Introduction to Inheritance:
Definition: Inheritance is the process by which genetic information is passed from one generation to the next
Historical Context: The foundational principles of inheritance were first elucidated by Gregor Mendel in the mid-19th century through his experiments with pea plants.
(toc)
Reasons for Mendel’s Success
Choice of Experimental Organism:
Mendel selected pea plants (Pisum sativum) due to their distinct, easily observable traits and the ability to control their mating.
Focus on Discrete Traits:
He concentrated on traits that exhibited clear-cut differences, such as flower color and seed shape, avoiding traits with continuous variation.
Controlled Cross-Pollination:
Mendel meticulously controlled pollination processes, ensuring accurate tracking of trait inheritance.
Large Sample Sizes:
By analyzing large numbers of offspring, Mendel ensured that his results were statistically significant.
Quantitative Analysis:
He applied mathematical rigor to his experiments, allowing for precise interpretation of inheritance patterns.
Replication and Consistency:
Mendel repeated his experiments multiple times, consistently obtaining similar results, which reinforced the reliability of his findings.
Why Gregor Mendel Chose Pea Plants
Ease of Cultivation: Pea plants are simple to grow and maintain.
Controlled Pollination: They naturally self-pollinate but can also be cross-pollinated, allowing precise control over breeding experiments.
Short Life Cycle: As annual plants, they have a quick generation time, enabling the study of multiple generations within a short period.
Distinct Traits: Pea plants exhibit clear, contrasting characteristics (e.g., flower color, seed shape), facilitating the tracking of inheritance patterns.
Mendel’s Experiments
1. Monohybrid Cross
Objective: To study the inheritance of a single trait.
Method:
- Crossed plants differing in one trait (e.g., tall vs. short).
- First-generation (F1) offspring all exhibited the dominant trait (e.g., all tall).
- Self-pollinated F1 plants to produce a second generation (F2).
Results: F2 generation displayed both dominant and recessive traits in a 3:1 ratio.
Conclusion: Led to the formulation of the Law of Dominance and the Law of Segregation.
2. Dihybrid Cross
Objective: To study the inheritance of two different traits simultaneously.
Method:
- Crossed plants differing in two traits (e.g., seed shape and color).
- F1 generation exhibited both dominant traits (e.g., round and yellow seeds).
- Self-pollinated F1 plants to produce F2 generation.
Results: F2 generation exhibited four trait combinations in a 9:3:3:1 ratio.
Conclusion: Led to the formulation of the Law of Independent Assortment.
Key Concepts from Mendel’s Findings
Genotype: The genetic makeup of an organism.
Phenotype: The observable physical traits of an organism.
Alleles: Alternative forms of a gene that determine specific traits.
Homozygous: Having two identical alleles for a particular gene.
Heterozygous: Having two different alleles for a particular gene.
Mendel’s Laws of Inheritance
1. Law of Dominance
Definition: In a heterozygote, one allele may conceal the presence of another.
Implication: The dominant allele determines the organism's appearance, while the recessive allele has no noticeable effect on the phenotype.
Example: In pea plants, the allele for purple flowers is dominant over the allele for white flowers; thus, a plant with at least one purple allele will have purple flowers.
Significance: Explains why certain traits appear in offspring even when the contrasting trait is also present.
Limitation: Not all traits exhibit complete dominance; some show incomplete dominance or codominance.
F2 generation: The phenotypes in the second generation show a 3 : 1 ratio.
In the genotype 25 % are homozygous with the dominant trait, 50 % are heterozygous genetic carriers of the recessive trait, 25 % are homozygous with the recessive genetic trait and expressing the recessive character. - Credits : Wikipedia
2. Law of Segregation
Definition: During gamete formation, the two alleles for a trait separate, so each gamete receives only one allele.
Process: Occurs during meiosis, where homologous chromosomes (each carrying different alleles) are separated into different gametes.
Outcome: Each gamete carries only one allele for each gene, ensuring genetic variability.
Recombination: Fertilization restores the diploid state, with offspring receiving one allele from each parent.
Predictability: Allows for the prediction of genotypic and phenotypic ratios in offspring.
3. Law of Independent Assortment
Definition: Alleles of different genes assort independently during gamete formation.
Mechanism: Occurs during metaphase I of meiosis when homologous chromosome pairs align independently at the cell's equator.
Result: This random alignment leads to the production of gametes with various combinations of maternal and paternal chromosomes.
Genetic Variation: Independent assortment contributes to genetic diversity by producing novel genetic combinations.
Limitation: This law applies only to genes located on different chromosomes or those far apart on the same chromosome; genes that are close together on the same chromosome (linked genes) do not assort independently.
References:
- Gardner, E. J., Simmons, M. J., & Snustad, D. P. (1991). Principles of genetics. New York: J. Wiley.
- Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.
- Mendelian Inheritance. Wikipedia. Available at: https://en.wikipedia.org/wiki/Mendelian_inheritance
- Gregor Mendel. Wikipedia. Available at: https://en.wikipedia.org/wiki/Gregor_Mendel
- Pea. Wikipedia. Available at: https://en.wikipedia.org/wiki/Pea
- History of Genetics. Wikipedia. Available at: https://en.wikipedia.org/wiki/History_of_genetics
- Particulate Inheritance. Wikipedia. Available at: https://en.wikipedia.org/wiki/Particulate_inheritance
