Chromosome mapping Flashcards
Manners in which genes are inherited during meiosis
-Independent Assortment
-Linkage without crossing over
-Linkage with crossing over
Independent Assortment
-Genes on two independent (different) chromosome Ms segregate independently
-No linkage exhibited
-Four genetically different gametes formed in equal proportions
Linkage without crossing over
-Genes linked on same chromosome but no crossing over occurs between genes
-2 different, genetically identical gametes formed
-complete linkage produces only two parental/non-crossover gametes but this rarely occurs
-usually when two genes are in very close proximity to each other
Linkage with crossing over
-Exchange between two nonsister chromatids of the four chromatids in a tetras
-Forms two crossover gametes and two noncrossover
-Frequency of crossing over is proportional to distance between loci
Linkage ratio for dihybrid
-organisms heterozygous at both loci are mates
-F2 ratio is unique as there is no independent assortment
-instead of 9:3:3:1 ration in F2, phenotypic. Ratio is 1:2:1
Example of linkage ratio
Heavy wing(hv) and brown eye (bw)
Wild type (hv+/bw+)
Determine distance between genes
-uses crossovers
-% of offspring with recombinant gametes can be used to estimate distance between 2 genes
-Chance of complete linkage is low as usually some offspring will have crossovers
-closer genes are, lower crossover frequency
-studies on X-linked genes illustrate how linkage can be used to map genes
-Recombination frequencies can be used to determine distance
Single crossovers
-only two nonsister chromatids involved in the exchange
-if a single crossover occurs 100% of the time, 50% of gametes are recombinat
-if a number of recombinants is 20%, then exchange has occurred in 40% of tetrads
Possibilities with singel crossovers
-single crossover occurs on nonsister chromatids but not between loci of interest, no recombinant gamete produced
-crossover occurs between alleles in nonsister chromatids, recombinant gamete produced
-cross over between sister chromatids which has no effect
Multiple crossover analysis
-occurs in tetras
-Enabled production of more extensive maps
-used to sequence 3 or more genes
-study with 3 gene pairs (heterozygous alleles and double crossover gametes)
Product law
Probability of a double crossover is the product of each individual probability
3 point mapping criteria
1:Genotype of organism must be heterozygous at all loci under consideration
2:Genotypes of all gametes can be accurately determined by observing phenotype of offspring
3: Sudficient number of offspring must be produced in the mapping experiment to recover a representation sample of all crossover classes
Determine order of genes in a sequence
-Gene sequence is not known in mapping experiments
-note all possible arrangements
-Examine observed phenotype frequencies in outcome of single and double crossovers
-Gametes with single swapped allele indicate which genes are located at edges or flanking regions
-double crossovers will have lowest frequency
-compare frequencies to order of possible arrangements and determine which is best fit
Current chromosome mapping
-More complex organisms with larger genomes are difficult to map using normal methods
-DNA markers or SNPS are used
DNA marker
Short segment of DNA with known sequence and location
-used as landmarks for mapping, E.g Restriction Fragment Length Polymorphisms, microsatellites, short tandem repeats
