Mendel and the Gene Idea: Principles of Inheritance

Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

The study of heredity and inheritance patterns began with Gregor Mendel's experiments on garden peas. Mendel's scientific approach led to the identification of fundamental laws governing the transmission of traits from parents to offspring. These principles form the basis of classical genetics and are essential for understanding how genetic information is passed through generations.

Concept 14.1: Mendel's Scientific Approach and Laws of Inheritance

Mendel used quantitative experiments to deduce the principles of heredity. He selected pea plants for their variety, short generation time, and ability to control mating. Mendel tracked characters (heritable features) and their traits (variants of characters) to study inheritance.

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

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

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

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

• P generation: True-breeding parent plants.

• F1 generation: Hybrid offspring of the P generation.

• F2 generation: Offspring produced when F1 individuals self- or cross-pollinate.

Mendel's Model of Inheritance

Mendel developed a model to explain the 3:1 inheritance pattern observed in the F2 generation. The model consists of four key concepts:

1. Alternative versions of genes (alleles) account for variations in inherited characters. Each gene resides at a specific locus on a chromosome.

2. For each character, an organism inherits two alleles, one from each parent. These alleles may be identical (homozygous) or different (heterozygous).

3. If the two alleles differ, the dominant allele determines the organism's appearance. The recessive allele has no noticeable effect on appearance.

4. The law of segregation: The two alleles for a heritable character separate during gamete formation and end up in different gametes. This corresponds to the distribution of homologous chromosomes during meiosis.

The model explains the 3:1 ratio observed in the F2 generation, which can be visualized using a Punnett square. Capital letters represent dominant alleles, and lowercase letters represent recessive alleles.

Useful Genetic Vocabulary

• Homozygote: An organism with two identical alleles for a gene (homozygous).

• Heterozygote: An organism with two different alleles for a gene (heterozygous).

• Phenotype: Physical appearance of an organism.

• Genotype: Genetic makeup of an organism.

For example, pea plants with genotypes PP and Pp both have purple flowers (same phenotype) but different genotypes.

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype. The individual is bred with a homozygous recessive partner. If any offspring display the recessive phenotype, the mystery parent must be heterozygous.

The Law of Independent Assortment

Mendel's second law was derived by following two characters simultaneously. Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation. A dihybrid cross determines whether two characters are transmitted together or independently.

• Law of Independent Assortment: Each pair of alleles segregates independently during gamete formation. This law applies to genes on different, nonhomologous chromosomes or those far apart on the same chromosome.

• Monohybrid: Heterozygous for one character.

• Dihybrid: Heterozygous for two characters.

Complex Patterns of Inheritance

Inheritance patterns are often more complex than simple Mendelian genetics. Many characters are not determined by only one gene with two alleles, but the principles of segregation and independent assortment still apply.

Extending Mendelian Genetics for a Single Gene

Single-gene inheritance may deviate from Mendelian patterns in several ways:

• Incomplete dominance: The phenotype of hybrids is intermediate between the two parental varieties.

• Codominance: Two dominant alleles affect the phenotype in separate, distinguishable ways.

• Multiple alleles: Most genes exist in populations in more than two allelic forms.

• Multiple phenotypes: A single gene may produce multiple phenotypes.

For example, the ABO blood group in humans is determined by three alleles (IA, IB, i), resulting in four phenotypes.

Degrees of Dominance

• Complete dominance: Phenotypes of the heterozygote and dominant homozygote are identical.

• Incomplete dominance: Phenotype of hybrids is intermediate.

• Codominance: Both alleles are expressed in the phenotype.

Multiple Alleles Example: ABO Blood Groups

The ABO blood group system in humans is determined by three alleles for the enzyme that attaches carbohydrates to red blood cells:

• IA: Adds A carbohydrate.

• IB: Adds B carbohydrate.

• i: Adds neither.

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

Character	Dominant Trait	Recessive Trait
Flower Color	Purple	White
Seed Shape	Round	Wrinkled
Seed Color	Yellow	Green
Pod Shape	Inflated	Constricted
Pod Color	Green	Yellow
Flower Position	Axial	Terminal
Stem Length	Tall	Dwarf

Key Equations

The expected ratio for a monohybrid cross (Mendel's law of segregation):

$\text{Phenotypic ratio} = 3:1$

The expected ratio for a dihybrid cross (Mendel's law of independent assortment):

$\text{Phenotypic ratio} = 9:3:3:1$

Example: Punnett Square for Monohybrid Cross

Cross between two heterozygous pea plants (Pp x Pp):

$\begin{array}{c|cc} & P & p \\ \hline P & PP & Pp \\ p & Pp & pp \end{array}$

Resulting phenotypes: 3 purple : 1 white

Example: ABO Blood Group Inheritance

Possible genotypes and phenotypes:

Genotype	Phenotype
IAIA, IAi	Type A
IBIB, IBi	Type B
IAIB	Type AB
ii	Type O

Additional info:

These principles are foundational for understanding more complex genetic phenomena, such as polygenic inheritance, gene linkage, and epistasis, which are covered in later chapters.