Two strains of flour beetles, both of which are pure breeding for the phenotype

Question

Two strains of flour beetles, both of which are pure breeding for the phenotype “weird egg”, were obtained by you from independent sources. In both cases the egg phenotype is determined by the maternal genotype (i.e., by the genotype of the female that lays the egg) and in both cases the locus involved is autosomal and the alleles for the weird egg phenotype are recessive. However you do not know whether the locus involved in this phenotype is the same in both strains, and you wish to determine this.

a. Explain why this pattern of maternal inheritance is not considered a form of cytoplasmic inheritance.

b. Design an experiment to determine whether the gene locus involved in the weird egg phenotype is the same locus or different loci in the two strains.

Explanation / Answer

* The term cytoplasmic inheritance is used only when transmission of genes that are outside the nucleus occurs. In this case, the inheritance is due to autosomes which are not outside the nucleus, and thus, such an inheritance is not called cytoplasmic inheritance. If the phenotype of offspring is determined by genotype of the mother, it is called maternal effect, not maternal inheritance. Maternal effect increases the firness of offspring.

* Cross the two different strains with a known homozygous dominant strain. If the two strains have the genes at the same locus, both the strains will produce similar patterns of phenotypes of the offspring. If the genes are at different loci, the phenotype pattern too changes.

For example, consider the two strains be A and B. You have crossed both the strains with a beetle homozygous dominant for the weired egg locus. If both of them have the gene at the same locus, both will produce heterozygous offspring which do not express the weird egg phenotype. On the other hand, if the offspring of strain B shows wierd egg phenotype but not those of strain A, it can be interpreted that both of them differ in the gene locus responsible for the trait.

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