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 abnormalities 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

Short Answer: Maternal effect inheritance is when an organism's phenotype is determined by the genotype of its mother, usually due to gene products in the egg cytoplasm, whereas organelle heredity involves the inheritance of cytoplasmic organelles such as mitochondria and chloroplasts and their DNA. Both inheritance types involve maternal inheritance, but the maternal effect is related to gene products while organelle heredity is related to organelle DNA.

Step-by-step solution

Understand maternal effect and organelle heredity

Maternal effect is a type of inheritance where an organism shows the phenotypic effect of a gene present in its mother, regardless of the organism's own genotype. This is usually due to the presence of the gene products in the egg cytoplasm. Organelle heredity, or extranuclear inheritance, is a type of inheritance where traits are passed from parents to offspring via cytoplasmic organelles such as mitochondria and chloroplasts instead of nuclear genes. These organelles have their own DNA, which is passed to offspring primarily through the maternal line.

Devise a set of parallel crosses for maternal effect

Let's consider two homozygous lines of Drosophila, one with a wild-type phenotype for the anterior-posterior axis (A), and the other with a mutant phenotype for the axis (a). Here, A represents the dominant wild type, and a represents the recessive maternal-effect mutation. We can represent the crosses as follows: 1. AA (♀) x AA (♂) - Results in all offspring with a wild-type phenotype 2. aa (♀) x AA (♂) - Results in all offspring with mutant phenotypes due to the maternal effect 3. AA (♀) x aa (♂) - Results in all offspring with wild-type phenotypes

Devise a set of parallel crosses for organelle heredity

Now let's consider two homozygous lines of Drosophila, one with healthy mitochondria, represented by (+) and the other with mutant mitochondria, represented by (m). We will represent the crosses as follows: 1. (+) (♀) x (+) (♂) - Results in offspring with healthy mitochondria due to maternal inheritance 2. (m) (♀) x (+) (♂) - Results in offspring with mutant mitochondria due to maternal inheritance 3. (+) (♀) x (m) (♂) - Results in offspring with healthy mitochondria due to maternal inheritance

Comparison of Expected Outcomes

In the maternal effect crosses, we notice that the mother's genotype directly affects the offspring's phenotype, while in organelle heredity crosses, offspring inherit their mitochondria only from the mother, leading to the expression of the phenotype associated with the mother's mitochondrial genotype. In both cases, paternal contribution does not affect the offspring's phenotype, which highlights the importance of maternal inheritance. However, they differ in that maternal effect involves gene products stored in the egg cytoplasm, whereas organelle heredity involves inheritance of cytoplasmic organelles and their DNA.