Question

A tomato plant with large leaves and round fruits was crossed to a plant with small leaves and oblong fruits. The F1 consisted of plants with large leaves; about half of these plants made round fruits and the other half made oblong fruits. Two F1 plants (one with round fruits and one with oblong fruits) were then analyzed through further crossing. The results were:

Cross

F1 plant #1 with round fruits × parent with round fruits

F1 plant #1 with round fruits × parent with oblong fruits

F1 plant #1 with round fruits × F1 plant #2 with oblong fruits

Progeny

35 large leaves, round fruits

12 large leaves, oblong fruits

17 large leaves, round fruits

20 large leaves, oblong fruits

7 small leaves, round fruits

5 small leaves, oblong fruits

27 large leaves, round fruits

26 large leaves, oblong fruits

a) What are the genotypes of the two original parents?

b) What are the expected phenotypic ratios in the progeny of a cross between “F1 plant

#1” and a tester?

c) What gamete genotypes can “F1 plant #2” produce?

Explain your reasoning behind each of your answers.

Answer 1

a) If parents are pure breeds they must produce half spring with dominant phenotype. But in the above problem, the dominant phenotype is expressed in offspring only for the single locus (leaf size) results in all F1 plants with large leaves. coming to the shape of the fruit only half of F1 population expressed dominant phenotype. Hence we can say the dominant parent must be heterozygote for this locus.

Here if leaf size is encoded by L gene with 'LL' represents the homozygous dominant, 'Ll' heterozygous dominant, 'll' homozygous recessive, and likewise if the shape of a fruit encoded by R gene then, RR is homozygous dominant round fruit, Rr heterozygotic dominant and rr must be homozygous recessive.

therefore the genotypes of parents must be- P1= llrr( small leaves and obligate fruit)

P2= LLRr( large leaves with round fruit).

b) If an F1 plant 1 and test crossed than

F1 -1 phenotype= Large leaf Round round fruit

Here this F1 plant has resulted from the parental cross = LLRr( heterozygotic parent) x llrr (recessive parent)

so the genotypes resulted from this cross

=LlRr ( F1-1: large leaf and round fruit) and Llrr (F1-2: large leaf and oblong fruit)

Here F1-1 crossed with recessive parent= LlRr X llrr

resulting genotypes= LlRr, Llrr, llRr and llrr in 1:1:1:1 ratio

resulting phenotypes= Large Round: Large oblong: small round: small oblong

in 1:1:1:1 ratio.

Hense the expected phenotypic ratio= 1:1:1:1

c) F1-2 plant genotype is Llrr

So there are two types of gametes produced by F1-2 that are= Lr and lr.