Question

The specification of the anterior-posterior axis in Drosophila embryos is initially controlled by various gene products that are synthesized and stored in the mature egg following oogenesis. Mutations in these genes result in abnormalitics of the axis during embryogenesis, illustrating maternal effect. How do such mutations vary from those involved in organelle heredity that illustrate extranuclear inheritance? Devise a set of parallel crosses and expected outcomes involving mutant genes that contrast maternal effect and organelle heredity.

Short answer

Answer: Maternal effect refers to the influence of the mother's genotype on the phenotype of her offspring because of gene products produced during oogenesis, which control early embryonic development. On the other hand, extranuclear inheritance, or organelle heredity, refers to the inheritance of traits determined by genes located outside the nucleus, such as those found in mitochondria and chloroplasts. Parallel crosses can illustrate these concepts by examining the offspring phenotypes based on the mothers' genotypes for maternal effect and the inheritance of organelle-related traits for extranuclear inheritance. For maternal effect, one cross would involve a female with one mutant allele paired with a male with normal alleles, resulting in all offspring displaying normal phenotypes. In another cross, a female homozygous for the mutant allele would cause all offspring to exhibit the affected phenotype, regardless of the male's genotype. For extranuclear inheritance, the offspring would inherit their mitochondrial genes exclusively from their mother, so even if the father carries normal alleles, offspring inherit the abnormal mitochondrial trait if the mother carries it.

Step-by-step solution

Understand Maternal Effect and Extracellular Inheritance

Maternal effect refers to the influence of the mother's genotype on the phenotype of her offspring due to the presence of gene products that are synthesized and stored in the mature egg during oogenesis. These gene products control early embryonic development. Mutations in these genes can lead to abnormalities in the developing embryo. Extranuclear inheritance, also known as organelle heredity, refers to the inheritance of traits that are determined by genes located outside the nucleus. This is usually due to genes present in the mitochondria or chloroplasts.

Design Parallel Crosses for Maternal Effect

To create parallel crosses to demonstrate maternal effect, we first need to identify a gene responsible for maternal effect and its mutant allele. Let's use gene A, in which the wild-type allele (A) results in normal axis formation in the embryo and the mutant allele (a) results in an abnormal axis during embryogenesis. Cross 1: Female: Aa Male: AA Expected outcome: All offspring will have a normal axis. The mutant allele from the female does not affect the offspring's phenotype because the mother's genotype produces normal gene products during oogenesis. Cross 2: Female: aa Male: AA Expected outcome: All offspring will have an abnormal axis. The mutant alleles from the female are unable to produce the correct gene products during oogenesis, thus affecting the embryonic development.

Design Parallel Crosses for Extracellular Inheritance

To create parallel crosses to demonstrate extranuclear inheritance, we first need to identify a relevant gene and its mutant allele. Let's use gene B, in which the wild-type allele (B) is responsible for healthy mitochondria function and the mutant allele (b) results in abnormal mitochondrial function leading to reduced energy production. Cross 3: Female: BB (healthy mitochondria) Male: Bb (abnormal mitochondria) Expected outcomes: Since mitochondrial DNA is inherited maternally, all offspring will inherit healthy mitochondria from the female. Cross 4: Female: Bb (abnormal mitochondria) Male: BB (healthy mitochondria) Expected outcomes: Despite the presence of a normal nuclear gene from the male parent, all offspring will inherit the mutant mitochondria from the female parent due to extranuclear inheritance. All offspring will exhibit reduced energy production because of the inherited abnormal mitochondria from the mother. These parallel crosses illustrate the difference between maternal effect and extranuclear inheritance. In maternal effect, the mother's genotype affects the phenotype of her offspring through the production of gene products during oogenesis, while extranuclear inheritance involves traits that are determined by genes located outside the nucleus, such as those found in mitochondria and chloroplasts.