L6 Flashcards
What is the classic way of making maps
using three point test crosses; by utilizing the information derived from crossing a triple heterozygote with a triply recessive tester
true or false, recombination frequencies from each pair of genes can be used to locate their relative positions
true; also allows us to establish whether these genes belong to the same or different linkage groups
true or false, three point test cross allows us to determine whether crossing over in one region of the chromosome has any effect on crossing over in another region
true
what do you call the possible interaction in a three point test cross
interference
why are using a pair of genes for mapping a chromosome not as efficient as when three genes are used
this is because numerous two point crosses must be carried out and because double crossovers are missed
why is three point test cross more efficient in comparison to a two point test cross
with three genes the order of the genes can be determined using a single set of offspring and double crossovers can usually be detected, this provides a more accurate map distances
what does it mean when the recombination frequency of the two genes are significantly less than 50%
means they are linked
what does it mean when the two genes are greater than 50%
means that the genes are independently being assorted and are thus unlinked and must be drawn on separate chromosomes
what type of genotype do we want our homozygous recessive test cross to be crossed to
w+/w r+/r (can be other symbols, but one wild type and one recessive)
true or false, map distances are not exactly additive, eg. distances between v and ct and ct and cv add up to 19.6, this is greater than the calculated 18.5
true
why are map distances not exactly additive
this is because the progeny of double crossovers are the same as the parents with respect to the two outside markers
true or false, recombinant progeny of a double cross over have their outer loci the same as the parental type
true
how do you account for the double cross over phenomenon in two genes; how do you adjust for their map units
put the observed output for the double crossovers twice in the equation then sum it up and divide by the total number of progeny
Since map distances are based on the recombination rates, how will multiple crossovers that have remained undetected, how can we overcome this implication
Can be overcome in two ways:
- insert more markers between the genes in question
- use mapping functions that correct for multiple cross over products
what is interference
it is when the cross over in one region inhibits the cross over in other region
how do you calculate the expected frequency of recombination
multiply the two pairs of genes that are not crossing over together; their recombination frequency (not in %)
How do you calculate the expected double crossover values
multiply the expected frequency of crossing over with the total number of progeny
how do you calculate the coefficient of coincidence
observed number of double crossovers (total)/ expected number of double cross overs
How do you calculate the Interference value
1 - coefficient of coincidence
What does the value of interference mean, eg. if I= 34%
This indicates that 34% of the expected double crossover progeny were not observed because of interference
What does it mean when I= 0
this means that there is no interference ( observed and expected frequencies are equal)
What does it mean when I = 1
this means that there is complete interference, with no observed double recombinants
true or false, I is usually < 1, that is there is fewer double recombinants expected
true
