GENE3200 - Linkage 2 (class 12)

recombinant offspring…

will always be less than (or equal to) 50% of the total offspring

recombinant offspring will always be less than or equal to 50% of the total offspring

-if crossing over occurs between the two loci, we are observing, M1 → M2 → 50% parental gametes, 50% recombinant gametes

-if crossing over occurs outside the two loci we are observing → M1 → M2 → parental gametes, parental gametes (with respect to the A and B locus)

upper limit to recombinant gametes

-maximum proportion of recombinant gametes that can be produced from a dihybrid is 50%

-meiosis with a single crossover results in half of the gametes being recombinant and the other half being recombinant

-even if crossing over between two genes takes place every meiosis, only 50% of the resulting gametes are recombinants

-because the location of crossovers is essentially random along the chromosome, the greater the distance between two loci, the more likely a crossover will occur between them

what does RF of 0.50 mean?

indicative of loci that are either on separate chromosomes, or are located very far apart on the same chromosome

mapping loci using RF

-maximum map distance than can be scored in a single cross = 50 m.u. (when loci are unlinked)

-unlinked loci may be on different chromosomes (linkage groups), or far apart on same chromosome

-two loci greater than 50 m.u. apart on the same chromosome can be mapped using intervening loci

double crossovers

can only be observed among three or more loci (between three loci, one or more crossovers can occur)

step 1:

-identify the types of progeny classes

-nonrecombinants are the largest 2 groups

-single crossovers have the middle numbers of progeny

-double crossovers are the smallest 2 groups

step 2

-identify the gene order

-the double crossovers help us identify the gene order

-since a crossover happens between each outside locus and the middle locus, whichever locus changes in the DCO (double crossovers) offspring is the middle locus

step 3

identify which offspring are recombinants between the outside loci and the middle locus

step 4

calculate the map distances between the outside loci and the middle locus

3 factor cross: point to keep in mind

there is more than one way to make triple heterozygote: don’t assume phase