Chapter 14 – Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with garden peas established the foundational principles of heredity. His work disproved the "blending" hypothesis and introduced the concept of particulate inheritance, where traits are passed as discrete units called genes.

• Blending Hypothesis: The outdated idea that parental traits mix in offspring, producing intermediate forms.

• Particulate Hypothesis: Mendel's model where parents pass on distinct heritable units (genes) that retain their identity across generations.

• Scientific Method: Mendel used controlled experiments and quantitative analysis to deduce laws of inheritance.

Mendel’s Experimental Approach

Mendel selected pea plants for their advantageous traits and controlled breeding. He tracked characters (genes) with clear, alternative forms (traits/alleles).

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

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

• Advantages of Peas: Short generation time, large offspring numbers, controlled mating, and no ethical concerns.

• True-breeding: Plants that consistently produce the same trait when self-pollinated.

Mendel’s Crosses and Generations

Mendel performed hybridization by crossing true-breeding plants. He defined three generations:

• P Generation: True-breeding parents.

• F1 Generation: Hybrid offspring of P generation.

• F2 Generation: Offspring from self- or cross-pollinated F1 hybrids.

• Observation: F1 hybrids showed only the dominant trait; F2 showed a 3:1 ratio of dominant to recessive traits.

Mendel’s Model of Inheritance

Mendel proposed four key concepts to explain the 3:1 ratio observed in F2 generation:

1. Alleles: Alternative versions of genes account for trait variation. Each gene is located at a specific locus on a chromosome.

2. Inheritance: Each organism inherits two alleles for each gene, one from each parent. Homozygous individuals have identical alleles; heterozygous have different alleles.

3. Dominance: If alleles differ, the dominant allele determines phenotype; the recessive allele is masked.

4. Law of Segregation: Two alleles for a character segregate during gamete formation, so each gamete carries only one allele.

Punnett Squares and Genetic Vocabulary

Punnett squares are used to predict genotype and phenotype ratios in offspring. Key terms include:

• Homozygote: Individual with two identical alleles (e.g., PP or pp).

• Heterozygote: Individual with two different alleles (e.g., Pp).

• Phenotype: Physical appearance (e.g., purple flowers).

• Genotype: Genetic makeup (e.g., PP, Pp, or pp).

Testcross

A testcross determines 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 parent is heterozygous.

• Testcross: Breeding a mystery individual with a homozygous recessive individual to reveal genotype.

Law of Independent Assortment

Mendel’s second law states that alleles of different genes assort 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 heterozygotes for two characters.

• Phenotypic Ratio: Dihybrid crosses yield a 9:3:3:1 ratio if genes assort independently.

Probability in Genetics

Mendelian inheritance follows rules of probability. The multiplication rule calculates the probability of independent events occurring together; the addition rule calculates the probability of mutually exclusive events.

• Multiplication Rule:

• Addition Rule:

• Application: Used to predict genotype probabilities in crosses.

Complex Patterns of Inheritance

Inheritance patterns can deviate from simple Mendelian genetics due to various factors:

• Incomplete Dominance: Heterozygotes show intermediate phenotype.

• Codominance: Both alleles are fully expressed in phenotype.

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

• Pleiotropy: One gene affects multiple phenotypic traits.

• Epistasis: One gene affects the expression of another gene.

• Polygenic Inheritance: Multiple genes contribute to a single trait, often resulting in continuous variation.

Environmental Impact on Phenotype

Phenotype can be influenced by environmental factors, especially for polygenic traits. Traits affected by both genes and environment are called multifactorial.

• Example: Hydrangea flower color varies with soil pH.

Human Genetics and Pedigree Analysis

Pedigrees are used to trace inheritance patterns in families. Many human traits follow Mendelian patterns, but ethical and practical constraints limit experimental crosses.

• Pedigree: Diagram showing family relationships and trait inheritance across generations.

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

Recessive and Dominant Inherited Disorders

Genetic disorders can be inherited in recessive or dominant manners. Recessive disorders appear only in homozygous individuals, while dominant disorders can appear in heterozygotes.

• Recessive Disorders: Albinism, cystic fibrosis, sickle-cell disease.

• Dominant Disorders: Achondroplasia (dwarfism), webbed digits.

• Carrier Testing: Genetic tests can identify carriers and help predict risk.

• Multifactorial Disorders: Diseases influenced by both genetic and environmental factors (e.g., heart disease, cancer).

Summary Table: Mendel's F1 Crosses for Seven Characters in Pea Plants

This table summarizes Mendel's results for seven pea plant characters, showing dominant and recessive traits, F2 generation counts, and ratios.

Key Terms and Concepts

• Gene: Unit of heredity; segment of DNA encoding a trait.

• Allele: Alternative form of a gene.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Pleiotropy: One gene affects multiple traits.

• Epistasis: One gene affects expression of another gene.

• Polygenic Inheritance: Multiple genes affect a single trait.

• Pedigree: Family tree tracing inheritance of traits.

• Carrier: Heterozygous individual carrying a recessive allele.

Summary of Mendelian Principles

• Mendel’s laws of segregation and independent assortment form the basis of classical genetics.

• Exceptions to Mendelian genetics include incomplete dominance, codominance, multiple alleles, pleiotropy, epistasis, polygenic inheritance, and environmental effects.

• Pedigree analysis and probability rules are essential tools for predicting inheritance patterns.