Biology 1 - Lesson 13: Mendelian Genetics
Gregor Mendel’s groundbreaking experiments in the mid-19th century established many of the fundamental principles of inheritance. By studying pea plants (Pisum sativum), he deduced that discrete “factors” (now known as genes) are passed from parents to offspring in predictable ratios. His work on dominant and recessive traits, segregation, and independent assortment laid the foundation for modern genetics.
Key Terminology
Before delving into Mendel’s laws, it is essential to understand some core genetic terms:
- Gene: A heritable unit that affects a trait (e.g., gene for flower color).
- Allele: Alternative versions of the same gene (e.g., purple-flower allele vs. white-flower allele).
- Genotype: The genetic makeup of an organism (e.g., PP, Pp, or pp for a flower-color gene).
- Phenotype: The observable characteristics resulting from the genotype (e.g., purple or white flowers).
- Homozygous: Having two identical alleles for a gene (PP or pp).
- Heterozygous: Having two different alleles for a gene (Pp).
- Dominant Allele: Masks the expression of another allele (represented by uppercase letter, e.g., P).
- Recessive Allele: Expressed only when homozygous and no dominant allele is present (represented by lowercase letter, e.g., p).
Mendel’s Laws of Inheritance
- Law of Segregation: The two alleles for a heritable character segregate during gamete formation, ending up in different gametes. Thus, each gamete carries only one allele for each gene.
- Law of Independent Assortment: Alleles of different genes assort independently of each other during gamete formation, provided they are on different chromosomes or far apart on the same chromosome. This law explains how traits can be inherited independently (e.g., seed shape does not dictate seed color).
Monohybrid Cross Example
A monohybrid cross considers a single trait. For instance, crossing a true-breeding purple-flowered plant (PP) with a true-breeding white-flowered plant (pp) yields all purple offspring (Pp) in the F1 generation. Self-pollinating the F1 plants produces an F2 generation with a 3:1 ratio of purple:white.
| P | p | |
|---|---|---|
| P | PP (Purple) | Pp (Purple) |
| p | Pp (Purple) | pp (White) |
Dihybrid Cross Example
A dihybrid cross explores the inheritance of two traits simultaneously—for example, seed color (Yellow = Y, Green = y) and seed shape (Round = R, Wrinkled = r). Crossing two doubly heterozygous plants (YyRr × YyRr) typically yields a 9:3:3:1 phenotypic ratio in the F2 generation:
- 9 with both dominant traits (Yellow, Round)
- 3 with one dominant, one recessive trait (Yellow, Wrinkled)
- 3 with the other dominant, other recessive trait (Green, Round)
- 1 with both recessive traits (Green, Wrinkled)
This ratio exemplifies the Law of Independent Assortment—alleles for seed color sort independently of those for seed shape.
YyRr)) -- x --> B((Parent 2
YyRr)) B --> C[Gamete Combinations
YR, Yr, yR, yr] A --> C C --> D{"F2 Offspring
(9:3:3:1)"}
Test Crosses
A test cross is used to determine the genotype of an individual that expresses a dominant trait. By crossing this unknown individual with a homozygous recessive individual (e.g., pp for flower color), the phenotypes of the offspring can reveal whether the mystery genotype is homozygous dominant (PP) or heterozygous (Pp).
Applications and Extensions
Mendelian genetics laid the groundwork for understanding how traits are passed through generations, but many complexities exist in real-world inheritance:
- Incomplete Dominance (blended phenotypes, e.g., red × white flowers produce pink offspring)
- Codominance (both alleles expressed equally, e.g., human ABO blood groups)
- Multiple Alleles (more than two alleles for a gene in a population, e.g., ABO blood types)
- Pleiotropy (one gene affecting multiple traits, e.g., cystic fibrosis)
- Polygenic Inheritance (multiple genes influencing a single trait, e.g., skin color, height)
These more complex inheritance patterns will be explored in subsequent lessons, illustrating that while Mendel’s rules are foundational, real genetic systems often have additional layers of regulation and variability.
Conclusion
Mendel’s experiments with pea plants remain a cornerstone of genetics. His laws of segregation and independent assortment illuminate how traits can be predicted, tested, and quantified in controlled crosses. While more intricate patterns of inheritance exist, Mendelian principles are essential for understanding heredity, predicting genotypic and phenotypic outcomes, and guiding studies in modern molecular biology, medicine, and evolutionary research.
