Mendel and the Gene: Foundations and Extensions of Mendelian 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 peas 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: Expressed in the phenotype when present.

• Recessive allele: Expressed only when two copies are present.

Self-Fertilization and Cross-Fertilization in Peas

Pea plants can reproduce by self-fertilization (pollen from the same plant fertilizes ovules) or cross-fertilization (pollen from one plant fertilizes ovules of another). Mendel controlled these processes to study inheritance patterns.

The Principle of Segregation

Mendel's principle of segregation states that two members of each gene pair segregate into different gametes during egg and sperm 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).

Monohybrid Crosses and Punnett Squares

Monohybrid crosses involve parents that differ in one trait. Mendel's experiments showed that the F2 generation exhibited a 3:1 ratio of dominant to recessive phenotypes, supporting the principle of segregation.

Dihybrid Crosses and the Principle of Independent Assortment

Mendel's dihybrid crosses (involving two traits) demonstrated that alleles of different genes assort independently during gamete formation. The expected phenotypic ratio in the F2 generation is 9:3:3:1, supporting the principle of independent assortment.

Testcrosses

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

Chromosomal Basis of Mendel’s Principles

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

• Segregation: Homologous chromosomes separate during meiosis I, explaining allele segregation.

• Independent Assortment: Nonhomologous chromosomes assort independently, explaining the independent transmission of genes located on different chromosomes.

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 used to estimate the distance between genes, forming the basis of genetic mapping.

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 red blood cell surface polysaccharides.

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, heterozygotes display an intermediate phenotype between the two homozygotes. For example, crossing red-flowered and white-flowered plants produces pink-flowered offspring.

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 trait expression.

Quantitative Traits

Some traits, called quantitative traits, vary continuously rather than falling into discrete categories. These traits are typically influenced by multiple genes (polygenic inheritance) and environmental factors, resulting in a bell-shaped distribution of phenotypes.

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 help determine the genetic basis of traits and diseases.

Autosomal vs. Sex-Linked Inheritance

• Autosomal traits: Located on non-sex chromosomes; affect males and females equally.

• Sex-linked traits: Located on sex chromosomes (usually X); often show different patterns in males and females.

Pedigree Analysis

Pedigrees can distinguish between autosomal dominant, autosomal recessive, X-linked recessive, and X-linked dominant inheritance patterns. For example, X-linked recessive traits (like red-green color blindness) are more common in males, while X-linked dominant traits affect both sexes but do not skip generations.

Inheritance Pattern	Key Features
Autosomal Dominant	Appears in every generation; affected individuals have at least one affected parent
Autosomal Recessive	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; does not skip generations; affected males pass trait to all daughters

Additional info: The above table summarizes the main inheritance patterns discussed in human genetics and their distinguishing features.