Second division segregation (SDS) indicates
Second division segregation indicates crossover occurred between gene locus and centromere, keeping allelic differences heterozygous after first division despite reductional segregation of centromeres. Daughter nuclei at end meiosis I still contain both alleles on sister chromatids, so alleles separate only at meiosis II equational division producing alternating spore patterns like AAaa or 2:2:2:2 alternation displaying recombination. Presence SDS demonstrates recombination between centromere and gene, diagnostic for distal position. Frequency SDS increases with gene-centromere distance, allow
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 5: SDS Indicates Gene-Centromere Crossover