Summary
Highlights
Gregor Mendel is known as the father of modern genetics for discovering the basic principles of heredity through breeding garden peas (pisum sativum). Before Mendel, the blending hypothesis was prevalent, suggesting offspring were a blend of parents. Mendel's work disproved this, showing traits are passed on discretely. He studied science under notable figures like Christian Doppler and conducted experiments in the abbey gardens.
Pea plants were ideal for Mendel's experiments due to the wide variety of available 'characters' (heritable features), such as flower color, seed color, and shape, with distinct 'traits' (variants like purple or white flowers). Other advantages included a short generation time, large numbers of offspring, and controlled mating through self-pollination or cross-pollination. A single pea flower contains both male (anthers forming pollen with sperm) and female (carpel containing eggs) reproductive parts, allowing for both self-fertilization and cross-fertilization experiments.
Mendel observed that some purple pea plants, when self-fertilized, always produced purple offspring ('true breeders'). However, other purple plants would produce a mix of purple and white offspring ('non-true breeders'). White flower plants, when self-fertilized, consistently produced only white offspring and were always true breeders. This observation was the starting point for his groundbreaking crosses.
Mendel's monohybrid cross involved crossing true-breeding purple flowers with true-breeding white flowers (P generation). The first filial generation (F1) consisted entirely of purple flowers, disproving the blending hypothesis. When these F1 purple flowers were self-fertilized, the F2 generation exhibited a 3:1 phenotypic ratio of purple to white flowers. This indicated that the white trait was not lost but masked, leading to the principle of segregation: individuals carry two alleles for each gene, and these alleles segregate randomly into gametes, so offspring inherit one allele from each parent. Purple was identified as a dominant trait and white as recessive.
Genotype refers to the allele combination (e.g., Big P Little P for purple and white alleles), while phenotype is the observable trait (e.g., purple flower color). True breeders have two identical alleles (homozygous: Big P Big P or Little P Little P). Non-true breeders have two different alleles (heterozygous: Big P Little P). Recessive traits like white (Little P Little P) can only be expressed when two recessive alleles are present, explaining why there are no non-true breeding white flowers.
A Punnett square is used to predict the genotypes and phenotypes of offspring. For the F1 generation self-cross (Big P Little P x Big P Little P), the Punnett square shows genotypes: 1 Big P Big P (true breeder purple), 2 Big P Little P (non-true breeder purple), and 1 Little P Little P (true breeder white). This results in a 1:2:1 genotypic ratio and confirms the 3:1 phenotypic ratio (3 purple, 1 white) observed in the F2 generation, further validating the principle of segregation.
Mendel then studied two characteristics simultaneously in a dihybrid cross, such as seed color (yellow/green) and seed shape (round/wrinkly). He crossed true-breeding round yellow pea plants (Big R Big R Big Y Big Y) with true-breeding wrinkly green pea plants (Little R Little R Little Y Little Y). The F1 generation showed 100% round yellow peas, establishing round and yellow as dominant traits.
When the F1 generation (Big R Little R Big Y Little Y) was self-fertilized, the F2 generation produced new combinations of traits, such as round green and wrinkly yellow peas, which were not present in the P generation. This demonstrated that traits are not linked but assort independently. The observed phenotypic ratio in the F2 generation was 9:3:3:1 (9 round yellow, 3 round green, 3 wrinkly yellow, 1 wrinkly green). This led to Mendel's principle of independent assortment, stating that alleles for different genes assort independently of one another during gamete formation, a phenomenon later linked to independent assortment during meiosis.