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#genetic linkage

13 public questions tagged with this topic.

Plant Breeding 4

Linkage drag refers to:

Linkage drag describes physical association along chromosome between desired donor gene and neighboring deleterious or undesirable genes inherited together due to limited recombination near centromere or introgression segment. When donor parent contributes disease resistance gene for example, flanking chromosomal region spanning several megabases containing genes for poor quality, late maturity, low yield, or bitterness is co-transferred because crossing over between closely linked loci is rare per meiosis, frequency proportional to map distance in centimorgans. Until recombination breaks link

Ref: Hospital F. Linkage drag – theoretical; Young & Tanksley 1989 RFLP study of drag. Nature Biotechnology

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Microbial genetics

Co-transduction frequency is inversely proportional to

Co-transduction scoring after P1 or P22 generalized transduction measures how often two bacterial markers transfer together in same phage head. Packaging capacity limits fragment size; genes within same head length co-occur frequently. If distance between genes exceeds headful, they cannot fit together, co-transduction drops to zero. Intermediate separations show recombination between them inside recipient reducing co-inheritance. Therefore frequency is inversely proportional to map distance: very close genes show near 100% co-transduction, farther genes show lower percentages, enabling fine-s

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Cotransduction Frequency and Distance

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Microbial genetics

Genes close together show high frequency of

During natural transformation, competence apparatus imports fragments of extracellular DNA typically tens of kilobases long. If two genes lie close on same DNA piece, they enter same recipient together rather than independently. Close linkage shows high co-transformation frequency because single incoming molecule carries both alleles. As intergenic distance increases, probability molecule breaks between them rises, reducing co-inheritance. Co-transformation frequency thus inversely measures physical distance, analogous to cotransduction. Mapping exploits this to order genes and estimate spacin

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Transformation Mapping and Linkage

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Linkage and Mapping

LOD score of 3 indicates linkage is

LOD transformation uses base 10 logarithm, so LOD = log10[Odds]. Inverting logarithm gives Odds = 10^LOD. Therefore LOD 3 corresponds to 10^3 = 1000, meaning data observation is one thousand times more probable if loci are linked at estimated recombination fraction than if unlinked. This odds interpretation motivated choice threshold because random human genome loci have prior 50:1 probability unlinked, thus 1000:1 yields genome-wide significance approx p 0.05 after accounting multiplicity. LOD -2 similarly corresponds to 100:1 odds against linkage, serving exclusion criterion.

Ref: Hartl & Jones, Genetics: Principles and Analysis, 6th ed., Chapter: LOD Score Interpretation 1000:1

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Linkage and Mapping

LOD score is used to assess

LOD score, logarithm of odds, provides statistical evaluation of likelihood that two loci are linked at certain recombination fraction theta versus unlinked theta 0.5. In human pedigrees experimental crosses impossible, so likelihoods computed from family segregation data combining phase information, marker informativeness and penetrance models. Log10 ratio summed across families yields cumulative evidence for linkage across kindreds. Positive values favor linkage, negative values exclude linkage. Concept introduced by Morton and applied widely for mapping Mendelian disease genes before dense

Ref: Morton, LOD Score Method, Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 4: LOD for Human Linkage

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Linkage and Mapping

Recombination frequency is same in cis and trans because

Recombination frequency quantifies probability of exchange between two loci measured as proportion recombinant progeny. Exchange probability depends on physical length of DNA, chromatin structure, and map distance, not on how alleles are distributed across homologs in the parent. Cis AB/ab and trans Ab/aB both possess identical interlocus chromatin and identical chance of chiasma formation. Arrangement merely defines which gametes are labeled parental versus recombinant after meiosis in testcross. After correcting labeling, calculated RF identical because crossover mechanism occurs independent

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 5: Recombination Frequency Independent of Coupling Phase

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Linkage and Mapping

In trans configuration, parental gametes are

Trans configuration also termed repulsion places dominant allele of one locus and recessive allele of second locus on same homolog, genotype written Ab / aB. During meiosis without crossing over between loci, these chromosomes segregate intact, so gametes carrying Ab and aB constitute parental nonrecombinant class observed at high frequency. Recombinant class AB and ab appears only after single crossover exchanges segments. Cis configuration AB/ab reverses parental identities but does not alter physical distance, therefore recombination frequency remains governed by interval length and chromat

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 4: Cis-Trans Configurations and Gamete Types

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Linkage and Mapping

Cis configuration is also known as

Cis configuration termed coupling phase describes linked heterozygote where both wild-type alleles reside on one homologue and both mutant alleles on other, notation + + / a b. Trans or repulsion phase carries wild allele opposite mutant on each homologue, + b / a +. Phase influences which phenotypes appear as parental versus recombinant but does not change map distance between loci measured by recombination frequency. Understanding cis versus trans essential for pedigree risk prediction, DNA diagnostics and interpreting Morgan's Drosophila crosses where parental classes correspond to alleles

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 5: Cis as Coupling Phase; Griffiths et al., Chapter 5: Phase Terminology

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Linkage and Mapping

Genes separated by ≥50 cM show

Genes separated by fifty centimorgans or more experience frequent crossover events along interval including single and double exchanges that randomise chromatid combinations so parental and recombinant gametes appear equally frequent indistinguishable from independent assortment. Genetic map distance measured in centimorgans equals one percent recombination over short intervals but beyond fifty becomes non-additive without mapping function correction like Haldane or Kosambi. Hence genetically distant loci behave as unlinked even though physically on same chromosome showing fifty percent recomb

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 5: Large Map Distances Mimic Independent Assortment

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Linkage and Mapping

Genetic linkage refers to the tendency of genes to

Genetic linkage describes co-segregation of loci located on same chromosome during meiosis because parental allele combinations remain on same chromatid unless crossover separates them. Independent assortment traditionally applies to genes on different chromosomes where homologue segregation randomises combinations, or distant syntenic loci where recombination approaches fifty percent effectively randomising. Linked genes form linkage group showing excess parental gametes in test crosses. Thomas Hunt Morgan first documented linkage in Drosophila with body colour and wing size establishing chro

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 5: Linkage Co-inheritance; Morgan 1911 Science

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Recombination

Maximum recombination frequency between two loci is

Recombination frequency measured as percentage recombinant progeny increases with physical distance between loci because greater separation provides larger opportunity for crossover. Maximum theoretical frequency for loci on different chromosomes or far apart on same chromosome is one half, corresponding to independent assortment where 50 percent gametes recombinant and 50 percent parental. At this limit loci behave as unlinked, obeying Mendel's second law. Frequency plateau at 0.5 defines unlinked status important for constructing linkage maps and calculating genetic distances in centiMorgans

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 9: Maximum Recombination Frequency 0.5 Explanation

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