Please enable JavaScript.
Coggle requires JavaScript to display documents.
RECOMBINATION & CHROMOSOME MAPPING (Linkage maps (Map units…
RECOMBINATION & CHROMOSOME MAPPING
Chiasma formation
It is the visible manifestations of crossover
occurs during pachytene/zygotene of meiotic prophase 1
brings our the recombination
(Morgan)
Chromosomal recombination
Inter
Unlinked genes
follows the
Independent assortment
Testcross progeny 1:1:1:1 ratio
produces 50% of recombination
Intra
Linked genes
Testcross progeny 1:1:1:1 ratio
produces
less than
50% of recombination
bc the genes are close together ==> less chance of getting crossing over
Recombination frequency
Key of chromosome mapping
gives a clue to the distance between them
The further the distance between genes :arrow_right: The higher the frequency
Ranges from 0-50%
0%: No crossover => genes are close together :arrow_forward:
Complete Linkage
50%: crossover is likely to occur => genes are further away/ at different chromosomes
:warning: cannot exceed 50% bc the 2 outside chromatids don't recombine (
even in the unlinked genes
)
the frequency with which a single chromosomal crossover will take place between two genes during meiosis
Linkage maps
a table for a species or experimental population that shows the position of its known genes relative to each other in terms of
recombination frequency
based on recombination frequency
developed by Morgan & Sturtevant
:no_entry: relative position and physical position are not necessarily the same
Map units
mu
centimorgans(cM)
1% of recombination frequency = 1cM
Linear map :arrow_right: corresponds to the linear arrangement of genes :arrow_right: makes up the chromosome (
suggestion
)
Three point testcross
a cross of a trihybrid (triple heterozygote) to a triply recessive tester
produces 2 parental gametes BUT 6 recombinants
crossover
single
double
gives the rarest frequency of recombination
for recombination frequency => should count these as twice
only the middle genes are altered
produces double recombinant chromatids that have the parental allele combination.
Interference
crossover in one region inhibits crossovers in other region
Double cross is the exception of Interference
quantified by
coefficient of coincidence
= observed no. of double crossovers/ expected no. of double crossover
Interference = 1 -
coefficient of coincidence
I=0 :arrow_right: no interference :arrow_right: observed and expected are equal
I=1 :arrow_right: complete interference :arrow_right: no observed crossover
I<1 (usually) :arrow_right: observed < expected