Cracking the Code of Inheritance: Mendel's Traits Get a 21st-Century Twist
Have you ever wondered why some people are taller than others?
Why do children look like their parents, sharing similar eye colour, hair texture or even certain habits?
The answer lies in inheritance, a fundamental concept in genetics that has intrigued us for centuries. The foundation of modern genetics was laid by Gregor Johann Mendel through his pioneering work with pea plants. But the molecular basis of the traits he studied remained poorly understood until recently. A groundbreaking study published in Nature has now uncovered the specific genes underlying Mendel's observations. This breakthrough not only deepens our understanding of how inheritance works but also opens up new possibilities in plant breeding and beyond.
Popularly known as the father of genetics, Mendel was a botanist, teacher, and Augustinian prelate with a passion for plants. Born in a modest family in rural Silesia, Mendel’s initial years were not easy on him. Despite the challenges, he pursued his passion for learning and earned a degree in practical and theoretical philosophy and physics at the Philosophical Institute of the Palacký University of Olmütz, Czech Republic. On recommendation of professor Friedrich Franz, his physics teacher, Mendel eventually joined the Augustinian St. Thomas' Abbey in Brno. The extensive library and two hectare experimental garden at the abbey sparked his interest in plant breeding, setting the stage for his revolutionary research.
Mendel's Legacy: Understanding inheritance even before chromosomes were known
Mendel focused a lot of his experiments on pea plants in the monastery’s garden, carefully choosing seven key characters.
These included simple features like plant height, pea shape, pea colour, pod colour, pod shape, flower colour, and flower position – features that appeared to be inherited independent of one another.
Between 1856 to1863, his meticulous experiments revealed the fundamental principles of inheritance, including the laws of segregation and independent assortment. We must not forget that the word ‘gene’ was coined only in the early 20th century and the concept was unknown at time of Mendel. Even the idea of chromosomes was unknown back then. He, therefore, used the term ‘trait’ to talk about the characters he studied.
Mendel tracked how these traits are passed on across generations without knowing the underlying chromosomal basis. He then used precise mathematical formula to explain the frequency of inheritance of each trait in the subsequent generations. Although Mendel’s work was originally published in the ‘Proceedings of the Brunn Society for the Study of Natural Sciences’ in 1866, it was largely unnoticed by the scientific community for decades. On the occasion of the sesquicentennial anniversary of this remarkable discovery, a translation of Mendel’s original paper has been republished in 2016.
Rediscovery of Mendel’s works marks the beginning of modern genetics
Mendel’s revolutionary work was lost in the ashes of obscurity for more than three decades until it was rediscovered by Hugo de Vries, Carl Correns, and Erich Tschermak von Seysenegg in early 1900s. These three researchers were working independently on plant hybridisation trying to understand the laws of inheritance. During literature review for drafting their own manuscripts, they came across Mendel’s paper that laid down the laws of inheritance as early as 1866. By this time chromosomes have been discovered and they could add a physical attribute to Mendel’s abstract concept of ‘trait’ in inheritance. Mendel’s work was further brought into limelight by William Bateson through his book ‘Mendel’s Principles of Heredity’. Bateson was in fact one of the greatest advocates of Mendel’s discovery and coined the term ‘genetics’ in 1905. This ‘rediscovery’ marks a major milestone in our understanding of modern genetics.
Genetic secrets of Mendel's peas revealed
For centuries, scientists have been fascinated by the seven traits that Mendel studied, which formed the cornerstone of modern genetics. It was not until 1990 that the first gene responsible for one of these traits was discovered. This was the rugosus (r) locus of the pea genome that encodes for the starch-branching enzyme (SBEI). Pea plants that can synthesise this enzyme have round seeds while plants lacking this enzyme have wrinkled seeds.
Subsequently, the genetic basis of plant height was linked to the length (le) locus in the genome. This locus precisely modulates synthesis of the plant growth regulator, gibberellin, resulting in tall or dwarf plants. Another trait described in Mendel’s seminal paper was the cotyledon colour (I) of pea. Later on this feature was found to be governed by the Staygreen (sgr) gene that interferes with cotyledon senescence. A mutation in this gene disables senescence or aging resulting in green cotyledons in contrast to yellow cotyledons in the normal plants. The flower colour trait was next decoded, with the A locus identified as key regulator. This locus codes for a transcription factor essential for producing purple anthocyanin pigment. If this locus is mutated pigment production is hampered and the flowers appear white.
The last lap in cracking the code
Another 15 years passed before the genetic basis of the remaining three traits was unveiled. A collaborating group of researchers from John Innes Centre, UK and Agricultural Genomics Institute at Shenzhen, China has uncovered this mystery only last week. The study used cutting-edge technology to sequence the pea genome and pinpoint the genetic variations underlying Mendel's observations. They have identified the genetic basis for pod colour, pod shape, and flower position – traits that Mendel described in his experiments. Their study has revealed that the yellow pod colour (Gp locus) in Mendel’s experiment was produced due to a deletion that interfered with chlorophyll biosynthesis.
Moreover, variation in pod shape in peas, described as 'constricted’ or ‘inflated’ by Mendel, has been attributed to two genes, P and V, which regulate sclerenchyma formation in pods. Sclerenchyma is a type of plant tissue consisting of dead cells with thick walls and give rigidity and strength to the plant. Disruption of P and V genes due to mutations or retrotransposon insertions can lead to impaired sclerenchyma development, causing pods to become constricted. Finally, the terminal or axillary flower position is regulated by the Fa locus, which encodes a cell membrane-localised senescence-associated receptor-like kinase (CIK2/3). This gene regulates shoot apex development and meristem structure and its mutation results in terminal flower development in pea.
This journey to unravel the genetic secrets of Mendel's experiments has been a long and winding one. It took 159 years for scientists to fully understand the genetic basis of the traits he studied. This breakthrough not only pays tribute to Mendel's pioneering work but also has far-reaching implications for agriculture, biotechnology, and our understanding of the natural world.
Dr. Riddhi Datta is a Molecular Biologist and Assistant Professor in the Post Graduate Department of Botany, Barasat Government College. She is a member of the Indian National Young Academy of Science, New Delhi.
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