Model Answer
0 min readIntroduction
Plastid inheritance, also known as extra-nuclear inheritance, deviates from the traditional Mendelian patterns of inheritance. It refers to the transmission of genes located within organelles like chloroplasts and mitochondria. These organelles possess their own DNA and replicate independently within the cell. The four o'clock plant, *Mirabilis jalapa*, served as a crucial model system for understanding this phenomenon. Carl Correns, in the early 20th century, meticulously documented the inheritance of leaf variegation in this plant, providing compelling evidence that certain traits are not governed by nuclear genes but by factors residing within the plastids. This discovery challenged the prevailing understanding of heredity and expanded the scope of genetic inheritance.
Plastid Inheritance: An Overview
Unlike nuclear genes which undergo segregation during meiosis and are inherited from both parents, plastid genes are typically inherited maternally in higher plants. This is because, during fertilization, the sperm cell contributes very little cytoplasm (and therefore few or no plastids) to the zygote. The zygote primarily receives its plastids from the egg cell. Consequently, the phenotype determined by plastid genes is usually that of the maternal parent.
Correns’ Experiments with *Mirabilis jalapa*
Carl Correns conducted a series of crosses using *Mirabilis jalapa* plants exhibiting different leaf variegation patterns. The plants displayed three distinct phenotypes:
- Green Leaves: Plants with entirely green leaves.
- White Leaves: Plants with entirely white leaves.
- Variegated Leaves: Plants with leaves containing both green and white sectors.
The Crosses and Observations
Correns performed reciprocal crosses between these phenotypes. The results were striking and differed significantly from Mendelian expectations:
- Cross 1: Green (♀) x White (♂): All F1 progeny had variegated leaves.
- Cross 2: White (♀) x Green (♂): All F1 progeny had green leaves.
- Cross 3: Variegated (♀) x Green (♂): F1 progeny showed a 50% variegated and 50% green leaf phenotype.
- Cross 4: Variegated (♂) x Green (♀): F1 progeny showed a 50% variegated and 50% green leaf phenotype.
- Cross 5: Variegated (♀) x White (♂): F1 progeny showed a 50% variegated and 50% white leaf phenotype.
- Cross 6: Variegated (♂) x White (♀): All F1 progeny had variegated leaves.
These results clearly indicated that the inheritance pattern was not based on nuclear genes. If it were, reciprocal crosses would yield identical results. The maternal parent’s phenotype consistently determined the F1 phenotype, particularly when white leaves were involved.
Explanation of the Inheritance Pattern
The variegation in *Mirabilis jalapa* is caused by a mutation in a chloroplast gene that affects chlorophyll synthesis.
- Green Plants: Possess normal chloroplasts capable of producing chlorophyll.
- White Plants: Have mutated chloroplasts that cannot produce chlorophyll. These plants rely on the few functional chloroplasts they may have or on photosynthesis by other parts of the plant.
- Variegated Plants: Contain a mixture of normal and mutated chloroplasts in their cells. The green sectors represent cells with functional chloroplasts, while the white sectors represent cells with mutated chloroplasts.
During meiosis in the female parent (egg formation), the cytoplasm (and therefore the chloroplasts) is distributed unequally among the egg cells. Some egg cells receive mostly normal chloroplasts, while others receive mostly mutated chloroplasts. This leads to the observed phenotypic ratios in the F1 generation. The male parent contributes negligible plastids, making its phenotype largely irrelevant in determining the F1 phenotype.
Molecular Basis of Plastid Inheritance
The plastid genome is circular and encodes genes essential for photosynthesis and other plastid functions. Mutations in these genes can lead to altered phenotypes. The maternal inheritance pattern is due to the limited contribution of plastids from the pollen grain during fertilization. The number of plastids per cell can also influence the expression of plastid genes, contributing to the mosaic variegation observed in some plants.
Conclusion
Correns’ experiments with *Mirabilis jalapa* provided foundational evidence for the existence of plastid inheritance, demonstrating that traits can be inherited through organelles outside the nucleus. This discovery expanded our understanding of heredity beyond Mendelian genetics and highlighted the importance of cytoplasmic factors in determining phenotypic expression. Further research into plastid inheritance has revealed its significance in plant evolution, adaptation, and breeding, and continues to be a vital area of study in plant biology.
Answer Length
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