Two GenesUnlinked

Aa Bb x aa bb

AB Ab aB ababAaBb AabbaaBbaabb

1AB: 1Ab: 1aB: 1ab

Two GenesLinked

A B/a b x ab/ab

AB abab AB/abab/ab

1AB: 1ab

Perform Test Crosses to Determine if Genes Are Linked

Prophase IPaired Homologous Chromosomes

(tetrad of 4 chromatids)

Chiasmata(Crossing-over points)

F yy ww x M y+ w+ Yyellow body, brown bodywhite eyes red eyes

P

F1

If genes independently assort1y+ w+ Y: 1y+ w Y: 1 y w+ Y: 1y w Ybrown body brown body yellow body yellow body

red eyes white eyes red eyes white eyes

F2

F y+y w+wbrown body

red eyes

M y w Yyellow bodywhite eyes

x

Actual Data4484 y+ w+ Y:4413 y w Y: 76 y w+ Y: 53y+ w Y

brown body yellow body yellow body brown bodyred eyes white eyes red eyes white eyes

“Parental Classes” “Recombinant Classes”4484 y+ w+ Y 76 y w+ Y

brown body, red eyes yellow body, red eyes

4413 y w Y 53 y+ w Y yellow body, white eyes brown body, white eyes

4484 + 4413 x 100 = 99% 76 + 53 x 100 = 1% 9026 9026

F yy ww x M y+ w+ Yyellow body, brown bodywhite eyes red eyes

P

F2:

F y+y w+w x M y w Y brown body,

red eyes

F1

y w

Crossing Over in the F1 femalesGenerates the Recombinant Gametes

y+ w+ y+ w y w+

X

F2

F w+ m+/w+ m+ x M w m/Yred eyes, white eyes,wt wings miniature wings

P

F1 F w+ m+/w mred eyeswt wings

M w+ m+/Y red eyes wt wings

x

Parental: w+ m+/Y 412red eyes, wt wings

w m/Y 389white eyes, miniature wings

Recombinant: w+ m/Y 206red eyes, miniature wings

w m+/Y 185white eyes, wt wings

412+389 x 100= 67.2% 1192

206+185 x 100= 32.8% 1192

}

}

Fig. 4.4

F b c+/b c+ x M b+ c/b+ cblack body, wt body,

wt wings curved wingsP

F1 F b c+/b+ cwt body (b/b+)wt wings (c+/c)

M b c/b cblack body,

curved wings

x Test Cross!

Fig. 4.5

Crossing-Over in theF1 Female b c+/b+ c

b c+

b+ c

b c+

b c

b+ c

b+ c+

b c+

b c

b+ c+

b+ c

Prophase I Metaphase II Gametes

F2

F b c+/b c+ x M b+ c/b+ cblack body, wt body body,

wt wings curved wingsP

F1 F b c+/b+ cwt body (b/b+)wt wings (c+/c)

M b c/b cblack body,

curved wings

x Test Cross!

black body, wt wings 2934

black body, curved wings871

wt body, curved wings2768

wt body, wt wings 846

Parental: black body, wt wings 2934

Recombinant: black body, curved wings871

Parental: wt body, curved wings 2768

Recombinant: wt body, wt wings846

Parental: black body, wt wings 2934b c+/b c

Recombinant: black body, curved wings871b c/b c

Parental: wt body, curved wings 2768b+ c/b c

Recombinant: wt body, wt wings846b+ c+/b c

total=7419

% Recombination =(871 + 846)/7419 x 100

= 23%

Fig. 4.5

Recombination Frequency Reflects Distances between Genes

if closely linked:cross-overs less likely to occur between A&B

if farther apart:cross-overs more

likely to occur between A&B

Sturtevant’s Data Sety- w 1.1% Recombination Frequencyw-m 32.8% Recombination Frequencyy-m 34.3% Recombination Frequency

The First Chromosome Map

1.1 32.8

34.3

w-m 32.8

A Series of Two Point Crosses Can Generate A Chromosome Map

Fig. 4.11

Three Point Mapping Cross: Order of Genes Unknown

P ¡ vg b pr/vg b pr x ¢ vg+ b+ pr+/vg+ b+ pr+

vestigial wings wt wingsblack body wt bodypurple eyes wt eyes

F1 vg b pr/vg+ b+ pr+

all wildtype

Test Cross Heterozygous F1:¡ vg b pr/vg+ b+ pr+ x ¢ vg b pr/vg b pr

F2:vestigial, black, purple 1779

wildtype 1654

black, purple 252

vestigial 241

black 131

vestigial, purple 118

vestigial, black 13

purple 9

4197

vg b pr

vg+ b+ pr+

vg+ b pr

vg b+ pr+

vg+ b pr+

vg b+ pr

vg b pr+

vg+ b+ pr

F

F

M

M

Fig. 4.12

vg b pr

vg+ b+ pr+

vg b+ pr

vg+ b pr+

vestigial, purple

black

b vg pr

b+ vg+ pr+

b vg+ pr

b+ vg pr+

black, purple

vestigial

vg pr b

vg+ pr+ b+

vg pr+ b

vg+ pr b+

vestigial, black

purple

Three Point Mapping Cross: Order of Genes Unknown

P ¡ vg b pr/vg b pr x ¢ vg+ b+ pr+/vg+ b+ pr+

vestigial wings wt wingsblack body wt bodypurple eyes wt eyes

F1 vg b pr/vg+ b+ pr+

all wildtype

Test Cross Heterozygous F1:¡ vg b pr/vg+ b+ pr+ x ¢ vg b pr/vg b pr

F2:vestigial, black, purple 1779

wildtype 1654

black, purple 252

vestigial 241

black 131

vestigial, purple 118

vestigial, black 13

purple 9

4197

vg b pr

vg+ b+ pr+

vg+ b pr

vg b+ pr+

vg+ b pr+

vg b+ pr

vg b pr+

vg+ b+ pr

F

F

M

M

Fig. 4.12