25. A plant that produces disc-shaped fruit is crossed with another plant that produces long fruit. All the F1 plants gave disc-shaped fruits. When the F1 were intercrossed, F2 progeny were produced in the following ration: 9/16 plants with disc-shaped fruits; 6/16 plants with spherical fruits and 1/16 plants having long fruits. Which one of the following options gives correct genotype of spherical fruits obtained in F2?
(1) A_bb only            (2) aaB_ only
(3) A_bb and aaB_    (4) A_B_ and aabb

The given problem involves fruit shape inheritance in plants with a dihybrid cross and a phenotypic ratio in the F2 generation of 9/16 disc-shaped fruits, 6/16 spherical fruits, and 1/16 long fruits. This 9:6:1 ratio indicates an interaction between two genes showing polygenic or complementary gene effects rather than simple dominance.

Explanation of Phenotypic Ratio (9:6:1)

• 9/16 disc-shaped fruits: plants having at least one dominant allele at both loci (A_B_)

• 6/16 spherical fruits: plants having either dominant allele at one locus but homozygous recessive at the other (A_bb or aaB_)

• 1/16 long fruits: plants homozygous recessive at both loci (aabb)

This genetic model suggests that two genes A and B control fruit shape, where both dominant alleles contribute to disc shape. The absence of dominant alleles at both loci results in long fruit.

Genotypes Corresponding to Spherical Fruits

• Spherical fruits occur when either gene A or gene B is homozygous recessive while the other gene has at least one dominant allele.

• Hence, spherical fruits genotypes are A_bb and aaB_.

Analysis of Options

• (1) A_bb only: Incomplete, spherical fruit arises not only from this genotype.

• (2) aaB_ only: Incomplete, spherical fruit arises either from A_bb or aaB_.

• (3) A_bb and aaB_: Correct, both genotypes explain the spherical fruit phenotype.

• (4) A_B_ and aabb: Incorrect; A_B_ are disc-shaped fruits; aabb are long fruits.

Introduction:

Fruit shape inheritance in plants is a fascinating example of gene interactions where two genes influence the final phenotype. The characteristic 9:6:1 phenotypic ratio observed in certain crosses highlights the effects of polygenic inheritance or complementary gene action. Understanding how genotypes correspond to disc, spherical, and long fruit shapes aids in solving classical genetic problems.

Explanation of Genotypes:

• The 9:6:1 ratio means disc-shaped fruits arise when both dominant alleles (A_B_) are present.

• Spherical fruits occur when one locus is homozygous recessive but the other has a dominant allele (A_bb or aaB_).

• Long fruits occur only when both loci are homozygous recessive (aabb).

Evaluation of Options:

• Option (3) correctly identifies spherical fruit genotypes as both A_bb and aaB_.

• Options (1) and (2) are partial.

• Option (4) mixes phenotypes unrelated to spherical fruits.

This understanding is essential for students and educators tackling classical Mendelian genetics with modifications due to gene interactions. It clarifies the molecular basis of complex inheritance beyond simple dominant-recessive patterns.

For anyone studying plant genetics, this example illuminates predicting phenotypes in offspring from genotypic combinations in dihybrid crosses.

This explanation should help to grasp the inheritance of fruit shape and correctly identify spherical fruit genotypes in the F2 progeny. Let me know if you want a more detailed stepwise genetic cross diagram or additional examples.