Basics of Linkage and the Crossing Over.pptx

Basics of Linkage and the Crossing Over.pptx

• 1.
  Linkage and crossingover 1. Introduction to linkage 2. Morgan’s work 3. Difference between homologous and non homologous chromosomes 4. Difference between linked genes and unlinked genes 5. Comparison between complete and incomplete linkage 6. Parental and recombinant types 7. Crossing over 8. Cytological basis of crossing over 9. Cis and trans configuration of linked genes
• 2.
  Introduction • 2 genes(A and B): – Responsible for different traits of plant – Located on 2 different chromosomes (non-homologous) – Each gene has: • Two alternative forms (i.e. for Gene A there are 2 alleles= A and a ; for Gene B there are 2 alleles= B and b ) – These genes will independently assort to give F1 generation of genotype AaBb – The F1 generation of selfing produce 4 types of gametes equally (i.e. AB, Ab, ab, ab) 2 genes A and B located on Non- homologous chromosomes A B
• 3.
  • It wasMorgan who first discovered the phenomenon of Linkage • He crossed yellow bodies with white eyes females with wild type gray bodies and red males Drosophila – F1 females were wild type – F1 Males expressed both mutant traits • In F2 generation, – 98.7% of total offsprings showed the parental phenotype yellow bodied with white eyed flies and gray bodied with red flies – The remaining 1.3% of flies were either yellow bodied with red eyes or gray bodied with white eyes • He proposed that, – 1.3% flies with recombinant feature developed as a result of separation of genes • He postulated, – The separation of linked genes occurs due to formation of chiasmata which represent the point of genetic exchange – The term crossing over to describe the physical exchange leading to recombination – Two genes located relatively close to each other along a chromosome are less likely to have chiasmata between them as compared to the two genes which are farther apart on the chromosome Morgan’s work
• 4.
  A B Aand B genes Independently assort and produce F 1 generation AaBb a b A B a b A A a a B B b b AB AB ab ab
• 5.
  A A A A B B BB a a a a b b b b It produces gametes like AB, Ab, aB, ab equally • But 2 genes of two different traits not always present on two chromosomes (non- homologous) •2 / more genes- Reside on same chromosomes is said to be linked •These genes may be linked together on autosomal chromosomes or sex chromosomes •Linked genes= genes on same chromosome •Transmission pattern of these linked genes= Linkage A B 2 genes A and B located on Non- homologous chromosomes = Unlinked genes A A B B 2 genes A and B located on same chromosomes = Linked genes AB AB ab ab
• 6.
  •A and Bgenes- on same chromosome •So, In Dihybrid cross, AABB ✗aabb •Segregation happens in a way, •Genes tend to remain together •Do not exhibit independent assortment •Gametes formed by F1 progeny (AaBb) have 2 possibilities: • All gametes are of parental type • No recombination • Gametes of both parental type and recombination type • Recombination happened Complete linkage Incomplete linkage Transmission pattern of linked genes= Linkage •0 % recombinants •Genes linked closely •Shows parental type (genotype) •No crossing over •0-5% recombinants • Genes are linked but not in strong contact • Shows both parental and recombinant types (genotypes) •Crossing over occurs
• 7.
  A a B b AaBb ab a b a b a b A B A B A B A B A B A B a b a b a b A b a B A b A B A B a b Complete linkage Incomplete linkage Crossing over Note: A and B genes are linked together A B
• 8.
  Therefore recombinants areproduced by 2 different cellular processes: 1. Independent assortment 2. Crossing over Independent assortment: It always produces 50 %recombinants Crossing over: • Also produces recombinants • Does not occur between two specific genes in all meiosis • But when occur, half of the products of that meiosis are recombinants • Meiosis+ no crossing over • Between genes • Genes with percent recombination less than 50% are present in same chromosomes (Linked genes) • Genes with percent recombination equal to 50% either are in non-homologous chromosomes or are located far apart on single chromosomes(Unlinked genes) Produce only parental genotype for this genes A b A B A B a b Parental type (50%) Recombinant type / cross over type (50%)
• 9.
  Source: Benjamin APierce, Genetics, A conceptual approach, Page no.162, 165
• 10.
  Cytological basis ofcrossing over During meiosis, (Prophase I stage is generally subdivided into 5 stages i.e. 1. Leptotene- Bouquet stage 2. Zygotene- Synapsis, intimate pairing 3. Pachytene- Crossing over 4. Diplotene- Chiasmata appearance 5. Diakinesis- Proceeding to metaphase I Especially in Pachytene stage, As soon as homologous chromosome (Pairing) is done- Synapsis (intimate pairing) The cell may remain for days- A large recombination nodule (Large protein complexes, contain Rad51) appears at intervals on synaptonemal complex which mediates Crossing over It leads to recombination of genes present on 2 homologous chromosomes Many regions along the length of chromosome pair where crossing over frequencies are higher/ lower. • Region of higher crossing over frequencies= Hotspots • Region of lower crossing over frequencies= Cold spots
• 11.
  Source: Benjamin APierce, Genetics, A conceptual approach, Page no. 32 Peter J Russell, iGenetics A molecular approach, 3rd edition, Page no. 335 In, Diplotene, •Desynapsis begins •2 homologous chromosomes in bivalent to pull away from each other •Each bivalent remains joined by one/more chiasmata- represents sites where crossing over has occurred
• 12.
  Cis and Transconfiguration Source: Snusted, Simmons (2012), Principles of genetics, John Wiley & Sons, Inc, Sixth edition, Page no. 138 • Doubly heterozygous genotypes can be in two different configurations • When 2 linked genes on each chromosome are of the same type (i.e. both dominant, AB or both recessive, ab), the arrangement is called-coupling/ cis • When 2 linked genes on each chromosome are of different types (i.e. one dominant and one recessive allele, aB or Ab), the arrangement is called- repulsion/trans – In cis configuration, chromosome carrying allele combination either AB or ab- Parental combination – In cis configuration, chromosome carrying allele combination either Ab or aB- Recombinant combination – In trans configuration, chromosome carrying allele combination either Ab or aB- Parental combination – In cis configuration, chromosome carrying allele combination either AB or ab- recombinant combination