chapter 7 three point test cross

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region 2 map distances

# SCO + DCO / total progeny * 100

the amount of interference varies

- from one chromosome to another - from one part of the chromosome to another - with gene location on the chromosome arm - with species - when genes regulate crossovers

steps in analyzing data from a three-point test cross

1. identify the parents (most frequent class) 2. identify the double crossovers (least frequent class) 3.determine gene order (which gene is in the middle) 4. re-write the gametes with the correct gene order 5.determine products of crossovers in each of region 1 and region 2 6. determine recombination frequencies in each of region 1 and region 2 7. map the genes

trihybrid testcross

8 different classes are in this cross

region 1 map distances

= # SCO + DCO / total progeny * 100

region 1 map distances

= % recombinants between p and j

coefficient of coincidence

= observed frequency of double crossovers / expected frequency of double crossovers

region 2 map distances

=% recombinants between j and r

decreases

Accuracy of our genetic map _________ if (1) genes are far apart and (2) there are double crossovers

to determine the origin

__________- of the other two recombinant classes, we need to look at the effect of crossovers between pairs of genes arranged in the correct order

genetic map distance

_______________ equals recombination frequency if genes are close together and there are no multiple crossovers between the genes

lack of independence

a ______________ in these two events (region 1 and 2 crossovers). once a crossover occurs in region 1 the probability of a crossover in region 2 is reduced (or vice versa) this is probably due to physical interference cause by breaking and redoing the chromatids

3 point test cross

a test cross involving three linked genes in a small region of the chromosome

3 point test cross

a test cross of a trihybrid

to get better maps (more accurate map)

a) study genes that are closer together b) use a 3-point test cross (trihybrid test cross) c) both a and b increase the accuracy of the genetic map

DCO

always the least frequent

parental phenotypes

always the most frequent

double crossover

for our map the probability of a ___________ is 0.208 * 0.10 = 0.0208 or 2.08% -this would be the expected number of __________

there are no multiple crossovers genetic distances between genes are small

genetic map distance = recombination frequency when

genes are far apart on the chromosome or there are multiple crossovers between genes

genetic map distance does not equal recombination if

independent

if a crossover is __________ the probability of a double crossover is the product of the probabilities of the two ____________ events (product rule)

independent

if double crossovers are _____________ we would expect 2.08%

interference

is measured by the coefficient of coincidence

interference

is when we observe fewer double crossovers than we expected based on the map distances between two genes

DCO

means double crossover

SCO

means single crossover

double crossover

occurs in both region 1 and region 2 and therefore is added into the recombination frequencies for the regions

two strand double crossover

problem occurs when the genes are far enough apart that more than one cross-over occurs between them

double crossover

recombinant chromosomes resulting from the _________ have only the middle gene altered

interference

reduces DCOs - when region 1 or region 2 prevents the other from happening.

region 2 map distances

the distance between j and r

region 1 map distances

the distance between p and j

with a third gene in the middle and a double crossover

the middle allelic pair changes orientation relative to the outside allelic pairs

3 point test cross

used to determine the recombination frequencies among the 3 genes and therefore, the relative distances between these genes.

3 point test cross

used to map genes to linkage groups

trihyrbid (a+ c+ b+/ a c b)

when mapping genes in a _________ the gametes in which one gene changes its orientation relative to the other two tells you which of the 3 genes in the ________ is in the middle


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