R_ x rr

1
Test Cross Back cross of offspring to its
homozygous recessive parent
R_ x rr
Either RR or Rr
Definitely rr
Homozygous Dominant Or Heterozygous
Homozygous Recessive
2
Any offspring variation in cross with dominant
means that dominant individual is variable
Heterozygous Yy
3
Solving Genetics Problems A) Divide and
Conquer B) Multiplication Rule
R Round r Wrinkled Y Yellow y Green T
Tall t Short
4
RRYYTT x rryytt RYT ryt 100 RrYyTt RrYyTt
x RrYyTt Phenotypic ratio of F2s?
P1 x P1 Gametes F1 F1 X F1 F2
5
Phenotypic ratio from a trihybrid cross A)
Divide and Conquer RrYyTt x RrYyTt
RrYy x RrYy And Tt x Tt
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27/64 R_Y_T_ 9/64 R_Y_tt
3/4 T_ 1/4 tt
9/16 R_Y_ 3/16 R_yy 3/16 rrY_ 1/16 rryy
3/4 T_ 1/4 tt
9/64 R_yyT_ 3/64 R_yytt
3/4 T_ 1/4 tt
9/64 rrY_T_ 3/64 rrY_tt
3/4 T_ 1/4 tt
3/64 rryyT_ 1/64 rryytt
7
27/64 R_Y_T_ 9/64 R_Y_tt
Round, Yellow, Tall
Round, Yellow, Short
Round, Green, Tall Round, Green,
Short Wrinkled, Yellow, Tall Wrinkled, Yellow,
Short Wrinkled, Green, Tall Wrinkled, Green
Short
9/64 R_yyT_ 3/64 R_yytt
9/64 rrY_T_ 3/64 rrY_tt
3/64 rryyT_ 1/64 rryytt
8
Another Way to Solve B) Multiplication Rule
RrYyttAa X RrYyTtAa
What is the probability of a wrinkled, green,
short, axial progeny?
RrYyttAa X RrYyTtAa
9
RrYyttAa X RrYyTtAa RrYy x RrYy rryy 1/16 tt
x Tt tt 1/2 Aa x Aa A_ 3/4
1/16 x 1/2 x 3/4 3/128 rryyttA_ 3/128
wrinkled, green, short, axial progeny!
10
Another Example of Divide and Conquer A Round,
Yellow, Tall plant is test crossed. R_Y_T_ x
rryytt
Testcross progeny Round, Yellow, Tall
250 Round, Green, Tall 248 Wrinkled, Yellow,
Tall 252 Wrinkled, Green, Tall 250 Total 1000
11
Testcross progeny Round, Yellow, Tall 250 Round,
Green, Tall 248 Wrinkled, Yellow,
Tall 252 Wrinkled, Green, Tall 250 Total 1000
Progeny Round Rr 498 Wrinkled rr 502
Ambigous Parent RrYyTT
Yellow Yy 502 Green yy 498
Tall Tt 1000 Short tt 0