Mendelism: Basic Principles of Inheritance

Topic Outline from Dr. Alessandro Siani

  • Mendel’s Study of Heredity

  • Applications of Mendel’s Principles

  • Formulating and Testing Genetic Hypotheses

  • Mendelian Principles in Human Genetics

  • Non-Mendelian Inheritance

Note: The inheritance patterns described in this lecture refer to eukaryotic organisms with sexual reproduction (i.e., NOT bacteria!).

The Puzzle of Inheritance

  • Artificial Selection:

    • Important practice since before recorded history.

    • Breeders were unable to explain the disappearance and reappearance of traits in subsequent generations.

    • Traits were thought to be transmitted through a fluid mechanism, blending in offspring.

    • Paradox of Blending:

    • If blending occurs, why do individuals not look alike?

Gregor Mendel (1822–1884)

  • Chose to study pea plants due to several reasons:

    • Previous research showed that pea hybrids could be produced.

    • Availability of many pea varieties.

    • Small size of pea plants made them easy to cultivate.

    • Peas possess the ability to self-fertilize or be cross-fertilized.

    • Personal note: “Peas go really well with fish and chips.”

The Garden Pea (Pisum sativum)

  • Structure of the Pea Plant:

    • Pollen recipient: emasculated stigma (part of the flower).

    • Pollen donor: includes anther (part of the stamen).

Mendel’s Experimental Method

  • The experimental method comprised usually three stages:

    1. Produce True-Breeding Strains:

    • Develop pure strains for each trait studied.

    1. Cross-Fertilization of True-Breeding Strains:

    • Cross true-breeding strains representing different forms of the specific trait.

    1. Self-Fertilization of Hybrid Offspring:

    • Allow the hybrid offspring (F1 generation) to self-fertilize for several generations.

    • Count offspring showing traits.

Terms Defined

  • Phenotype:

    • An observable characteristic or trait (e.g., blue eyes).

  • Genotype:

    • The genetic makeup concerning a particular trait, meaning the genes that code for traits like eye color.

F1 Generation (First Filial Generation)

  • Characteristics of F1 Generation:

    • Produced by crossing two true-breeding (homozygous) strains.

    • For every trait analyzed, all F1 plants resembled only one parent (dominant phenotype).

    • No intermediate characteristics were observed.

    • Such traits are called "Mendelian traits."

F2 Generation (Second Filial Generation)

  • Characteristics of F2 Generation:

    • Resulting from the self-fertilization of F1 (heterozygous) plants.

    • The recessive trait, hidden in the F1 generation, appeared among some F2 individuals.

    • Mendel counted proportions of differing traits.

Dominant vs. Recessive Traits in F2 Generation

  • Counted Ratios for Dominant and Recessive Traits:

    • Traits counted with their forms:

    • Flower Color: Purple (705) vs. White (224) → Ratio 3.15:1

    • Seed Color: Yellow (6022) vs. Green (2001) → Ratio 3.01:1

    • Seed Shape: Round (5474) vs. Wrinkled (1850) → Ratio 2.96:1

    • Pod Color: Green (428) vs. Yellow (152) → Ratio 2.82:1

    • Pod Shape: Inflated (882) vs. Constricted (299) → Ratio 2.95:1

    • Flower Position: Axial (651) vs. Terminal (207) → Ratio 3.14:1

    • Plant Height: Tall (787) vs. Dwarf (277) → Ratio 2.84:1

Conclusions from Mendel’s Studies

  • Key Findings:

    • Mendel’s plants did not show intermediate traits.

    • For each trait, one phenotype was dominant and the other was recessive.

    • In the F2 generation, alternative traits were expressed at a ratio of ¾ dominant to ¼ recessive.

Five-Element Model of Inheritance

  1. Discreteness of Factors:

    • Parents transmit discrete factors (called genes).

  2. Gene Copies:

    • Each individual receives one copy of a gene from each parent.

  3. Allelic Variation:

    • Not all copies of a gene are identical; different versions are called alleles.

  4. No Blending of Alleles:

    • Alleles remain discrete, with no blending occurring.

  5. Expression of Alleles:

    • The presence of an allele does not guarantee its expression.

Mendel’s 1st Law: Principle of Segregation

  • States that two alleles for a given gene segregate (separate) during the formation of gametes and are randomly rejoined during fertilization.

  • Physical Basis:

    • The behavior of chromosomes during meiosis underpins allele segregation.

Segregation Principle: The Modern Explanation

  • Mechanisms during Meiosis I and Meiosis II ensure the segregation of alleles into haploid gametes:

    • Homologous chromosomes align and separate appropriately.

