Genetic Linkage & Two-Point Mapping - 06

how are chromosomes inherited?

• as units

• far more gene loci than there are chromosomes

• Mendel’s Second Law can’t always apply

what did Bateson & Punnet find out

• flower colour and seed shape, sweet peas cross

• observed an excess of parental and shortage of recombinant class compare to 9:3:3:1

  • must be coupling (now 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 size

who developed the test cross for linkage in Drosophila?

• TH Morgan

• test for deviations from free recombination in F² individuals

• better than tests that look only for deviations from a simple 9:3:3:1 ratio, as each phenotypic class is genotypically homogenous and all expected ratios are the same size

• two recessive mutants - PR vs PR+ vs VG vs VG+

  • PR+ is dominant to PR and VG+ is dominant to VG

what is the difference between a test cross and a back cross?

• backcross - a cross with a parental genotype

• testcross - a cross with a double mutant

what are the F² proportions expected on free recombination?

• equal frequencies of each phenotypic class in the F² - a 1:1:1:1 ratio if there is free recombination

• classes A & D - parental classes

• classes B & C - recombinant classes (new classes)

• observed - an excess of parentals and an excess of recombinants

what is Morgan’s hypothesis?

• the loci are linked together and are inherited to some extent as a single unit

• though, this deviation from Mendel’s Second Law has something to do with properties of the mutants (epistatic interactions), rather than being related to their physical association.

what does this experiment & the second experiment show?

• mutants in coupling (two mutants or two wild types in each parental strain)

why is Morgan’s hypothesis proved?

• the hypothesis that the deviation from Mendel’s second law is due to the physical association of the mutants, not to some intrinsic property of their genes themselves is proved.

• this association, whether in coupling or repulsion is LINKAGE.

• later studies from the physical association of the two loci on the same chromosome

why would you get recombinants at all?

• during meiosis, each chromosome replicates itself

• two of the homologous chromosomes (one maternal, one paternal) zip up (synapsis) and exchange DNA (form chiasmata) due to a physical process of breaking and joining

• if chiasmata form between two loci, you will form a recombinant gamete

• if chiasmata form outside of the two loci, you will form a parental genotype

what is the frequency of chiasma formation dependent on?

• dependent on the distance of the two loci

• the further apart the two loci are, the greater the probability of chiasma formation between the loci

  • the greater the frequency of recombinant phenotypes in the F2

what is Sturtevant’s leap?

• use the frequency of recombinant loci to calculate the relative distance between two loci

• recombination frequency = recombinants/total