Mendel and the Gene Idea: Principles of Inheritance

Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with garden peas established the fundamental principles of heredity, forming the basis of classical genetics. His work demonstrated that inheritance is governed by discrete units called genes, which are passed from parents to offspring in predictable patterns.

Historical Context and Mendel's Approach

• Blending Hypothesis: Suggested that parental traits blend in offspring (e.g., blue and yellow paint make green).

• Particulate Hypothesis: Mendel's idea that parents pass on discrete heritable units (genes) that retain their identity in offspring.

• Experimental Organism: Mendel used Pisum sativum (garden pea) due to its short generation time, large number of offspring, and controlled mating.

Mendel's Experimental Design

• Character: A heritable feature (e.g., flower color).

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

• True-breeding: Plants that produce offspring of the same variety when self-pollinated.

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

• P Generation: Parental generation (true-breeding).

• F1 Generation: First filial generation (hybrids).

• F2 Generation: Offspring of F1 individuals (self- or cross-pollinated).

Results of Mendel's Crosses

Mendel observed consistent ratios in the F2 generation for seven pea plant characters, supporting the particulate hypothesis.

Character	Dominant Trait	Recessive Trait	F2 Ratio
Flower color	Purple	White	3.15:1
Seed color	Yellow	Green	3.01:1
Seed shape	Round	Wrinkled	2.96:1
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 of Inheritance

Mendel proposed a model with four key concepts to explain the 3:1 ratio in the F2 generation:

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.

3. If the alleles differ, the dominant allele determines the phenotype; 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 Square: Diagram for predicting the results of a genetic cross between individuals of known genotype.

• Homozygous: Two identical alleles for a gene (e.g., PP or pp).

• Heterozygous: Two different alleles for a gene (e.g., Pp).

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

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 is heterozygous.

Law of Independent Assortment

Mendel's second law states that each pair of alleles segregates independently during gamete formation. This law applies to genes on different chromosomes or those far apart on the same chromosome.

• Monohybrid Cross: Cross between heterozygotes for one character.

• Dihybrid Cross: Cross between individuals heterozygous for two characters, producing a 9:3:3:1 phenotypic ratio in the F2 generation.

Probability in Genetics

• Multiplication Rule: Probability of two independent events occurring together is the product of their individual probabilities.

• Addition Rule: Probability of any one of two or more mutually exclusive events is the sum of their individual probabilities.

Complex Patterns of Inheritance

Inheritance patterns can be more complex than Mendel predicted:

• Incomplete Dominance: Heterozygotes have an intermediate phenotype (e.g., pink flowers from red and white parents).

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

• Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood groups).

• Pleiotropy: One gene affects multiple phenotypic traits (e.g., sickle-cell disease).

Epistasis and Polygenic Inheritance

• Epistasis: One gene affects the expression of another gene (e.g., coat color in Labrador retrievers).

• Polygenic Inheritance: Multiple genes independently affect a single trait, resulting in continuous variation (e.g., human skin color).

Environmental Impact on Phenotype

Phenotype can be influenced by both genotype and environmental factors. Traits affected by multiple genes and the environment are called multifactorial traits.

Human Genetics and Pedigree Analysis

• Pedigree: Family tree that describes the inheritance of a trait across generations.

• Pedigrees are used to predict the probability of genetic disorders in offspring.

Recessively and Dominantly Inherited Disorders

• Recessive Disorders: Expressed only in homozygous individuals (e.g., albinism, cystic fibrosis, sickle-cell disease).

• Dominant Disorders: Expressed in heterozygotes (e.g., achondroplasia, Huntington's disease).

• Carriers: Heterozygotes who carry a recessive allele but are phenotypically normal.

Genetic Testing and Counseling

• Genetic Counseling: Uses family history and probability rules to assess risk of genetic disorders.

• Carrier Testing: Identifies individuals who carry recessive alleles for genetic diseases.

• Fetal Testing: Includes amniocentesis and chorionic villus sampling (CVS) to detect genetic abnormalities before birth.

• Newborn Screening: Routine tests for genetic disorders (e.g., phenylketonuria, PKU).

Summary Table: Key Terms and Concepts

Term	Definition
Gene	Unit of heredity; segment of DNA encoding a trait
Allele	Alternative form of a gene
Genotype	Genetic makeup of an organism
Phenotype	Observable traits of an organism
Homozygous	Two identical alleles for a gene
Heterozygous	Two different alleles for a gene
Pleiotropy	One gene influences multiple traits
Epistasis	One gene affects the expression of another gene
Polygenic inheritance	Multiple genes contribute to a single trait