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Chapter 7 – Linkage, Recombination, and Eukaryotic Gene Mapping

Chapter 7 – Linkage, Recombination, and Eukaryotic Gene Mapping. Genetic Principles. Principle of Segregation Diploid organisms have 2 alleles for each gene Separate during meiosis – only one gamete enters each gamete Principle of Independent Assortment

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Chapter 7 – Linkage, Recombination, and Eukaryotic Gene Mapping

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  1. Chapter 7 – Linkage, Recombination, and Eukaryotic Gene Mapping

  2. Genetic Principles • Principle of Segregation • Diploid organisms have 2 alleles for each gene • Separate during meiosis – only one gamete enters each gamete • Principle of Independent Assortment • 2 alleles of a gene separate independently from alleles at other loci/other genes

  3. Chromosomes • Chromosomes follow independent assortment IF: • Genes are located of different chromosomes BUT: • If genes are on the same chromosome, they tend to travel together • Linked genes – close together on the same chromosome

  4. Sweet peas – dihybrid cross • P generation purple, long x red, round • F1 generation – all purple,long • Prediction for F2 generation – ratio of 9:3:3:1

  5. Sweet pea – dihybrid cross cont • Expected F2 phenotype ratios is not observed • Conclusion – genes for flower color and pollen shape must be located close together on the same chromosome • Why are any recombinant progeny seen?

  6. Crossing over • If 2 genes are on the same chromosome, but far apart, crossing over can allow for recombination of gametes • Genes very far apart on the same chromosome will always be separated by crossing over, and are not considered to be linked

  7. Notation for linked genes • Horizontal lines indicate actual chromosome A_________B a b *individual heterozygous for 2 different genes where both dominant alleles are on one chromosome, and both recessive alleles are on its homologous chromosome • Can be abbreviated by AB/ab

  8. Testcross for linkage • For determination if two genes are linked (close together on the same chromosome) or not • Set-up: • One individual heterozygous for both traits x individual homozygous recessive for both traits

  9. Testcross for linkage cont • MmDd x mmdd • If not closely linked, alleles will assort independently • MmDd individual can form 4 different types of gametes • 50% recombinant offspring/50% non-recombinant offspring

  10. Testcross for linkage cont • MD/md x md/md • If closely linked, 2 alleles will always travel together • all offspring are non-recombinant

  11. Testcross for linkage cont • Can be separated by crossing over • Small number of recombinant progeny/chromosomes is seen

  12. Crossing over • Single cross over produces 50% nonrecombinant chromosomes (same configuration as parental chromosome) and 50% recombinant chromosomes (new allelic combination)

  13. Recombination frequency • = number of recombinant progenyx 100 total number of progeny Values from slide #11 8 + 7 15 55+53+8+7 = 123 = 12.2% or .122 • Smaller the recombination frequency = more closely linked

  14. Coupling and Repulsion • For heterozygous individuals • Cis configuration/coupling • Both wildtype alleles are on one chromosome; both mutant alleles are on the homologous chromosome • Trans configuration/repulsion • Each chromosome has one wildtype allele and one mutant allele

  15. Recombination • Interchromosomal • Between genes on different chromosomes • Independent assortment/random segregation during Metaphase/Anaphase I • Produces 50% recombinant/50% non-recombinant gametes • Intrachromosomal • Between genes on same chromosome • Crossing over during Prophase I • Usually produces recombinant gametes less than 50% • Unless very far apart on the same chromosome

  16. Genetic mapping • Relative position of different genes based on recombination rates • Does NOT state actual chromosome, or position (locus) • Distance measured in map units or centimorgans (cM) • 1 m.u. (or cM) = 1% recombination

  17. Genetic mapping example • A and B = 5 m.u. • A and C = 15 m.u. • B and C = 10 m.u. • A and D = 8 m.u. • B and D = 13 m.u. • C and D = 23 m.u. • Any genes with 50% recombination are either on different chromosomes, or very far apart on the same chromosome (crossing over always separates them)

  18. Physical mapping • Locates gene to a specific chromosome/region of chromosome • Deletion mapping • Chromosome deletion studies – how phenotype is affected/what genes may be missing • Duchenne m.s. • X linked disease – but where on X? • Some affected males have small deletions – common deleted area must be where gene is located

  19. Somatic cell hybridization • Fusion of 2 cell types (altered by viruses or tumor cells to allow cell lines – uninhibited growth) • Somatic cells • Heterokaryon – 2 distinct nuclei • Eventually fuse • Most chromosomes are lost (differentially from one type) • Human chromosomes usually lost, only a few remain • Human genes expressed in hybrid cell lines must be located on retained chromosomes • deletion studies can give more specific location on chromosome

  20. Molecular Analysis • Fluorescence In Situ Hybridization (FISH) • Probe complementary to gene sequence will bind to DNA • Gene sequence/partial sequence must be known • DNA sequencing • Yields base pair distance between two genes

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