Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the foundational principles of heredity, which explain how traits are transmitted from parents to offspring. His scientific approach led to the identification of two key laws of inheritance: the law of segregation and the law of independent assortment.

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

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

• Allele: Alternative versions of a gene responsible for different traits.

Mendel’s Experimental Approach

Mendel used garden peas for his experiments due to their short generation time, large number of offspring, and controllable mating. He tracked characters with two distinct forms and started with true-breeding varieties.

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

• P Generation: True-breeding parents.

• F1 Generation: Hybrid offspring of the P generation.

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

Mendel’s Model of Inheritance

Mendel proposed a model explaining the inheritance of traits based on the behavior of alleles.

• Alleles: Genes exist in alternative forms (alleles) at specific loci on chromosomes.

• Dominant vs. Recessive: If two alleles differ, the dominant allele determines appearance; the recessive allele is masked.

• Law of Segregation: The two alleles for a character separate during gamete formation, so each gamete receives only one allele.

• Punnett Square: Diagram used to predict allele combinations in offspring.

Genetic Vocabulary

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

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

• Phenotype: Physical appearance of an organism.

• Genotype: Genetic makeup of an organism.

The Law of Independent Assortment

Mendel's second law states that alleles of different genes assort independently during gamete formation. This 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.

Probability Laws in Genetics

Mendelian inheritance follows the rules of probability, similar to coin tossing or dice rolling. The outcome of one event does not affect another, and alleles segregate independently.

Degrees of Dominance

• Complete Dominance: Heterozygote and dominant homozygote have identical phenotypes.

• Incomplete Dominance: Heterozygote phenotype is intermediate between parental phenotypes.

• Codominance: Both alleles affect the phenotype in distinguishable ways.

Example: Incomplete Dominance in Snapdragon Color

Crossing red and white snapdragons produces pink hybrids, demonstrating incomplete dominance.

Example: ABO Blood Groups

Multiple alleles exist for the ABO blood group gene, with codominance between A and B alleles.

Pleiotropy

Most genes have multiple phenotypic effects, known as pleiotropy. For example, the gene responsible for sickle-cell disease affects multiple symptoms.

Extension of Mendelian Genetics: Two or More Genes

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

• Polygenic Inheritance: Multiple genes independently affect a single trait.

Example: Epistasis in Labrador Retrievers

Coat color depends on two genes: one for pigment color and one for pigment deposition. The phenotypic ratio is modified from the expected 9:3:3:1 to 9:3:4.

Example: Polygenic Inheritance of Skin Color

Skin color is determined by the additive effects of multiple genes.

Mendelian Patterns in Human Genetics

Although humans are not ideal for genetic research, Mendelian genetics forms the basis of human genetics. Pedigree analysis is used to study inheritance patterns in families.

Pedigree Analysis

A pedigree is a family tree that tracks the inheritance of traits across generations.

Recessive Alleles and Genetic Disorders

• Recessive Disorders: Only appear in individuals homozygous for the allele.

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

• Albinism: Recessive disorder causing lack of pigmentation.

• Cystic Fibrosis: Most common lethal genetic disease in the U.S., caused by defective chloride channels.

• Sickle-Cell Disease: Caused by a single amino acid substitution in hemoglobin; heterozygotes have a selective advantage against malaria.

Example: Sickle-Cell Disease and Trait

Homozygotes have abnormal hemoglobin and severe symptoms; heterozygotes are generally healthy but resistant to malaria.

Key Equations

• Probability of a specific genotype:

• Law of Segregation:

• Law of Independent Assortment: