Meiosis, linkage and crossing over

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Meiosis, Linkage, Crossing over

Meiosis, Linkage, Crossing over

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  • 1. MEIOSIS. LINKAGE. and. CROSSING OVER
  • 2. LINKED GENES • Genes located close together on the same chromosome • Linked genes travel together during meiosis, eventually arriving at the same destination (the same gamete), and are not expected to assort independently
  • 3. LINKAGE • Describes the tendency of genes to be inherited together as a result of their location on the same chromosome; measured by percent recombination between loci. • Linked genes do not assort independently
  • 4. Concepts • Recombination is the sorting of alleles into new combinations. • Inter-chromosomal recombination, produced by independent assortment, is the sorting of alleles on different chromosomes into new combinations. • Intra-chromosomal recombination, produced by crossing over, is the sorting of alleles on the same chromosome into new combinations.
  • 5. Notation for Crosses with Linkage • In analyzing crosses with linked genes, we must know not only the genotypes of the individuals crossed, but also the arrangement of the genes on the chromosomes
  • 6. Complete Linkage Compared with Independent Assortment
  • 7. Crossing Over with Linked Genes • Linkage is rarely complete—usually, there is some crossing over between linked genes (incomplete linkage), producing new combinations of traits • Crossing over, which takes place in prophase I of meiosis, is the exchange of genetic material between non-sister chromatids
  • 8. Coupling and Repulsion • In crosses for linked genes, the arrangement of alleles on the homologous chromosomes is critically important in determining the outcome of the cross
  • 9. Coupling • Wild-type alleles are found on one chromosome and mutant alleles are found on the other • Allelic arrangement in which mutants are on the same chromosome and wild-type alleles on the homologue • The cis configuration
  • 10. Coupling phase (cis)
  • 11. Repulsion • Each chromosome contains one wild-type and one mutant allele • Allelic arrangement in which each homologue chromosome has mutant and wild-type allele • The trans configuration
  • 12. Repulsion phase (trans)
  • 13. Calculation of Recombination Frequency • The percentage of recombinant progeny produced in a cross is called the recombination frequency, which is • calculated as follows:
  • 14. Diskusi Suatu organisme pentahibrid yang mempunyai genotip Aa Bb Cc Dd Ee. Diketahui bahwa gen B dan C berangkai tidak sempurna secara coupling phase pada satu kromosom, sedangkan gen D dan E berangkai sempurna secara repulsion phase pada kromosom lainnya. Adapun gen A tidak berangkai dengan gen-gen lainnya , pada kromosom yang lain a) Apabila individu tersebutmembentuk gamet, ada berapamacamgamet yang dibentuk dan sebutkan macam- macamgamet tersebut b) Gambarkan secara jelas kemungkinan-kemungkinan sinapsis yang terjadi saatmetafase I (gambar tersebut berupa skema kromosomdengan gen-gen yang dibawa)
  • 15. A two-point test cross to detect linkage in fruit flies
  • 16. RESULTS AND CONCLUSION • In the 2300 offspring (bottom row) of an actual cross, about 1909 of the offspring belong to each of the two parental classes (83% total), and 391 belong to each of the two recombinant classes (17% total) • Thus, loci for wing length and body color are linked on a homologous chromosome pair
  • 17. Calculating the frequency of crossing over reveals the linear order of linked genes on a chromosome
  • 18. Concepts • During a single meiotic division, crossing-over may occur at several different points along the length of each homologous chromosome pair • In general, crossing-over is more likely to occur between two loci if they lie far apart on the chromosome and less likely to occur if they lie close together • A genetic map of the chromosome can be generated by converting the percentage of recombination to map units • 1% recombination between two loci equals a distance of 1 map unit
  • 19. Result • By convention, 1% recombination between two loci equals a distance of 1 map unit, so the loci in our example are 17 map units apart
  • 20. Gene Mapping • Gene order (that is, which locus lies between the other two) is determined by the percentage of recombination between each of the possible pairs
  • 21. Gene Mapping • In this hypothetical example, the percentage of recombination between locus A and locus B is 5% (corresponding to 5 map units) and that between B and C is 3% (3 map units). • There are two alternatives for the linear order of these alleles.
  • 22. Predicting the Outcomes of Crosses with Linked Genes • In cucumbers, – smooth fruit (t) is recessive to warty fruit (T) and – glossy fruit (d) is recessive to dull fruit (D) • Geneticists have determined that these two genes exhibit a recombination frequency of 16%.
  • 23. Predicting the Outcomes of Crosses with Linked Genes • Suppose we cross a plant homozygous for warty and dull fruit with a plant homozygous for smooth and glossy fruit and then carry out a testcross by using the F1 • What types and proportions of progeny will result from this testcross?
  • 24. Solution • Four types of gametes will be produced by the heterozygous parent – two types of non-recombinant gametes ( T D and t d ) – two types of recombinant gametes ( T d and t D ) • The recombination frequency tells us that 16% of the gametes produced by the heterozygous parent will be recombinants
  • 25. Solution • Because there are two types of recombinant gametes, each should arise with a frequency of 16% : 2 = 8% • All the other gametes will be nonrecombinants; so they should arise with a frequency of 100% - 16% = 84% • Because there are two types of nonrecombinant gametes, each should arise with a frequency of 84% : 2 = 42%
  • 26. Solution • The progeny of the cross result from the union of two gametes, producing four types of progeny • The expected proportion of each type can be determined by using the multiplication rule, multiplying together the probability of each uniting gamete