Castle-Mendel's Law of Heredity
Mendel's Law of Heredity
Introduction
Author: W. E. Castle
Source: Proceedings of the American Academy of Arts and Sciences, Vol. 38, No. 18 (Jan., 1903)
Discovery by Gregor Mendel, an Austrian monk, in the early 1860s
Initially overlooked for about 30 years due to the dominance of Darwinian theory
Rediscovery by de Vries, Correns, and Tschermak in 1900
Bateson demonstrated its significance in 1902
Mendel's Laws
1. Law of Dominance
Dominant character is expressed in the offspring, recessive character is masked.
Example: Crossing white mice with gray mice results in all gray offspring.
Dominance in pea plant traits:
Yellow cotyledons are dominant over green
Round seeds are dominant over wrinkled seeds
Violet flowers are dominant over white
2. Peculiar Hybrid Forms
Not all hybrid forms express simple dominance; some display intermediate characteristics.
Crossing tall and dwarf pea varieties can yield hybrid offspring taller than dwarf yet shorter than tall.
Some hybrids may exhibit entirely new traits not seen in either parent.
Example: Crossing spotted and albino mice yields gray offspring, differing from both.
3. Purity of Germ Cells
Hybrids produce germ cells that carry only one allele from each parent, not both.
This principle is known as the law of segregation.
Result: In hybrid crosses, pure forms (AA and BB) and hybrids (AB) appear in a 1:2:1 ratio.
Empirical Observations
Mendel’s experiments demonstrated that in the second generation of hybrids, three combinations are possible:
AA, AB, and BB with expected frequency ratios depending on dominance.
For instance, crossing yellow with green peas results in yellow (dominant) and green (recessive) seeds in a 3:1 ratio in the subsequent generation.
Complex Traits and Genetic Interactions
A. Multiple Traits
When crossing parents differing in two traits, a variety of combinations can occur:
Dominant combinations AB, Ab, aB, ab arise in the second generation.
Example: Sixteen combinations arise from two traits, with one individual of each type on average.
B. Exceptions to Mendel's Principles
Mosaic Inheritance: Both dominant and recessive traits can appear side by side in hybrids (e.g. spotted mice).
Stable Hybrid Forms: Some hybrids consistently produce offspring with their unique characteristics.
Coupled Characters: Two or more traits that are inherited together (e.g. plant height with flower color).
Disintegration of Characters: Traits that segregate and become independently heritable (e.g. coat colors in animals).
Reversal and Adjustments in Dominance
Cases of reversal in dominance can occur (e.g. where a recessive trait appears instead of the dominant one).
Often referred to as "false hybridization" or induced parthenogenesis.
Conclusion: Implications of Mendel's Work
Mendel's principles deepen our understanding of heredity and evolution.
New types of organisms exhibit greater variability due to novel combinations of traits.
Over time, natural selection may lead to the stability of some mutations while others persist as latent traits.
Bibliography
Bateson, W.: Mendel's Principles of Heredity, Cambridge.
Correns, C.: Mendel's Rules on the Inheritance of Hybrids.
Mendel, G.: Experiments on Plant Hybridization.
Tschermak, E.: On Artificial Crossbreeding in Peas.
Vries, H. de.: On the Law of Separation of Hybrids.
Mendel's Law of Heredity
Introduction
Author: W. E. Castle
Source: Proceedings of the American Academy of Arts and Sciences, Vol. 38, No. 18 (Jan., 1903)
Discovery: Gregor Mendel, an Austrian monk, conducted experiments on pea plants in the early 1860s, laying the groundwork for the field of genetics. His work remained largely unrecognized for about 30 years, overshadowed by the prevailing Darwinian theory of evolution.
Rediscovery: Mendel's principles were rediscovered in 1900 by scientists Hugo de Vries, Carl Correns, and Erich von Tschermak, who independently confirmed his findings. The British biologist William Bateson later demonstrated the significance of Mendel's work in 1902, coining the term "genetics" in the early 20th century.
