Fig. 6: The impact of the juxtaposition effect on generating genetic variability and its application in crop breeding.

a, Stimulating crossovers in heterozygous regions enables the creation of more allelic combinations in populations with high levels of homozygosity (that is, with high inbreeding coefficient). When large chromosomal segments are homozygous, crossovers within these regions produce gametes that retain the parental allele configuration (top panel). By contrast, crossovers in heterozygous regions always result in recombinant gametes, generating novel allelic combinations (bottom panel). b, Potential application of the juxtaposition effect in commercial crop breeding. The diagram illustrates a scenario in which a breeder aims to transfer valuable variation (QTL1) from one parental line into another. After crossing, doubled haploids are generated, each representing a unique recombinant product of the two parents. Due to the short genetic distance between QTL1 (from parent 1) and QTL2 (from parent 2), the likelihood of obtaining a recombinant is very low. Selection of doubled haploid (DH) lines based on polymorphism patterns allows identification of ideal partners for subsequent crosses in the breeding program. The resulting progeny show a Juxtaposed configuration in the region encompassing QTL1 and QTL2, substantially increasing the chances of obtaining a recombinant. Alternative doubled haploid lines are shown in grey; these facilitate the QTL1/QTL2 combination while retaining a genetic background predominantly from parent 1. A second round of doubled haploid production facilitates the development of a new elite line enriched with QTL1.