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#genetic selection

3 public questions tagged with this topic.

Plant Breeding 4

MAS is most effective when markers are:

Efficiency of marker-assisted selection hinges on recombination frequency between marker and causal gene influencing linkage disequilibrium persistence. If marker is loosely linked at 20 cM, crossover occurs in 20% gametes each generation, decoupling marker allele from desired trait allele, leading to false positive selections and loss of target and reduced genetic gain. Tightly linked markers situated within 1-2 cM or preferably gene-based intragenic markers such as functional SNPs in coding region show recombination below 1%, ensuring near-complete co-segregation with trait across breeding cycles. This reduces need for large populations to identify recombinants and maintains diagnostic accuracy across diverse germplasm and genetic backgrounds without breaking association. For foreground selection in backcrossing, flanking markers within few hundred kilobases minimize linkage drag and confirm intact gene presence. Advances in whole genome sequencing identified perfect markers derived from causal polymorphism itself, providing 100% selection accuracy. Marker distance therefore directly determines reliability and economic benefit of MAS pipeline and adoption success. Haplotype based selection using tightly linked SNP haplotypes improves diagnostic power beyond single marker, especially in diverse germplasm where single marker may lose linkage phase; constructing haplotype blocks around

Ref: Michelmore RW. 1995 Molecular dissection – tight linkage. Collard & Mackill 2008 tightly linked effectiveness

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Plant Breeding 4

MAS selects plants based on:

Marker-assisted selection replaces unreliable phenotypic screening with direct interrogation of DNA sequence polymorphisms linked to trait-controlling loci. Instead of measuring disease severity in field that depends on weather and inoculum load, MAS uses polymerase chain reaction amplification of SSR, SNP, or SCAR markers situated within few centimorgans of target gene, detecting presence of donor allele regardless of environment, plant stage, or dominance interactions. Segregating individuals are genotyped at seedling stage, selection made before flowering, accelerating backcross programs and enabling pyramiding of multiple resistance genes that are phenotypically indistinguishable. DNA markers are codominant, neutral, abundant across genome, and not influenced by G×E interaction, so breeding value predicted from marker haplotype reflects genotype faithfully. This paradigm shift from phenotype to genotype selection improves efficiency for traits difficult to score such as root characters, quality, and recessive alleles and reduces phenotyping costs. Functional markers derived from causal genes such as Pi21, Wx1, and opaque2 provide perfect selection accuracy; KASP and TaqMan SNP assays enable high throughput automated genotyping in breeding pipeline, allowing selection at seedling stage and rapid cycling without field phenotyping or biochemical assays for quality traits.

Ref: Tanksley SD 1989 BioTechnology – RFLP; Collard & Mackill 2008 Philos Trans – MAS based on DNA markers

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Plant Breeding 3

Single Seed Descent (SSD) emphasizes selection in:

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.

Ref: Goulden CH. SSD method – Crop Breeding; Nature Plants Speed breeding, Watson et al. 2018

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