Chapter 14: Mendel & Inheritance – Principles of Classical Genetics

Historical Views of Inheritance

Pre-Mendelian Theories

Before the discovery of modern genetics, several theories attempted to explain inheritance:

• Homunculus Theory (pre-1900): Proposed that each sperm contained a tiny human.

• Hippocrates (~400 B.C.): Suggested that particles from both parents traveled to sex organs and merged, resulting in offspring as a mixture.

• Blended Inheritance: The belief that parental traits blended in offspring, which failed to explain the persistence of distinct traits.

Gregor Mendel: The Father of Genetics

Contributions and Experiments

Gregor Mendel, an Austrian monk, conducted systematic research in genetics using garden peas (Pisum sativum). He identified two fundamental laws of inheritance and is recognized as the founder of modern genetics.

• Studied physics, botany, and mathematics.

• Discovered the process of heredity through controlled breeding experiments.

Genetics Terminology

Key Definitions

• Chromosome: Structure in the nucleus made of DNA, containing genetic information.

• Homologous Chromosomes: Maternal and paternal copies of a chromosome.

• Gene: A segment of DNA that encodes a specific protein (Mendel called it a "factor").

• Allele: Alternate forms of a gene, arising from mutations.

• Each person inherits two alleles for each gene—one from each parent.

• Humans have approximately 25,000 genes, each located on homologous chromosome pairs.

Dominant and Recessive Alleles

Allelic Interactions

• Dominant Allele: Exerts its effect whenever present.

• Recessive Allele: Masked if a dominant allele is present; often codes for a non-functional protein.

• The most common allele is not always the dominant one (e.g., blue eyes are common in northern Europe but recessive).

• Except for identical twins, every individual has a unique combination of alleles.

Mendel's Pea Plant Experiments

Experimental Design and Terminology

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

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

Why Peas?

• Easy to grow, rapid development, many offspring.

• Many traits exist in two forms; easy to control mating.

• Each flower has male (stamens) and female (stigma, style, ovules) parts; normally self-pollinate but can be artificially cross-pollinated.

True-Breeding and Hybridization

Definitions

• True-breeding: Generations of self-fertilization produce offspring identical to parents.

• Hybridization: Mating of two true-breeding plants with different traits.

Characters Used by Mendel

Traits and Crosses

• Studied 7 true-breeding traits (one type of allele each).

• Monohybrid Cross: Crossing two plants differing in a single trait.

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

Mendel's Experimental Methods

Artificial Pollination

• Cross fertilization by transferring pollen from one plant to another.

• Set up all possible combinations of crosses to analyze inheritance patterns.

Generational Terminology

• P Generation: Parental, true-breeding plants.

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

• F2 Generation: Offspring of self-fertilized F1 plants; showed a 3:1 ratio of dominant to recessive traits.

Genetic Models and Punnett Squares

Segregation and Ratios

• Mendel's segregation model explains the 3:1 ratio in F2 generation.

• Punnett Square: Diagram for predicting genetic cross results.

Parent Genotype	Gametes	F1 Genotype	F2 Genotype
PP x pp	P, p	Pp	PP, Pp, pp

Testcross: Determining Genotype

Method and Interpretation

• Testcross: Breeding an individual with dominant phenotype to a homozygous recessive individual.

• If any offspring display the recessive phenotype, the parent is heterozygous.

• Cannot be used in humans; pedigree analysis is used instead.

Mendelian Inheritance

Law of Segregation

• Two alleles of each gene separate randomly during gamete formation.

• Each gamete receives only one allele of each gene.

Law of Independent Assortment

• Alleles of different genes assort independently into gametes.

• Transmission of alleles for one trait does not affect transmission of alleles for another trait.

• Law holds for most genes, but exceptions exist (e.g., linked genes).

Monohybrid and Dihybrid Crosses

Monohybrid Cross

• Mating between two heterozygotes for one gene.

• F2 generation shows a 3:1 phenotypic ratio.

Dihybrid Cross

• Tracks inheritance of two genes at once.

• Cross between two heterozygous F1 plants yields four phenotypes in a 9:3:3:1 ratio.

Genotype	Phenotype	Ratio
YYRR	Round, Yellow	9
YYrr	Round, Green	3
yyRR	Wrinkled, Yellow	3
yyrr	Wrinkled, Green	1

Summary of Mendel's Laws

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes on different chromosomes are inherited independently.

Key Equations

• Probability of two independent events (Multiplication Rule): $P(A \text{ and } B) = P(A) \times P(B)$

• Probability of either of two mutually exclusive events (Addition Rule): $P(A \text{ or } B) = P(A) + P(B)$

Additional info:

• Pedigree analysis is used in humans to deduce genotypes across generations.

• Linked genes on the same chromosome may not assort independently due to physical proximity.