Mendel and the Gene Idea: Principles of Inheritance

Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with garden peas established the fundamental principles of heredity, which form the basis of classical genetics. His quantitative approach and careful breeding experiments allowed him to deduce how traits are transmitted from parents to offspring.

Mendel’s Experimental, Quantitative Approach

Key Features of Mendel’s Experiments

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

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

• Mendel used pea plants due to their many varieties, short generation time, large number of offspring, and the ability to control mating (self- or cross-pollination).

• He tracked characters with two distinct forms and started with true-breeding varieties (offspring identical to parents when self-pollinated).

Hybridization and Generations

• 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.

The Law of Segregation

Discovery and Explanation

• Mendel observed that crossing true-breeding purple and white flowered plants produced all purple F1 hybrids, contradicting the blending hypothesis.

• Self-pollination of F1 hybrids produced F2 plants with a 3:1 ratio of purple to white flowers.

• Mendel concluded that the purple trait was dominant and the white trait was recessive.

• The "heritable factor" is now known as a gene.

Mendel’s Model of Inheritance

Four Key Concepts

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

2. Each organism inherits two alleles for each gene, one from each parent. These alleles may be identical (homozygous) or different (heterozygous).

3. If the alleles differ, the dominant allele determines the organism’s appearance; the recessive allele has no noticeable effect.

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

Punnett Squares and Genetic Ratios

• Punnett squares illustrate possible combinations of alleles from parental gametes.

• Capital letters represent dominant alleles; lowercase letters represent recessive alleles.

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.

• Genotype: Genetic makeup.

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. If any offspring display the recessive phenotype, the unknown parent is heterozygous.

The Law of Independent Assortment

Monohybrid and Dihybrid Crosses

• Monohybrid cross: Cross between heterozygotes for one character.

• Dihybrid cross: Cross between individuals heterozygous for two characters.

• Law of independent assortment: Each pair of alleles segregates independently during gamete formation. Applies to genes on different chromosomes or far apart on the same chromosome.

Probability Laws in Mendelian Inheritance

Rules of Probability

• Multiplication rule: Probability of two independent events occurring together is the product of their individual probabilities.

• Addition rule: Probability of any one of two or more mutually exclusive events is the sum of their individual probabilities.

Complex Patterns of Inheritance

Degrees of Dominance

• Complete dominance: Heterozygote and dominant homozygote are phenotypically identical.

• Incomplete dominance: Heterozygote phenotype is intermediate between the two parental varieties.

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

Multiple Alleles

• Most genes exist in more than two allelic forms.

• Example: ABO blood groups in humans are determined by three alleles (IA, IB, and i).

Pleiotropy

• Most genes have multiple phenotypic effects (pleiotropy).

• Example: Cystic fibrosis and sickle-cell disease are caused by pleiotropic alleles.

Epistasis

• One gene affects the expression of another gene.

• Example: Coat color in Labrador retrievers is determined by two genes—one for pigment color and one for pigment deposition.

Polygenic Inheritance

• Multiple genes independently affect a single trait (polygenic inheritance).

• Quantitative characters, such as height and skin color, show continuous variation.

Environmental Impact on Phenotype

• Phenotype can be influenced by both genotype and environment (multifactorial traits).

• Example: Hydrangea flower color varies with soil pH.

Mendelian Inheritance in Humans

Pedigree Analysis

• Pedigree: Family tree describing the inheritance of a trait across generations.

• Used to predict probabilities of future offspring inheriting certain traits.

Recessively Inherited Disorders

• Disorders appear only in individuals homozygous for the recessive allele.

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

• Example: Albinism is a recessive condition characterized by lack of pigmentation.

Dominantly Inherited Disorders

• Caused by dominant alleles; rare if lethal.

• Examples: Achondroplasia (dwarfism), Huntington’s disease (late-onset neurodegenerative disorder).

Multifactorial Disorders

• Many diseases (e.g., heart disease, cancer) have both genetic and environmental components.

• Lifestyle choices can significantly affect phenotype, regardless of genotype.

Genetic Testing and Counseling

• Genetic counselors provide information to prospective parents about inherited diseases.

• Fetal and newborn testing can detect genetic disorders early.

• Techniques include amniocentesis, chorionic villus sampling (CVS), and ultrasound.