Zea mays (maize, or corn) was originally domesticated in central Mexico at least 7000 years ago from an endemic grass called teosinte. Teosinte is generally unbranched, has male and female flowers on the same branch, and has few kernels per 'cob,' each encased in a hard, leaf-like organ called a glume. In contrast, maize is highly branched, with a male inflorescence (tassel) on its central branch and female inflorescences (cobs) on axillary branches. In addition, maize cobs have many rows of kernels and soft glumes. George Beadle crossed cultivated maize and wild teosinte, which resulted in fully fertile F₁ plants. When the F₁ plants were self-fertilized, about 1 plant in every 1000 of the F₂ progeny resembled either a modern maize plant or a wild teosinte plant. What did Beadle conclude about whether the different architectures of maize and teosinte were caused by changes with a small effect in many genes or changes with a large effect in just a few genes?

Verified step by step guidance

1. Understand the context of the problem: The question is asking about the genetic basis of the differences in plant architecture between maize and teosinte. Specifically, it asks whether these differences are due to many genes with small effects or a few genes with large effects.
2. Analyze the experimental setup: George Beadle crossed maize and teosinte to produce F₁ plants, which were fully fertile. This indicates that maize and teosinte are closely related species capable of producing viable offspring.
3. Interpret the F₂ generation results: When the F₁ plants were self-fertilized, the F₂ progeny showed a segregation pattern where approximately 1 in 1000 plants resembled either the modern maize or wild teosinte phenotype. This suggests that the traits distinguishing maize and teosinte are controlled by a small number of genes, as a large number of genes with small effects would result in a more continuous variation rather than distinct phenotypes.
4. Relate the findings to genetic principles: The observed ratio of 1 in 1000 indicates that the differences in plant architecture are likely due to a few genes with large effects. This is because such a low frequency of parental phenotypes in the F₂ generation is consistent with the segregation of a small number of Mendelian loci.
5. Conclude Beadle's interpretation: Based on the data, Beadle concluded that the dramatic differences in architecture between maize and teosinte were caused by changes in a small number of genes with large effects, rather than many genes with small effects.

Key Concepts

Genetic Variation

Genetic variation refers to the differences in DNA sequences among individuals within a population. This variation can arise from mutations, gene flow, and sexual reproduction, leading to diverse traits. Understanding genetic variation is crucial for studying how traits are inherited and how they can change over generations, particularly in the context of domestication and selective breeding.

Quantitative vs. Qualitative Traits

Traits can be classified as quantitative or qualitative. Quantitative traits, like kernel number in maize, are influenced by multiple genes and show continuous variation, while qualitative traits are typically controlled by a single gene and exhibit discrete categories. This distinction is essential for understanding the genetic architecture of traits and how they evolve through natural or artificial selection.

Mendelian Inheritance

Mendelian inheritance describes the patterns of inheritance first outlined by Gregor Mendel, which include concepts such as dominant and recessive alleles. In the context of Beadle's experiment, understanding Mendelian principles helps explain how traits are passed from parents to offspring and how the segregation of alleles can lead to the emergence of distinct phenotypes in the F₂ generation.