Ch 14. 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

Mendel's Experimental Design

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 started with true-breeding varieties, which consistently produce offspring identical to themselves when self-pollinated.

Hybridization and Generations

Mendel mated two contrasting, true-breeding varieties in a process called hybridization. The parental plants are the P generation, their hybrid offspring are the F1 generation, and the offspring of F1 individuals (by self- or cross-pollination) are the F2 generation.

Results of Mendel's Crosses

Mendel observed consistent ratios in the F2 generation for various traits, such as flower color, seed shape, and pod color. These results led to the formulation of Mendel's 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	3.01:1
Flower position	Axial	Terminal	3.14:1
Stem length	Tall	Dwarf	2.84:1

Key Genetic Concepts

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

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

• Allele: Alternative versions of a gene that account for variations in inherited characters.

• Locus: The specific location of a gene on a chromosome.

Mendel's Four Concepts

1. Alternative versions of genes (alleles) account for variations in inherited characters.

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

3. Dominance: If two 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 Ratios

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. The classic monohybrid cross yields a 3:1 phenotypic ratio in the F2 generation.

Genotype and Phenotype

• Genotype: The genetic makeup of an organism (e.g., PP, Pp, or pp).

• Phenotype: The observable physical or biochemical characteristics (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 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

Some genes have more than two alleles in the population, such as the ABO blood group system in humans.

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 the other.

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 Genetics

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, though these are rare if lethal. Examples include achondroplasia (a form of dwarfism) and Huntington's disease (a late-onset neurodegenerative disorder).

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	Phenotypic expression of one gene affects another	Coat color in Labradors
Polygenic inheritance	Single trait affected by two or more genes	Human skin color