Mendel and the Gene Idea: Principles of Inheritance (Chapter 14)

Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the fundamental principles of inheritance, which explain how traits are transmitted from parents to offspring. These principles form the basis of classical genetics and are essential for understanding heredity in all organisms.

• Trait Transmission: Traits are inherited as discrete units called genes.

• Alleles: Alternative versions of a gene that account for variations in inherited characters.

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

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

How Traits Are Transmitted from Parents to Offspring

Alleles and Chromosomes

Each parent has two alleles for each gene, located at the same locus on homologous chromosomes. During gamete formation, these alleles segregate so that each gamete receives only one allele for each gene.

• Homologous Chromosomes: Chromosome pairs, one from each parent, that carry the same genes at the same loci.

• Gamete Formation: The process by which sperm and eggs are produced, each carrying only one allele for each gene.

• Fertilization: Offspring inherit one allele from each parent, restoring the pair of alleles for each gene.

Law of Segregation

The Law of Segregation states that the two alleles for a heritable character separate during gamete formation and end up in different gametes. This explains why offspring inherit one allele from each parent.

• Segregation occurs during meiosis, corresponding to the distribution of homologous chromosomes.

• Each gamete contains only one allele for each gene.

Mendel's Experiments and Laws

Experimental Design

Mendel used true-breeding pea plants with distinct traits and performed controlled crosses to study inheritance patterns.

• P Generation: Parental generation, true-breeding for specific traits.

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

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

Law of Independent Assortment

The Law of Independent Assortment states that each pair of alleles segregates independently of other pairs during gamete formation. This law applies to genes located on different chromosomes or those far apart on the same chromosome.

• Explains the inheritance of two or more traits simultaneously.

• Demonstrated by dihybrid crosses (crosses involving two characters).

Genotype and Phenotype

Definitions

• Genotype: The genetic makeup of an organism; the specific alleles present (e.g., PP, Pp, or pp).

• Phenotype: The observable physical or biochemical characteristics of an organism (e.g., purple or white flowers).

Punnett Squares

Punnett squares are used to predict the possible combinations of alleles in offspring from a genetic cross.

• Capital letters represent dominant alleles (e.g., P for purple).

• Lowercase letters represent recessive alleles (e.g., p for white).

Types of Dominance

Complete Dominance

In complete dominance, the phenotype of the heterozygote is identical to that of the dominant homozygote.

Incomplete Dominance

In incomplete dominance, the phenotype of heterozygotes is intermediate between the phenotypes of the two homozygotes.

Codominance

In codominance, both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits distinctly.

Multiple Alleles and Pleiotropy

Multiple Alleles

Some genes have more than two alleles in the population. A classic example is the ABO blood group system in humans.

Genotype	Blood Group	Surface Carbohydrates
IAIA or IAi	A	A
IBIB or IBi	B	B
IAIB	AB	A and B
ii	O	None

Pleiotropy

Pleiotropy occurs when one gene affects multiple phenotypic traits. For example, the gene responsible for sickle-cell disease affects multiple systems in the body.

Epistasis and Polygenic Inheritance

Epistasis

Epistasis is the interaction between genes, where the expression of one gene affects or masks the expression of another gene at a different locus.

Polygenic Inheritance

Polygenic inheritance occurs when a single phenotypic character is affected by two or more genes, resulting in continuous variation (e.g., human height, skin color).

Human Inheritance Patterns

Recessively Inherited Disorders

Recessive genetic disorders only appear in individuals who are homozygous for the recessive allele. Carriers are heterozygous and do not show symptoms but can pass the allele to offspring.

Dominantly Inherited Disorders

Some disorders are caused by dominant alleles. These are less common and often result from new mutations. An example is achondroplasia (a form of dwarfism) and Huntington's disease (a late-onset neurodegenerative disorder).

Summary Table: Relationships Among Inheritance Patterns

Pattern	Description	Example
Complete Dominance	Heterozygote phenotype same as homozygous dominant	Purple flower color in peas
Incomplete Dominance	Heterozygote phenotype intermediate between homozygotes	Pink snapdragon flowers
Codominance	Both phenotypes expressed in heterozygotes	AB blood group
Multiple Alleles	More than two alleles in the population	ABO blood group
Pleiotropy	One gene affects multiple traits	Sickle-cell disease
Epistasis	One gene affects expression of another gene	Coat color in mice
Polygenic Inheritance	Multiple genes affect one trait	Human height

Key Equations and Concepts

• Probability of a genotype in a monohybrid cross:

$P = \frac{\text{Number of ways to obtain genotype}}{\text{Total number of possible outcomes}}$

• Phenotypic ratio in a monohybrid cross (complete dominance):

$3:1$ (dominant:recessive)

• Genotypic ratio in a monohybrid cross:

$1:2:1$ (homozygous dominant : heterozygous : homozygous recessive)

• Phenotypic ratio in a dihybrid cross (independent assortment):

$9:3:3:1$

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the summary tables and equations.