Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 14.1 Mendel Used the Scientific Approach to Identify Two Laws of Inheritance
Gregor Mendel’s experiments with pea plants revealed the fundamental principles of heredity. By applying a scientific and quantitative method, Mendel discovered the laws of segregation and independent assortment—cornerstones of classical genetics.
Mendel’s Experimental Method
Mendel studied pea plants, which self-fertilize and have easily distinguishable traits
He used true-breeding lines: plants that consistently produced the same trait over generations
Mendel controlled crosses by removing stamens and transferring pollen manually
Tracked only characters with two distinct traits (e.g., purple vs. white flowers)
Observed inheritance patterns over multiple generations: P → F₁ → F₂
Used large sample sizes and quantitative analysis to derive reliable ratios
Key Terminology
Character: a heritable feature (e.g., flower color)
Trait: a variant of a character (e.g., purple or white)
P generation: true-breeding parent plants
F₁ generation: hybrid offspring of the P generation
F₂ generation: offspring from self- or cross-pollinated F₁ hybrids
Alleles: alternative versions of a gene
Homozygous: identical alleles (e.g., PP or pp)
Heterozygous: different alleles (e.g., Pp)
Phenotype: observable trait (e.g., purple flowers)
Genotype: genetic makeup (e.g., PP, Pp, pp)
Mendel’s Model & The Law of Segregation
Each character is determined by a gene, which comes in alleles (e.g., purple vs. white flower allele)
Each organism inherits two alleles, one from each parent
If alleles differ: the dominant allele determines appearance; the recessive allele is masked
During gamete formation, the two alleles segregate into different gametes
This explains why recessive traits reappear in the F₂ generation
Punnett squares predict outcomes of genetic crosses (e.g., 3:1 ratio in F₂ for dominant:recessive)
The Testcross
Determines whether an individual showing a dominant phenotype is PP or Pp
Crossed with a recessive homozygote (pp)
If all offspring show the dominant trait → the unknown is PP
If a 1:1 ratio appears → the unknown is Pp
Mendel used testcrosses to verify genotypes and inheritance patterns
The Law of Independent Assortment
From dihybrid crosses (e.g., seed color and seed shape), Mendel observed a 9:3:3:1 ratio in F₂
Concluded that alleles for different genes segregate independently during gamete formation
This law applies when genes are on different chromosomes or far apart on the same chromosome
Independent assortment produces new combinations not seen in either parent (e.g., yellow wrinkled seeds)
In a Nutshell
Through careful cross-breeding of pea plants, Mendel discovered the law of segregation and the law of independent assortment. His use of statistics, controlled experiments, and large sample sizes laid the foundation for modern genetics.