Mendel and the Gene: Principles of Inheritance

Chapter 14: Mendel and the Gene

Introduction to Mendel and the Gene

Gregor Mendel established the foundational rules of inheritance through experiments on pea plants. His work led to the Chromosomal Theory of Inheritance, later expanded by Sutton and Boveri, which linked the transmission of genetic information to meiosis and asserted that genes are located on chromosomes.

• Key Concept: Genes are the units of inheritance, and their behavior during meiosis explains patterns of inheritance.

Historical Context: Competing Hypotheses

Before Mendel, two main hypotheses attempted to explain inheritance:

• Blending Inheritance: Parental traits blend in offspring, producing intermediate traits.

• Inheritance of Acquired Characteristics: Traits modified through use are passed to offspring (Lamarck).

Model Organism: The Garden Pea (Pisum sativum)

Mendel used garden peas due to their practical advantages and polymorphic traits (traits with two or more common forms, e.g., purple vs. white flowers). The observable features of an individual are called its phenotype.

Trait	Form 1	Form 2
Seed Shape	Round	Wrinkled
Seed Cotyledon Color	Yellow	Green
Flower Color	White	Violet
Pod Form	Full	Constricted
Pod Color	Green	Yellow
Stem Place	Axial	Terminal
Stem Size	Tall	Short

Mendel's Experimental System

• Self-fertilization: Peas naturally self-pollinate.

• Cross-fertilization: Mendel controlled mating by transferring pollen from one plant to another, enabling precise genetic crosses.

Key Genetic Terms

Term	Definition	Example/Comment
Gene	Hereditary factor influencing a trait	Flower color gene
Allele	Alternative form of a gene	Allele for purple or white flowers
Genotype	Combination of alleles in an individual	RR, Rr, or rr
Phenotype	Observable traits	Round or wrinkled seeds
Homozygous	Two identical alleles	RR or rr
Heterozygous	Two different alleles	Rr
Dominant	Allele expressed in phenotype	R (round)
Recessive	Allele masked in heterozygote	r (wrinkled)

Mendel's Experiments and Principles

Monohybrid Crosses: Principle of Segregation

Mendel crossed plants differing in a single trait (e.g., seed shape). He observed that traits did not blend but appeared in predictable ratios.

• F1 Generation: All offspring showed the dominant trait.

• F2 Generation: Traits appeared in a 3:1 ratio (dominant:recessive).

Principle of Segregation: The two alleles for a gene segregate during gamete formation (anaphase I of meiosis).

Example: Seed Shape

• Cross RR (round) x rr (wrinkled): All F1 are Rr (round).

• Cross Rr x Rr: F2 genotypes are 1 RR : 2 Rr : 1 rr; phenotypes are 3 round : 1 wrinkled.

Dihybrid Crosses: Principle of Independent Assortment

Mendel examined inheritance of two traits simultaneously (e.g., seed shape and color).

• Hypothesis of Independent Assortment: Alleles of different genes assort independently during gamete formation.

• F2 Generation: Phenotypic ratio of 9:3:3:1 (for two heterozygotes).

Punnett Square: Used to predict offspring genotypes and phenotypes.

Testcross

• Crossing an individual with a homozygous recessive to determine the unknown genotype.

Chromosome Theory of Inheritance

Sutton and Boveri connected Mendel's principles to chromosome behavior during meiosis:

• Principle of Segregation: Explained by separation of homologous chromosomes in anaphase I.

• Principle of Independent Assortment: Explained by random alignment of chromosomes in metaphase I.

Extensions to Mendel's Principles

Sex Linkage and Testing the Chromosome Theory

Thomas Hunt Morgan used fruit flies (Drosophila melanogaster) to test the chromosome theory. He discovered sex-linked inheritance, where genes are located on sex chromosomes (X or Y).

• Wild type: Most common phenotype.

• Mutant: Phenotype caused by mutation.

• Sex linkage: Inheritance pattern differs between males and females (e.g., white-eyed mutation in flies is X-linked).

Gene Linkage and Crossing Over

Linkage: Genes located close together on the same chromosome tend to be inherited together.

• Crossing over during meiosis can separate linked genes, producing recombinants.

• The frequency of recombination can be used to map the relative positions of genes (genetic map).

Multiple Alleles, Codominance, and Incomplete Dominance

• Multiple Allelism: More than two alleles exist for a gene (e.g., ABO blood types in humans).

• Codominance: Heterozygotes express both alleles (e.g., AB blood type).

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

Pleiotropy and Polygenic Inheritance

• Pleiotropy: One gene influences multiple traits (e.g., Marfan syndrome).

• Polygenic Inheritance: Multiple genes contribute to a single trait, often producing continuous variation (e.g., human height, skin color).

Gene-Environment Interaction

• The expression of many genes depends on environmental factors (e.g., phenylketonuria [PKU] can be managed with diet).

Human Inheritance and Pedigrees

Pedigrees are diagrams used to study inheritance patterns in families. They help determine whether a trait is autosomal or sex-linked, and whether it is dominant or recessive.

• Autosomal Dominant: Trait appears in every generation; affected offspring have at least one affected parent.

• Autosomal Recessive: Trait can skip generations; affected offspring often have unaffected carrier parents.

• X-linked Recessive: More common in males; trait never passed from father to son; daughters of affected males are carriers.

• X-linked Dominant: Both males and females affected; all daughters of affected males are affected.

Summary Table: Key Inheritance Patterns

Pattern	Key Features	Example
Autosomal Dominant	Appears in every generation; both sexes equally affected	Huntington disease
Autosomal Recessive	Can skip generations; both sexes equally affected	Cystic fibrosis
X-linked Recessive	More males affected; skips generations	Red-green color blindness
X-linked Dominant	Both sexes affected; all daughters of affected males affected	Hypophosphatemia

Key Equations and Concepts

• Monohybrid Cross Genotypic Ratio: $1 : 2 : 1$ (RR : Rr : rr)

• Monohybrid Cross Phenotypic Ratio: $3 : 1$ (dominant : recessive)

• Dihybrid Cross Phenotypic Ratio: $9 : 3 : 3 : 1$

• Testcross: Used to determine the genotype of an individual with a dominant phenotype by crossing with a homozygous recessive.

Additional info: These notes integrate foundational Mendelian genetics with modern extensions, including gene linkage, multiple alleles, and human inheritance patterns, providing a comprehensive overview for exam preparation.