LINKAGE: violates “independent assortment”

1/22/92, rvsd 1/26/94, 1/19/96, 1/22/97, 1/23/98, 19 Jan 00, 19 Jan 01, 21 Jan 04, 19Jan05, 24Jan08

SGML p. 95-, GMSLG: p. 123-, 7^th: pp 142-153, 9^th: 129-140

Exceptions (violations) of Mendelian-predicted behavior can yield new insights

Bateson and Punnett studying sweet pea,

             cross: PP.LL x pp.ll                                       phenotype               expected:          observed

             Purple/white, Long/round grains:                  PL                            3911                  4831

             PL x pl: PpLl (p 142), then selfed                  Pl                             1303                  390

             F₂ (selfing) did not produce 9:3:3:1            pL                            1303                  393 

             ratio: (note excess of parental types)            pl                             435                    1338

postulated “coupling”                                                                                Total:               6952

Morgan found similar prob in Drosophila (purple eyes, vestigial wings x w.t.),

introduced test cross technique

page 131: pr/pr.vg/vg x pr+/pr+.vg+/vg+. F1: pr/pr+.vg/vg+ (then crossed times pr/pr.vg.vg)

TEST CROSSED: (notation: + = wildtype)                phenotype               expected:          observed

allows direct analysis of genotype of parent               ++                            710                    1339

(use doubly homozygous recessive)                            pr vg                        710                    1195      

should get 1:1:1:1 if independent, got:                     + vg                         710                    151

(excess of parental types)(p. 142)                              pr +                          710                    154

                                                                                                                     Total:               2839

Morgan postulated both genes on same chromosome, segregate together (violated Mendel’s 2^nd law)

If genes are located on same chromosome, how do they separate, as in the rare progeny above?

RECOMBINATION:  meiosis generates 1N genotype different from either of parental 1N genotypes.

Evidence: Can be seen as chiasmata, mark exchanges between non-sister homologous chromosomes

Note difference between     interchromosomal recombination:      same as Mendel’s independent assortment

                                            intrachromosomal recombination:      by recombination

“Recombinant type” phenotype defined as one trait from one parent, one trait from other parent

Recombination frequency: (total recombinant events/total progeny) = distance between in %

             Interchromosomal freq =      50%     (25% of each recombinant type in test cross)

             Intrachromosomal freq =      <50%   zB: fr Morgans expt: (151 + 154)/2839 (total) = 0.107, (10.7 %)

Seen as chiasmata during meiosis

[2008: OMIT?:

Linkage of genes on X chromosome easier than autosomal:

              male is hemizygous, his phenotype = genotype

(y = yellow body) (+ = brown) / (+ = red eye) (w = white eye) P: yw+/yw+ fem x y+w/Y male       

zB: heterozygous female x male                                          phenotype     expected:          observed

 will produce sons reflecting females genotype:     yw:                      1128                                          43

Drosophila:                                                                          y⁺w                1128                                 2146      

                                                                                             yw⁺                1128                                 2302 

                                                                                             y⁺w⁺              1128                                 22

Count only males:                                                                                       total                                 4513

              F₁: yw⁺/y⁺w x yw⁺/Y:  rec freq: (43 + 22)/4513 = 1.4%

LINKAGE MAPS: (p 129 and 139)

Sturtevant (undergrad student of Morgan) proposed linear relationship on chromosome:

             greater distance loci, more prob of cross over:

             genetic map unit = distance producing 1% recombinants.