Meiosis, linkage and crossing over
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Transcript of Meiosis, linkage and crossing over
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MEIOSIS.
LINKAGE.
and.
CROSSING OVER
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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
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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
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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.
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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
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Complete Linkage Comparedwith Independent Assortment
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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
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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
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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
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Coupling phase (cis)
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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
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Repulsion phase (trans)
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Calculation of Recombination Frequency
• The percentage of recombinant progeny produced in a cross is called the recombination frequency, which is
• calculated as follows:
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DiskusiSuatu 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 laina) 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)
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A two-point test cross todetect linkage in fruit flies
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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
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Calculating the frequency of crossing overreveals the linear order of linked
genes on a chromosome
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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
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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
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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
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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.
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Predicting the Outcomes ofCrosses 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%.
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Predicting the Outcomes ofCrosses 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?
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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
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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%
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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