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3.2 describe outcomes of dihybrid crosses involving simple dominance using Mendel’s explanations

describe outcomes of dihybrid crosses involving simple dominance using Mendel’s explanations

a) Pure breeding, double homozygous tall stem with round seed plant is crossed with double homozygous short stem with wrinkled seed:
Parents: TTRR x ttrr
Possible gametes: TR, TR, TR, TR and tr, tr, tr, tr
F1 : genotypes: all TtRr
phenotypes: all tall stem, round seed

Breed two F1 individuals together:
F1: TtRr x TtRr
Possible gametes: TR, Tr, tR, tr and TR, Tr, tR, tr
F2:

gametes TR Tr tR tr
TR TTRR TTRr TtRR TtRr
Tr TTRr TTrr TtRr Ttrr
tR TtRR TtRr ttRR ttRr
tr TtRr Ttrr ttRr ttrr

 

Genotypes Number Phenotypes
TTRR 1 Tall stem, round seed
TTRr 2 Tall stem, round seed
TtRR 2 Tall stem, round seed
TtRr 4 Tall stem, round seed
TTrr 1 Tall stem, wrinkled seed
Ttrr 2 Tall stem, wrinkled seed
ttRR 1 Short stem, round seed
ttRr 2 Short stem, round seed
ttrr 1 Short stem, wrinkled seed

The phenotypic ratio of F2 individuals is:

9 tall round : 3 tall wrinkled : 3 short round : 1 short wrinkled

According to Mendel, because chromosomes segregate independently and all genes are inherited independently, there is an equal chance of the four different types of possible gametes being produced to result in this outcome.

b) Parents: TtRr (tall round) x ttrr (short wrinkled)
Possible gametes: TR, Tr, tR, tr tr

F1:

gametes TR Tr tR tr
tr TtRr Ttrr ttRr ttrr

25% TtRr : 25% Ttrr : 25% ttRr : 25% ttrr