9.8: Linkage and Mapping
- Page ID
- 27268
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)As we learned in Chapter 6, Mendel reported that the pairs of loci he observed behaved independently of each other; for example, the segregation of seed color alleles was independent from the segregation of alleles for seed shape. This observation was the basis for his Second Law (Independent Assortment), and contributed greatly to our understanding of heredity. However, further research showed that Mendel’s Second Law did not apply to every pair of genes that could be studied. In fact, we now know that alleles of loci that are located close together on the same chromosome tend to be inherited together. This phenomenon is called linkage, and is a major exception to Mendel’s Second Law of Independent Assortment. Researchers use linkage to determine the location of genes along chromosomes in a process called genetic mapping. The concept of gene linkage is important to the natural processes of heredity and evolution.
- 9.8.1: Linkage
- Mendel’s Second Law does not apply to every pair of genes that could be studied. In fact, we now know that alleles of loci that are located close together on the same chromosome tend to be inherited together. This phenomenon is called linkage, and is a major exception to Mendel’s Second Law of Independent Assortment. Researchers use linkage to determine the location of genes along chromosomes in a process called genetic mapping and is important to natural processes of heredity and evolution.
- 9.8.2: Recombination
- In heredity, recombination is any process that results in gametes with combinations of alleles that were not present in the gametes of a previous generation. Interchromosomal recombination occurs either through independent assortment of alleles whose loci are on different chromosomes. Intrachromosomal recombination occurs through crossovers between loci on the same chromosomes . In both cases, recombination is a process that occurs during meiosis (mitotic recombination is relatively rare).
- 9.8.3: Linkage Reduces Recombination Frequency
- This page explains complete linkage in genetics, where closely situated loci on a chromosome segregate together during meiosis without crossover events, resulting in a recombination frequency of 0%. This rare occurrence leads to inheritance of parental combinations of alleles, with no recombinant types detected in the gametes.
- 9.8.4: Crossovers Allow Recombination of Linked Loci
- This page explains recombination frequencies in genetics, focusing on incomplete linkage between loci on a chromosome that are sufficiently spaced to allow crossover events during meiosis. It describes how crossovers occur during prophase I through DNA exchange between non-sister chromatids, resulting in recombinant genotypes.
- 9.8.5: Inferring Recombination From Genetic Data
- This page explores the difficulties of inferring allele arrangements in genetic studies without direct chromosome analysis. It highlights the importance of parental genotypes in determining allele configurations (cis or trans) and details an experiment with dihybrid organisms from pure-breeding lines. A testcross with recessive individuals helps identify gamete genotypes, enabling calculation of recombination frequencies from observed phenotypes.
- 9.8.6: Genetic Mapping
- This page explains that genetic maps use recombination frequencies to estimate distances between loci in centimorgans (cM), with 25% recombination reflecting roughly 25 cM apart. These maps help visualize locus arrangements, aiding gene identification through linkage analysis. Accuracy diminishes over longer distances due to multiple crossovers, which can lead to underestimation of distances. Genetic maps are valuable in crop breeding, evolutionary studies, and human disease research.
- 9.8.7: Mapping With Three-Point Crosses
- This page discusses the three-point cross method for mapping three linked genes simultaneously. It involves crossing pure breeding lines to create a trihybrid and testcrossing to observe phenotypic distributions, allowing for the identification of rare double crossover events. The method aids in calculating recombination frequencies, correcting for undetected double crossovers, and improving genetic mapping accuracy.
- 9.8.S: Linkage and Mapping (Summary)
- This page discusses recombination, a process that generates new allele combinations in gametes influenced by chromosomal proximity. Unlinked loci can have 50% recombination, while linked loci maintain parental combinations. Meiosis crossovers enable recombination and frequencies aid in genetic mapping, though double crossovers can lead to distance underestimation.
Contributors
Dr. Todd Nickle and Isabelle Barrette-Ng (Mount Royal University) The content on this page is licensed under CC SA 3.0 licensing guidelines.