Practice question

Single Seed Descent maintains genetic diversity by advancing one random seed from each F2 plant forward without selection until homozygosity increases. Active natural or artificial selection during early segregating generations would favor competitive or early flowering types and eliminate potentially superior recombinants whose phenotype is not yet expressed due to heterozygosity and dominance masking. By postponing rigorous phenotypic evaluation to F5-F6, when lines approach 94-97% homozygosity under selfing, additive genetic variance becomes fixed and heritability rises substantially. In F6, single plant progenies are grown in replicated plots, allowing accurate measurement of yield, quality, and disease resistance governed by multiple QTLs. This strategy concentrates breeder effort on lines that breed true, maximizes retention of rare recombinants, and aligns selection efficiency with the theoretical increase in homozygosity at 0.5 per generation of selfing while conserving space and labor during intermediate generations. Genomic selection can be overlayed on SSD populations to predict breeding values early, but phenotypic selection remains delayed to exploit maximal homozygosity and repeatable performance before final cultivar release decisions.