Mendel's Laws
1. Law of Dominance
In Mendelian inheritance, when an organism inherits two different alleles for a trait, the dominant allele masks the expression of the recessive allele.
Example: When crossing a purebred white mouse (homozygous recessive) with a purebred gray mouse (homozygous dominant), all offspring in the first generation exhibit the gray phenotype due to the dominance of the gray allele.
Dominance in Pea Plant Traits:
Yellow cotyledons (seed leaves) are dominant over green cotyledons.
Round seeds are dominant over wrinkled seeds.
Violet flowers are dominant over white flowers.
2. Peculiar Hybrid Forms
Mendel observed that not all hybrid forms follow the simple dominance pattern; some hybrids exhibit intermediate traits, leading to a broader understanding of inheritance.
Example: When crossing tall and dwarf pea plants, the hybrid offspring may be of intermediate height, showcasing traits that are blends of both parent plants, resulting in height characteristics that are neither fully tall nor fully dwarf. Further crossings may yield entirely new traits not present in either parent organism.
Example: Crossing spotted and albino mice can produce gray offspring, which display unique phenotypes that differ from either parent.
3. Purity of Germ Cells
Mendel established that during the formation of gametes (sperm and egg cells), the alleles are segregated, resulting in gametes that carry only one allele for each trait, a process known as the Law of Segregation.
This results in a predictable ratio of genotypes when hybrids are crossed, generally producing offspring in a 1:2:1 ratio of pure forms (AA and BB) to hybrids (AB).
Empirical Observations
Mendel's experiments revealed that in the second generation of hybrids, the three possible combinations of alleles are produced: AA, AB, and BB. The frequency ratios vary based on which trait is dominant.
Example: Crossing yellow seeds (genotype YY or Yy) with green seeds (yy) typically yields seed color in a 3:1 ratio in the subsequent generation, with three-quarters displaying the dominant yellow phenotype and one-quarter displaying the recessive green phenotype.
Complex Traits and Genetic Interactions
A. Multiple Traits
When two parents differing in two traits are crossed, combinatorial genetics allows for multiple trait combinations among the offspring.
Example: Crossing two pea plants differing in seed color (yellow and green) and seed shape (round and wrinkled) results in combinations such as AB (round yellow), Ab (round green), aB (wrinkled yellow), and ab (wrinkled green), leading to a potential 16 different combinations in the second generation.
B. Exceptions to Mendel's Principles
Mosaic Inheritance: Observed in certain hybrids where both dominant and recessive characteristics manifest simultaneously (e.g., spotted mice).
Stable Hybrid Forms: Some hybrids consistently produce offspring with distinctive traits, thereby preserving specific characteristics across generations.
Coupled Characters: In certain situations, two or more traits are inherited together due to their proximity on the chromosome (e.g., plant height correlated with flower color).
Disintegration of Characters: Refers to traits that segregate and become independently heritable, leading to variations (e.g., diverse coat colors in some animal species).
Reversal and Adjustments in Dominance
Cases wherein recessive traits appear instead of the expected dominant traits are referred to as reversals in dominance.
Such phenomena are often attributed to genetic mechanisms like "false hybridization" or induced parthenogenesis, highlighting the complexity of genetic inheritance.
Conclusion: Implications of Mendel's Work
Mendel's foundational principles enhance our comprehension of heredity and evolutionary processes.
The increased variability among organisms arises from novel combinations of traits generated through sexual reproduction.
Over time, natural selection acts on these variants, leading to the stabilization of some advantageous mutations while maintaining others as latent traits within populations, further informing modern genetics and breeding practices.
Bibliography
Bateson, W.: Mendel's Principles of Heredity, Cambridge.
Correns, C.: Mendel's Rules on the Inheritance of Hybrids.
Mendel, G.: Experiments on Plant Hybridization.
Tschermak, E.: On Artificial Crossbreeding in Peas.
Vries, H. de.: On the Law of Separation of Hybrids.