Chapter 5 - Chromosome Mapping

no linkage exhibited

formation of 4 genetically different gametes in equal proportions

complete linkage

produces only parental gametes (non-crossover gametes)

recombinant linkage

produces 2 new allele combinations (crossover gametes) and 2 chromatids parental

crossing over

results in recombination of alleles between homologs and always occurs during tetrad phase

the distance between the genes

the frequency of crossing over between any 2 loci on a single chromosomes is proportional to…

cross heterozygous with homozygous recessive → if not completely linked then 4 phenotypes will be produced

how to use test cross to prove is genes are linked

parental types

offspring that have the same phenotype as the parent

recombinants

offspring that have new combinations of phenotypes

50

once __% of the offspring are recombinants, then two genes are not linked, even if they are on the same chromosomes

linkage ratio

unique F2 phenotypic ratio when complete linkage exists between 2 genes because of close proximity and organisms heterozygous at both loci mated

determine distance between genes but NOT sequence of genes

single crossover events can held determine “what” but not “what”

1. genotype of organism producing crossover gametes must be heterozygous at all loci

2. cross must e constructed so that genotype of all gametes can be accurately determined by observing phenotypes

3. a sufficient number of offspring must be produced in experiment to recover a representative sample of all crossover classes

3 criteria for 3-point mapping