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 seedBreed 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 trF1:
gametes TR Tr tR tr tr TtRr Ttrr ttRr ttrr 25% TtRr : 25% Ttrr : 25% ttRr : 25% ttrr