12. 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
E. The trait is showing position effect variegation.
Which one of the following option is correct?
(1) A only (2) B only
(3) C only (4) D and E
Introduction:
In a unique genetic cross involving Drosophila, males homozygous for the dominant allele S+ (phenotype A) are crossed with females homozygous for the recessive allele s (phenotype B). Surprisingly, all F1 individuals show phenotype B despite carrying the dominant allele S+. However, when F1 are crossed among themselves, the F2 generation exhibits only phenotype A. This puzzling observation is explained by a phenomenon called genetic maternal effect, where the phenotype of the offspring is determined by the genotype or gene products supplied by the mother, not by the offspring’s own genotype.
Detailed Explanation of the Options:
A. Cytoplasmic inheritance:
This involves transmission of traits through cytoplasmic components such as mitochondria or plastids. Cytoplasmic inheritance is maternal because the egg provides the bulk of the cytoplasm, but it typically involves organelle genes, not nuclear alleles like S+/s. Since the trait here is linked to alleles on chromosomes and the F1 phenotype matches the mother, this option might partially explain maternal influence but not fully account for nuclear gene expression patterns seen here.
B. Genetic maternal effect:
This is the correct explanation. Maternal effect genes are expressed in the mother and the gene products (like mRNA or proteins) deposited in the egg influence the phenotype of the offspring. Here, the mother is s/s (phenotype B) and supplies mRNA for the recessive allele, leading all F1 (S+/s genotype) to show the recessive phenotype B despite having the dominant allele. In the F2, self-crossing of F1 produces offspring whose phenotypes are determined by their own genotypes without the maternal mRNA influence, showing phenotype A (dominant) uniformly.
C. Quantitative trait influenced by environment:
Quantitative traits show continuous variation influenced by multiple genes and environmental factors. Here, the trait is clearly Mendelian with distinct phenotypes A and B, not continuous variation. So, this option is incorrect.
D. Gene interaction:
Gene interaction involves interplay between different genes influencing phenotype. There is no evidence in this cross about interactions between multiple genes causing the observed phenotype pattern, so this option is less likely.
E. Position effect variegation:
This is a phenomenon where gene expression is suppressed due to the gene’s chromosomal position near heterochromatin, causing variegated phenotypes. The uniform phenotypes observed here in F1 and F2 do not support variegation.
Correct Option: B only
This case illustrates a classic example of genetic maternal effect in Drosophila, where maternally supplied gene products influence early phenotype expression overriding the offspring’s genotype initially.
Keywords: Drosophila genetics, maternal effect, cytoplasmic inheritance, gene interaction, position effect variegation, S+ allele, recessive phenotype
References: The explanation aligns with the genetic principles documented in genetic studies and specific Drosophila maternal effect cases showing phenotype determined by maternal mRNA and gene products.
