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 modern genetics through his experiments with garden peas (Pisum sativum). He 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 studying inheritance.

Key Terms in Mendelian Genetics

• Gene: A hereditary unit that influences a specific trait.

• Allele: Different forms of a gene found at the same locus on homologous chromosomes.

• Genotype: The genetic makeup of an organism (e.g., RR, Rr, rr).

• Phenotype: Observable characteristics resulting from genotype and environment.

• Homozygous: Having two identical alleles for a gene (e.g., RR or rr).

• Heterozygous: Having two different alleles for a gene (e.g., Rr).

• Dominant allele: Expressed in the phenotype even if only one copy is present.

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

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 segregate into gametes independently.

• Genotypic ratio in F2 generation: 1:2:1 (RR:Rr:rr).

• Phenotypic ratio in F2 generation: 3:1 (dominant:recessive).

Dihybrid Crosses and the Principle of Independent Assortment

Mendel extended his experiments to two traits at once (dihybrid crosses). He found that alleles of different genes assort independently during gamete formation, leading to new combinations of traits. This is the principle of independent assortment.

• Predicted phenotypic ratio in F2 generation: 9:3:3:1 for two traits.

• Genes on different chromosomes assort 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 offspring phenotypes reveal the unknown genotype.

• Confirms predictions 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 during the separation of homologous chromosomes.

Extending Mendel’s Rules

Not all traits follow simple Mendelian inheritance. Researchers discovered more complex patterns, including linkage, multiple alleles, codominance, and incomplete dominance.

Linkage and Crossing Over

Linkage refers to the tendency of genes located close together on the same chromosome to be inherited together. However, crossing over during meiosis can separate linked genes, producing recombinant offspring. The frequency of recombination can be used to create genetic maps showing the relative positions of genes.

Multiple Allelism

Some genes have more than two alleles in a population (multiple allelism). For example, the human ABO blood group is determined by three alleles (IA, IB, i), resulting in four blood types.

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 (AB blood type).

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 pink-flowered offspring.

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. Thus, phenotype is the result of the interaction between genotype and environment.

Quantitative Traits

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

Human Inheritance and Pedigree Analysis

In humans, inheritance patterns are studied using pedigrees (family trees). Pedigree analysis helps determine whether a trait is autosomal or sex-linked, and whether it is dominant or recessive.

• Autosomal traits: Located on non-sex chromosomes.

• Sex-linked traits: Located on sex chromosomes (usually X-linked).

• Dominant traits: Expressed when at least one dominant allele is present.

• Recessive traits: Expressed only when two recessive alleles are present.

X-Linked Recessive Traits

X-linked recessive traits (e.g., red-green color blindness) are more common in males, as they have only one X chromosome. Affected males inherit the allele from their carrier mothers.

X-Linked Dominant Traits

X-linked dominant traits affect both males and females, but all daughters of affected males will be affected, while sons will not (if the mother is unaffected).

Summary Table: Mendelian Genetics Terms

Term	Definition
Gene	Unit of heredity that influences a trait
Allele	Alternative form of a gene
Genotype	Genetic makeup of an organism
Phenotype	Observable characteristics
Homozygous	Two identical alleles
Heterozygous	Two different alleles
Dominant	Allele expressed in phenotype
Recessive	Allele masked in heterozygote

Summary Table: F2 Generation from Monohybrid Crosses

Genotype	Phenotype	Ratio
RR	Dominant	1
Rr	Dominant	2
rr	Recessive	1

Key Equations

• Probability of genotype in F2 (monohybrid cross): $P(\text{genotype}) = \frac{\text{number of individuals with genotype}}{\text{total number of individuals}}$

• Probability of phenotype in F2 (monohybrid cross): $P(\text{phenotype}) = \frac{\text{number of individuals with phenotype}}{\text{total number of individuals}}$

Additional info: These notes integrate foundational Mendelian genetics with modern extensions, including linkage, multiple alleles, and human inheritance patterns, to provide a comprehensive overview suitable for college-level biology students.