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In the 1860s, Gregor Mendel, an Augustinian monk, began meticulous breeding experiments with the common pea plant (Pisum sativum). By controlling cross‑pollination, he isolated traits that existed in two distinct forms—what we now call alleles—and demonstrated that each trait was inherited in a predictable, binary manner.
Mendel identified seven binary traits in peas, four of which he studied in depth: plant height (tall vs. short), pod shape (inflated vs. constricted), seed texture (smooth vs. wrinkled), and seed color (green vs. yellow). Each trait segregated independently, laying the groundwork for modern genetics.
1. Genes and Alleles – Traits are encoded by pairs of genes, one inherited from each parent. Alleles are alternative versions of a gene; for height, the alleles are T (tall) and t (short).
2. Dominance – When two different alleles are present, the dominant allele masks the recessive one in the phenotype. The recessive allele is expressed only in a homozygous recessive genotype (tt).
3. Segregation – Alleles separate during gamete formation so that each gamete carries only one allele of a given gene. Consequently, each parent contributes one allele, and a zygote becomes diploid again.
A monohybrid cross examines a single trait where both parents are heterozygous (Aa). Using a Punnett square—a 2 × 2 grid—geneticists predict the proportion of each genotype in the offspring.
For a true monohybrid cross, both parents possess one dominant and one recessive allele. For example, in a color trait where green (G) dominates blue (g), both parents are Gg.
Place one parent’s alleles along the top and the other’s along the side, forming a 2 × 2 grid:
| G | g | |
|---|---|---|
| G | GG | Gg |
| g | Gg | gg |
Each cell represents a possible genotype for an offspring.
The genotype determines the phenotype: GG and Gg both produce green, while gg produces blue. Thus, the expected ratio of green to blue offspring is 3:1. In large populations this ratio holds precisely, while small families may show natural variation.
When multiple traits are examined, the Punnett square expands to 4 × 4 or larger grids. Crosses involving homozygous parents (e.g., GG × gg) yield uniform phenotypes, illustrating how genotype combinations influence outcomes.
These principles underpin modern breeding, genetic counseling, and evolutionary biology, providing a clear, evidence‑based framework for understanding inheritance.
