Mendel and the Gene: Foundations of Classical Genetics

Chapter 14: Mendel and the Gene

The Garden Pea as a Model Organism

Gregor Mendel established the foundation for the chromosome theory of inheritance through his experiments with garden peas (Pisum sativum). Mendel selected peas because they were inexpensive, easy to grow, had a short generation time, produced many seeds, and allowed controlled mating. These features made the garden pea an ideal model organism for genetic studies.

Key Terms in Mendelian Genetics

Understanding Mendel's experiments requires familiarity with several key terms:

• Gene: A hereditary factor that determines a particular trait.

• Allele: Different forms of a gene.

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics of an organism.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

• Dominant allele: An allele that determines the phenotype in a heterozygote.

• Recessive allele: An allele whose phenotype is masked in a heterozygote.

Self-Fertilization and Cross-Fertilization in Peas

Pea plants can reproduce by self-fertilization (pollen from the same plant fertilizes its ovules) or cross-fertilization (pollen from one plant fertilizes the ovules of another). Mendel used both methods to control genetic crosses and analyze inheritance patterns.

The Principle of Segregation

Mendel's monohybrid crosses led to the principle of segregation, which states that two members of each gene pair segregate into different gametes during gamete formation. This explains why offspring inherit one allele from each parent. Mendel used letters to represent alleles (e.g., R for dominant, r for recessive).

The Principle of Independent Assortment

Through dihybrid crosses, Mendel discovered the principle of independent assortment: alleles of different genes are transmitted independently of one another. This principle predicts that four possible phenotypes will appear in a 9:3:3:1 ratio in the F2 generation of a dihybrid cross.

Testcrosses

A testcross involves crossing an individual with a dominant phenotype (but unknown genotype) with an individual that is homozygous recessive. The phenotypes of the offspring reveal the genotype of the unknown parent. Mendel used testcrosses to confirm the principle of independent assortment.

Meiosis Explains Mendel’s Principles

The behavior of chromosomes during meiosis explains Mendel’s principles. Genes located on different nonhomologous chromosomes assort independently, accounting for the independent assortment of traits. The segregation of homologous chromosomes during meiosis I explains the principle of segregation.

Extending Mendel’s Rules

Further research revealed exceptions and extensions to Mendel’s rules, including linkage, multiple alleles, codominance, and incomplete dominance.

Linkage and Crossing Over

Linkage refers to the tendency of genes located on the same chromosome to be inherited together. However, crossing over during meiosis can separate linked genes. The frequency of recombinant offspring is proportional to the distance between genes and can be used to construct genetic maps.

Multiple Allelism

Some genes have more than two alleles in a population, a phenomenon known as multiple allelism. For example, the human ABO blood group is determined by three common alleles (IA, IB, and i), each coding for a different enzyme that modifies the surface of red blood cells.

Codominance

In codominance, both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits. The AB blood type is an example, where both IA and IB alleles are expressed.

Incomplete Dominance

With incomplete dominance, the heterozygote displays an intermediate phenotype between the two homozygotes. For example, crossing red-flowered and white-flowered plants produces offspring with pink flowers.

Environmental Effects on Phenotype

Most phenotypes are influenced by both genes and the environment. Mendel minimized environmental variation in his experiments, but in nature, factors such as sunlight, water, and soil can affect the expression of traits.

Quantitative Traits

Some traits, called quantitative traits, do not fall into discrete categories but vary continuously (e.g., height, skin color). These traits are typically influenced by multiple genes (polygenic inheritance) and environmental factors, resulting in a bell-shaped distribution in populations.

Applying Mendel’s Rules to Human Inheritance

Human geneticists use pedigrees (family trees) to study inheritance patterns. The mode of transmission describes whether a trait is autosomal or sex-linked and whether it is dominant or recessive. Pedigrees can help distinguish between these patterns.

Autosomal vs. Sex-Linked Inheritance

• Autosomal traits are located on non-sex chromosomes.

• Sex-linked traits are located on sex chromosomes (usually the X chromosome).

Pedigree Analysis

Pedigrees can reveal whether a trait is dominant or recessive, and whether it is autosomal or X-linked. For example, X-linked recessive traits (such as red-green color blindness) are more common in males, while X-linked dominant traits affect both sexes but do not skip generations.

Additional info: The above notes synthesize and expand upon the provided content, integrating standard academic context for clarity and completeness. Tables referenced in the original material (e.g., summary of terms, Punnett squares, and genetic maps) are described in text and illustrated with relevant images where appropriate.