Punnett Square Explanation

  • Example:

    • Cross a purple-flowered plant with a white-flowered plant.

    • Define dominant allele: P = purple flowers; recessive allele: p = white flowers.

  • True-breeding white-flowered plant is pp; purple-flowered plant is PP.

Testcross to Determine Genotype

  • Testcross Process:

    • Cross the individual with unknown genotype (P_) with a homozygous recessive (pp).

    • Two possibilities arise from offspring resulting phenotype.

    • All offspring are purple if the unknown flower is homozygous dominant (PP).

    • Offspring show a 1:1 ratio of purple to white if the unknown flower is heterozygous (Pp).

The Principle of Independent Assortment

  • Analysis of two separate traits in a single cross illustrates independent assortment.

  • Producing two true-breeding lines for distinct traits (e.g., Pea shape and Pea color):

  • F1 Generation shows dominant phenotypes (ROUND, YELLOW).

  • Allow F1 to self-fertilize to produce F2, leading to all four possible phenotypes in the ratio of 9:3:3:1 for a dihybrid cross (RrYy x RrYy).

Mendel’s 2nd Law: Principle of Independent Assortment

  • The alleles of each gene assort into gametes independently from one another.

  • Segregation of different allele pairs occurs independently during alignment in meiosis.

  • Applies only to genes located on different chromosomes.

Key Concepts of Mendelian Genetics

  • Law of Dominance:

    • When crossing two true-breeding plants, only one phenotype arises in the F1 generation.

    • Example: PP x pp results in 100% Pp (purple).

  • Law of Segregation:

    • Each gamete receives one copy of the gene, explaining dominant trait disappearance in F1 and reappearance in F2:

    • Example: Pp x Pp yields 75% Purple and 25% White.

  • Law of Independent Assortment:

    • Alleles of different genes are sorted into gametes independently.

    • Example: RyRy x RyRy produces ratios of 9/16 Round & Yellow: 3/16 Round & Green: 3/16 Wrinkled & Yellow: 1/16 Wrinkled & Green.

Non-Mendelian Inheritance

  • Non-Mendelian traits often do not fit the clear dominant-recessive relationship.

  • Incomplete Dominance:

    • Heterozygous phenotype is different from both homozygous phenotypes (e.g., Red × White = Pink).

  • Codominance:

    • Both alleles are expressed simultaneously (e.g., AB blood type).

  • Extranuclear Inheritance:

    • Involves traits inherited from the mitochondrial genome.

  • Polygenic Traits:

    • Traits determined by multiple genes leading to continuous or quantitative variation (e.g., human height).

  • Pleiotropy:

    • A single gene affects multiple phenotypic traits.

Blood Groups as an Example of Codominance

  • ABO Blood Type Alleles:

    • Three alleles: IA, IB, and i.

    • IA and IB are codominant while both are dominant over allele i.

  • Genotypes and Corresponding Blood Groups:

    • Phenotypes B: IBIB or IBi

    • Phenotypes A: IAIA or IAi

    • Phenotype O: ii

    • Phenotype AB: IAIB

Mitochondrial Inheritance

  • Traits are derived from mitochondrial DNA.

  • Mitochondrial disorders can be present in every generation and affect both genders, but fathers do not pass mitochondrial traits to their children.

Analysis of Human Traits Using Pedigree

  • Traits can be observed through family trees (pedigrees) to track inheritance. Examples include:

  • Dominant Pedigree: Neurofibromatosis.

  • Recessive Pedigree: Sickle cell anemia.

Autosomal vs. X-Linked Inheritance

  • Traits discussed thus far are autosomal (located on autosomes 1-22):

    • Autosomal recessive traits occur in offspring from unaffected parents (e.g., cystic fibrosis).

    • Autosomal dominant traits arise in each generation and affect offspring equivalently (e.g., Huntington's disease).

X-Linked Inheritance Patterns

  • Male Bias in Disorders:

    • X-linked recessive diseases show prevalence in males and carriers in females.

    • Sons of affected males do not carry the disorder, while all daughters do.

  • X-Linked Dominant Disorders:

    • Affected males pass conditions to daughters, but not to sons.

Summary of Genetic Principles

  • Genetics encapsulates various principles of inheritance applicable to both Mendelian and non-Mendelian traits, providing a foundation for understanding biological inheritance in eukaryotic organisms.

  • Understanding the mechanisms of inheritance is crucial in areas such as genetic counseling, breeding practices, and medical genetics.

Worksheet and Breaktime Exercise

  • Students are encouraged to complete related worksheets at home or during class.

  • Assistance is available for those needing help.