In Drosophila, a cross was set between a male homozygous for alleles s⁺/s⁺ (phenotype A) and a female homozygous of s/s (phenotype B) ('s⁺' being a dominant allele and 's' a recessive allele). All of the F1 individuals thus obtained had the phenotype B. When F1 individuals were crossed among themselves, all progeny obtained were of phenotype A in F2.

The following explanations were proposed for the results obtained:

A. This is an example of cytoplasmic inheritance.
B. This is exhibiting genetic maternal effect.
C. This is a quantitative trait influenced by the environment.
D. This is exhibiting gene interaction with epistasis.
E. The trait is showing position effect variegation.

Which one of the following options is correct?

Correct option is B

Correct Answer: (2) B only

Explanation:

Let’s break down the scenario:

In Drosophila, a cross was done between:

• Male: s⁺/s⁺ (Phenotype A) — homozygous dominant

• Female: s/s (Phenotype B) — homozygous recessive

Result of Cross (P generation):

• All F1 progeny showed Phenotype B, even though they are s⁺/s (heterozygous).

• Normally, with s⁺ being dominant, s⁺/s should show Phenotype A, but here it shows Phenotype B.

This is an unexpected result under classical Mendelian inheritance.

F1 Self-Cross Result:

• F1 (s⁺/s) × F1 (s⁺/s) → All F2 progeny showed Phenotype A, which is expected if the dominant allele behaves normally.

What does this suggest?

This clearly indicates the initial maternal influence is responsible for the phenotype in F1 generation, where:

• Even though the dominant allele s⁺ is present, its effect is not seen in F1, because the mother was s/s.

• Once the maternal influence is gone (in F1 selfed), the dominant allele expresses normally in F2.

 This is a classic case of a Genetic Maternal Effect:

• In maternal effect, the phenotype of the offspring is determined not by their own genotype, but by the genotype of the mother.

• In this case, since the mother was s/s, all F1 individuals — regardless of their own genotype — showed Phenotype B.

Why other options are incorrect?

• A. Cytoplasmic inheritance: Would involve mitochondrial or chloroplast DNA — not applicable here.

• C. Quantitative trait influenced by environment: This is not a polygenic trait nor influenced by environment.

• D. Epistasis: There’s no gene-gene interaction shown; this is a single-locus maternal effect.

• E. Position-effect variegation: Involves gene silencing due to chromosomal location (like heterochromatin) — not supported here.