Mendel and the Gene Idea: Foundations of Classical Genetics

Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with garden peas established the basic principles of heredity, forming the foundation of classical genetics. By analyzing patterns of inheritance, Mendel identified key laws that explain how traits are transmitted from one generation to the next.

Concept 14.1: Mendel’s Experimental Approach and Laws of Inheritance

Experimental Design and Terminology

Mendel used garden peas (Pisum sativum) for his experiments due to their many varieties and ability to self-pollinate or cross-pollinate. He tracked characters with two distinct forms and began with true-breeding varieties, which consistently produce offspring identical to themselves when self-pollinated.

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white flowers).

• Hybridization: Mating two contrasting, true-breeding varieties.

• P Generation: Parental generation.

• F1 Generation: First filial generation, hybrid offspring of the P generation.

• F2 Generation: Offspring resulting from self- or cross-pollination of F1 individuals.

Results of Mendel’s Crosses

Mendel observed consistent ratios in the F2 generation for several characters, leading to the formulation of his laws.

Character	Dominant Trait	Recessive Trait	F2 Ratio
Flower color	Purple	White	3:1
Seed shape	Round	Wrinkled	2.96:1
Seed color	Yellow	Green	2.82:1
Pod shape	Inflated	Constricted	2.95:1
Pod color	Green	Yellow	2.82:1
Flower position	Axial	Terminal	3.14:1
Stem length	Tall	Dwarf	2.84:1

Mendel’s Model: The Law of Segregation

Mendel proposed four key concepts to explain his results:

1. Alternative versions of genes (alleles) account for variations in inherited characters. Each gene resides at a specific locus on a chromosome.

2. Each organism inherits two alleles for each gene, one from each parent. These alleles may be identical (homozygous) or different (heterozygous).

3. If the alleles differ, the dominant allele determines the organism’s appearance; the recessive allele has no noticeable effect.

4. Law of Segregation: The two alleles for a heritable character segregate during gamete formation and end up in different gametes.

Punnett Squares and Genetic Vocabulary

Punnett squares are used to predict the possible combinations of alleles in offspring. A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele.

• Genotype: The genetic makeup (e.g., PP, Pp, pp).

• Phenotype: The observable traits (e.g., purple or white flowers).

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. If any offspring display the recessive phenotype, the unknown parent must be heterozygous.

The Law of Independent Assortment

Mendel’s law of independent assortment states that each pair of alleles segregates independently of other pairs during gamete formation. This law applies to genes on different, nonhomologous chromosomes or those far apart on the same chromosome.

Concept 14.3: Complex Patterns of Inheritance

Degrees of Dominance

• Complete dominance: Heterozygote and dominant homozygote are phenotypically identical.

• Incomplete dominance: Heterozygote phenotype is intermediate between the two homozygotes (e.g., red and white flowers produce pink offspring).

• Codominance: Both alleles are fully expressed in the heterozygote (e.g., AB blood type).

Multiple Alleles and Codominance

Some genes have more than two alleles in the population. The ABO blood group in humans is determined by three alleles (IA, IB, i), with IA and IB being codominant.

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. For example, the gene responsible for sickle-cell disease affects multiple organs and functions.

Epistasis

In epistasis, the expression of one gene affects the expression of another gene. For example, coat color in Labrador retrievers is determined by two genes, where one gene can mask the expression of another.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes independently affect a single trait, resulting in continuous variation (e.g., human skin color, height).

Concept 14.4: Human Mendelian Disorders

Recessively Inherited Disorders

Recessive disorders only appear in individuals who are homozygous for the recessive allele. Heterozygotes are carriers but do not show symptoms. Examples include albinism and cystic fibrosis.

Dominantly Inherited Disorders

Some disorders are caused by dominant alleles. These are less common, especially if they are lethal. Examples include achondroplasia (a form of dwarfism) and Huntington’s disease, which has a late onset.

Summary Tables

Relationships Among Alleles and Genes

Relationship	Description	Example
Complete dominance	Heterozygote phenotype same as homozygous dominant	PP, Pp (purple flowers)
Incomplete dominance	Heterozygote phenotype intermediate	CRCW (pink flowers)
Codominance	Both phenotypes expressed in heterozygotes	IAIB (AB blood group)
Multiple alleles	More than two alleles in the population	IA, IB, i (ABO blood group)
Pleiotropy	One gene affects multiple traits	Sickle-cell disease

Relationship	Description	Example
Epistasis	One gene affects the expression of another	Labrador coat color
Polygenic inheritance	Multiple genes affect one trait	Human skin color

Additional info: Mendel’s principles remain the foundation for modern genetics, though molecular genetics has revealed further complexity in gene interactions and inheritance patterns.