GENE INTERACTION: ALTERNATIVE MENDELIAN RATIOS

rvsd 1/12/94, 1/20/95, 1/19/96, 1/17/97, 1/21/98, 14 Jan 00, 17 Jan 01, 22 Jan 01, 16 Jan 04, 14 Jan 05, 25jAN08

GMSLG: 7^th, p106-130, GWLC: pp 221-249

Alleles: alternate form of the same gene (from “other”) (I prefer “given form of a gene”) (p 224: haplosufficient)

CODOMINANCE: P 226  Blood groups, multiple alleles: I, I^A, I^B (A and B dominant over O, co-dominant in AB)

LETHAL RECESSIVE causes ratios in cross of 2:1 Pleiotropic effects genes affect phenotype, but also survivability

              yel x norm mice= 50/50, but yel x yel= 2/3 yel, 1/3 norm. p 227

              Cats: Manx x Manx=2 Manx + 1 norm, p 229

DIHYBRID RATIOS WHICH INTERACT CAN DISPLAY ALTERNATIVE TO MENDELIAN RATIOS:

Construct table of modified phenotypic ratios, explain that all are manifestations of the 9:3:3:1 ratio.

1)           classic:                  four phenotypes (AB, Ab, aB, ab, 9:3:3:1)*: yellow smooth/green wrinkled peas

              parakeets:            green times green: 9green, 3 blue, 3 yellow, 1 white

2)           Complimentary gene action (requires both A and B) thus A-B- (p . 107, 108)                                 9:7 ratio

              one of two phenotypes, either requires nutrient or does not [also color in peas, white vs purple]

                            purple flowers                    white flowers (precursor and intermediate colorless) (p 237, 238)

              Metabolic pathways: 9 A-B- grows on minimal medium, 7 require nutrient supplement (p 232)

              Test with complimentation test: form heterokaryon in fungi (239)

3)           Recessive epistasis (stands upon, masks out): masks effect of functional gene down stream:     9:3:4 ratio

              Labrador retrievers: (p 117) ee is recessive epistatic, unable to make precursor:

                            gold converted to brown by E, brown to black by B: [E permits expression of color genes]

                          E enz B enz

              gold----------------chocolate----------------black             (Also white to pink to blue Blue Eyed Marys)( p 243)

              Ratio: 9 black (B-:E-), 3 chocolate (bb:E-), 4 golden (any with -- ee)

              Also, albino in many animals: B-C- (black, bbC- (brown, B-cc (albino)

4)           Dominant epistasis (Repressor) (from Russell’s 4th, p.114) [p. 245]                                          12:3:1 ratio

              In summer squashes, suppressor W prevents pigmentation, Y is yellow, y is green

                                W blocks Y converts

              colorless----------||-----------green------------------------yellow

              F2:         12 white (W-,--), 3 yellow (ww,Y-), 1 green (ww,yy)

5)           Duplicate genes: either A or B will convert white to red flowers. [p. 119 in GMSLG]                  15:1 ratio

                            Only aabb will be white, thus 15:1

another example of classic 9:3:3:1:

Coat color in mice:    A gene                 distributes color in hair, produces mottled hair shaft       |AB:     agouti

pp 120-121                   B gene turns        in aa: brown pigment to black in undistributed                |Ab:     cinnamon

                                                                  in A-: cinnamon to agouti in distributed pigment            |aB:      black

                                                                                                                                                            |ab:      brown

              F₁:         AABB (agouti) x aabb (brown) or Aabb (cinnamon) x aaBB (black): 100% agouti

              F₂:         AaBb times self:           9 agouti, 3 cinnamon, 3 black and 1 brown 

              (Squash: AB = patty-pan (9), Ab or aB = round (6), ab = long(1))

[Poor example of dominant epistasis:

In Foxglove flowers:

         W makes repressor, prevents deposition of red pigment, i.e. W- -- will be white

         D- makes lots of anthocyanin pigment, dd low level

  W- -- prevents ww D- deposits reddish                ww dd:

white (12)------------------dark reddish (3)--------------------------light reddish (1)