A farmer cultivates a unique species of tomato plants. The two traits selected for breeding were the colour of the fruits and the height of the plants. The farmer initially crosses a homozygous tall plant bearing red fruits (TTRR) with a dwarf plant bearing yellow fruits (ttrr), and the genotype of all the offsprings of the F1 generation was TtRr. Next, he crosses two F1 generation plants (TtRr).
Explain the phenotype of the offsprings of the F1 generation obtained from the initial cross.
Topic: Dihybrid cross — F1 phenotype
Answer
ALL the F1 offspring are TALL PLANTS WITH RED FRUITS.
Although every plant carries the alleles for dwarfness (t) and yellow fruit (r), these are RECESSIVE and are masked by the DOMINANT alleles T (tall) and R (red). So the genotype is TtRr but the phenotype shows only the dominant characters.
The answer must explain the gap between GENOTYPE and PHENOTYPE — that is what the word "explain" is asking for.
All the F1 plants are TtRr, so each carries both recessive alleles, yet none of them shows dwarfness or yellow fruit. A recessive allele is expressed only when it is present in the HOMOZYGOUS state (tt or rr).
This is MENDEL’S LAW OF DOMINANCE, and naming it is worth doing: when two contrasting alleles are present together, only the dominant one expresses itself in the F1 generation.
It also explains why the recessive characters reappear in the F2 generation — they were never lost, only hidden.