Why cytoplasmic genes are always recessive and nuclear genes are dominant in plant breeding?

Cytoplasmic genes, which are primarily found in the chloroplasts and mitochondria, are typically inherited maternally in plants. These genes are often associated with essential cellular functions such as photosynthesis and respiration. In contrast, nuclear genes are inherited from both parents and control a wide range of traits, including plant morphology, development, and physiology.

The distinction between cytoplasmic genes being recessive and nuclear genes being dominant in plant breeding arises from their mode of inheritance and the nature of their interactions within the cell:

 

Cytoplasmic Genes:

Inheritance: Cytoplasmic genes are inherited maternally, meaning they are passed down from the maternal parent to the offspring through the egg cell.

Recessive Inheritance: Cytoplasmic genes are typically recessive because the presence of functional cytoplasmic genes in the offspring is determined by the maternal genotype. If the maternal parent carries a functional allele for a cytoplasmic gene, it will mask the effects of any non-functional alleles contributed by the paternal parent.

Nuclear Genes:

Inheritance: Nuclear genes are inherited from both parents, with alleles from each parent contributing to the genetic makeup of the offspring.

Dominant Inheritance: Nuclear genes can exhibit dominant or recessive inheritance patterns, depending on the specific alleles present. Dominant alleles are expressed phenotypically even in the presence of a recessive allele, while recessive alleles are only expressed phenotypically in the homozygous state (when present in two copies).

Genetic Interactions: Nuclear genes often interact in complex pathways and networks, and their phenotypic expression can be influenced by multiple genetic and environmental factors. Dominance relationships between alleles at nuclear loci can vary depending on the specific genetic background and environmental conditions.

In plant breeding, the distinction between cytoplasmic genes being recessive and nuclear genes being dominant is important for understanding inheritance patterns and predicting the phenotypic outcomes of crosses. By considering the mode of inheritance and dominance relationships of different genes, breeders can make informed decisions to develop improved plant varieties with desired trait.