and the linkage phases may vary across loci and between the parents of an experimental cross. The experimental design of inbreeding species often cannot be applied to outbreeding species. This chapter focuses on explaining in detail the genetic situations encountered in outbreeding species. It further describes the linkage analysis of a full-sib family of an outbreeding species.

Introduction

In the preceding chapters, we treated linkage mapping for experimental populations derived from a cross between two fully homozygous parents. Homozygosity is obtained by many generations of self-fertilization or sib mating, or by creating doubled haploids. For many plant species, self-pollination followed by self-fertilization (also called autogamy) is the normal mode of sexual reproduction. In some hermaphrodite species (i.e. species with organs of both sexes), a system of self-incompatibility or self-sterility causes an obstruction to self-fertilization. In dioecious species (dioecious means that individuals have organs of only one of the two sexes), self-fertilization is of course impossible. In outbreeding species, for which the normal mode of sexual reproduction is by crossing with other individuals (also called allogamy), forced inbreeding usually results in individuals with a poor viability. This phenomenon is called inbreeding depression. The consequence for such species is that linkage analysis can often only be performed using populations obtained by crossing relatively unrelated individuals.