Mendel and the Gene: Principles of Inheritance and Extensions

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 cultivate, 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

Mendelian genetics uses specific terminology to describe inheritance patterns:

• 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 traits of an organism.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

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

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

Mendel’s Monohybrid Crosses and the Principle of Segregation

Mendel’s monohybrid crosses involved parents differing in one trait. He discovered that two members of each gene pair segregate during gamete formation, so each gamete carries only one allele for each gene. This is known as the principle of segregation.

• Dominant and recessive alleles are represented by uppercase and lowercase letters, respectively (e.g., R and r for seed shape).

Mendel’s Dihybrid Crosses and the Principle of Independent Assortment

By crossing individuals differing in two traits (dihybrid cross), Mendel found that alleles of different genes assort independently during gamete formation. This is the principle of independent assortment. The Punnett square for a dihybrid cross predicts four possible phenotypes in a 9:3:3:1 ratio, supporting the idea that genes are transmitted independently.

Testcrosses

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. The phenotypes of the offspring reveal the unknown genotype. Mendel used testcrosses to confirm the principle of independent assortment.

Chromosomal Basis of Mendel’s Principles

Mendel’s principles are explained by the behavior of chromosomes during meiosis:

• Genes located on different nonhomologous chromosomes assort independently.

• Segregation of alleles occurs because homologous chromosomes separate during meiosis I.

Extending Mendel’s Rules

Further research revealed that not all traits follow Mendel’s simple patterns. Some traits are influenced by linkage, multiple alleles, codominance, incomplete dominance, environmental effects, and polygenic inheritance.

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, especially if they are far apart. The frequency of recombinant offspring can be used to estimate the distance between genes, creating a genetic map.

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, 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. For example, individuals with genotype IAIB have both A and B antigens on their red blood cells, resulting in the AB blood group.

Incomplete Dominance

In 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. Environmental factors such as sunlight, water, and soil can affect the expression of genetic traits. Mendel minimized environmental variation in his experiments, but in nature, gene-environment interactions are common.

Quantitative Traits and Polygenic Inheritance

Some traits, called quantitative traits, show continuous variation and are influenced by multiple genes (polygenic inheritance). These traits often display a normal distribution in populations, such as human height or skin color.

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 dominant: Trait appears in every generation; affected individuals have at least one affected parent.

• Autosomal recessive: Trait may skip generations; affected individuals can have unaffected parents.

• X-linked recessive: More common in males; affected males often have carrier mothers.

• X-linked dominant: Affects both sexes; affected males pass the trait to all daughters but no sons.

Mode of Inheritance	Key Features
Autosomal Dominant	Appears in every generation; both sexes equally affected; affected offspring have at least one affected parent.
Autosomal Recessive	May skip generations; both sexes equally affected; affected offspring can have unaffected parents.
X-linked Recessive	More males affected; trait often skips generations; affected males usually have carrier mothers.
X-linked Dominant	Both sexes affected; affected males pass trait to all daughters; does not skip generations.

Additional info: The above table summarizes the main differences between modes of inheritance as inferred from pedigree analysis.