Genetic Linkage And Two Point Mapping Flashcards
Bateson and Punnet
Flower colour (purple/red) and seed shape (long/short) Sweet Peas. Cross:
P1 Purple long x red short PPLL x pp ll
P2 All purple long PpLl
Self fertilise F1 Excpect 9:3:3:1
Look at booklet for specific numbers
Linkage
Chromosomes inherited as units; but far more gene loci than there are chromosomes. Mendel’s
Second Law (independent assortment) cannot always apply.
However, observed an excess of parental and shortage of recombinant classes compared to 9:3:3:1.
Must be “coupling” (now called linkage) of the two loci. However, testing for statistical deviations
from a 9:3:3:1 ratio is not efficient and requires very large sample sizes.
TH Morgan developed the test cross for linkage in drosophila
- test for deviations from free recombination in F2 individuals
Better than tests that look for deviations from a simple Mendelian 9:3:3:1 ratio, as each phenotypic class is genotypically homogenous and all expected ratios the same size
BACKCROSS
A cross with a parental genotype
TESTCROSS
A cross with a double mutant
Look at document for F2 proportions expected on free recombination (expect EQUAL FREQUENCIES of each phenotypic class in the F2 - a 1:1:1:1 ratio if there is free recombination
This is statistically easier to test
A and D, B and C
A and D – PARENTAL CLASSES (same combinations of phenotypes seen in the P1 individuals).
B and C – RECOMBINANT CLASSES (new combinations not seen in the P1 flies).
Observed: an EXCESS of PARENTALS (A + D) and a DEFICIENCY of RECOMBINANTS (B + C)
Look at document for actual numbers
Morgan’s hypothesis
The loci are linked together, and are inherited to some extent a single unit.
Perhaps though this deviation from Mendel’s second law has something to do with the properties of the mutants (epistatic interactions) rather than being related to their physical association
Look at document for actual again
Morgan’s hypothesis 2.0
the deviation from Mendel’s second law is due to the physical association
of the mutants and not to some intrinsic property of the genes themselves is PROVED. This
association, whether in coupling or repulsion, is now referred to as LINKAGE, and later studies
showed that it arises from the physical association of the two loci on the same chromosome.
Why do we get recombinant at all?
During meiosis each chromosome replicates itself. Two of the homologous chromosomes (one maternal and one paternal) ‘zip up’ (termed synapsis) and exchange DNA (form chiasmata) due a physical process of breaking and joining. If chiasmata form between the two linked loci, you will form a recombinant gamete. If chiasmata form outside of the two loci, you will form the parental genotype.
Frequency of chiasmata
The frequency of chiasma formation is dependent upon the distance of the two loci. The further
apart two loci are, the greater the probability of chiasma formation between the loci. I.e. the further
apart two loci are, the greater the frequency of recombinant phenotypes in the F2.
Sturtevant
Sturtevant’s leap
in knowledge was to use the frequency of recombinant loci to calculate the relative distance
between two loci:
P1 Purple vestigial (prpr vgvg) x Red normal (pr+pr+ vg+vg+)
F1 All red normal prpr+ vgvg+
Look at doc for all numbers
Recombination frequency
Recombination frequency = recombinant/total
For example:
44/400 = 0.11 (or 11%)
This means the relative map distance between the vg and pr loci is 11 cM (centi Morgans) or MU
(Map Units)
