Chapter 14: Mendel and the Gene

Chromosome Theory of Inheritance

The Chromosome Theory of Inheritance, proposed by Sutton and Boveri in 1902,

states that genetic information is encoded in genes located on chromosomes. This theory connects the physical process of

meiosis with the patterns of inheritance observed by Mendel.

• Pattern: All cells contain genetic information in genes on chromosomes.

• Process: Meiosis explains how genes are segregated and assorted into gametes.

Gregor Mendel and the Foundations of Genetics

Genetics is the branch of biology focused on the inheritance of traits. Gregor Mendel, an Austrian monk,

is considered the father of genetics due to his pioneering work on heredity using the garden pea (Pisum sativum).

• Heredity: Transmission of traits from parent to offspring.

• Trait: Any characteristic of an individual that is inherited, not learned.

• Model Organism: Mendel used peas because they are cheap, easy to grow, have a short reproductive cycle,

  and produce many offspring.

• Mendel controlled matings by artificial pollination and studied seven traits, each with two distinct phenotypes.

Mendel’s Experimental System

• Pure (True Breeding) Lines: Plants that produce offspring identical to themselves when self-fertilized.

• Hybrids: Offspring from crosses between different pure lines.

Monohybrid Crosses and Mendel’s Laws

A monohybrid cross examines the inheritance of a single trait (e.g., seed shape: round vs. wrinkled).

• P (Parental) Generation: Round seeds × wrinkled seeds.

• F1 Generation: All round seeds (dominant trait).

• F2 Generation: Both round and wrinkled seeds appear, typically in a 3:1 ratio.

Dominant Trait: Visible in the F1 generation. Recessive Trait: Reappears in the F2 generation.

Particulate Inheritance Hypothesis

• Hereditary determinants (genes) do not blend or become modified; they act as discrete,

  unchanging particles.

Genotype vs. Phenotype

• Genotype: The alleles an individual has for a trait (e.g., RR, Rr, rr).

• Phenotype: The physical expression of the genotype (e.g., round or wrinkled seeds).

The Principle of Segregation

• During gamete formation, alleles segregate into different gametes (from diploid to haploid).

• Fertilization restores the diploid state, bringing together two copies of each gene.

• Homozygous: Two identical alleles for a trait.

• Heterozygous: Two different alleles for a trait.

Punnett Squares and Probability Rules

• Punnett Square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

• Multiplication Rule: Probability that two independent events occur together is the product of their individual probabilities.

• Addition Rule: Probability that any one of several mutually exclusive events occurs is the sum of their individual probabilities.

Dihybrid Crosses and the Principle of Independent Assortment

A dihybrid cross examines the inheritance of two traits simultaneously. Mendel’s experiments led to the

Principle of Independent Assortment: alleles of different genes are transmitted independently of each other.

Testcrosses

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with

a homozygous recessive individual.

Significance of Mendel’s Work

• Use of appropriate model organisms and traits with discrete differences.

• Large sample sizes and application of probability rules.

• Identification of two fundamental patterns: segregation of alleles and independent assortment of genes.

The Chromosome Theory of Inheritance

Connecting Mendel’s Laws to Chromosomes

• Mendel’s principles were explained physically by the behavior of chromosomes during meiosis.

• Principle of Segregation and Principle of Independent Assortment are both explained by chromosome

  movement in meiosis.

Testing the Chromosome Theory: Morgan’s Experiments

• Thomas Hunt Morgan used Drosophila melanogaster (fruit fly) as a model organism.

• Discovered sex-linked inheritance through eye color experiments.

• Wild-type (red eyes) vs. mutant (white eyes); white eyes are recessive and X-linked.

Sex Chromosomes and Sex-Linked Inheritance

• Nettie Stevens discovered sex chromosomes in beetles.

• Females: XX; Males: XY.

• X and Y chromosomes do not carry the same genes.

• X-linked inheritance: Gene is on the X chromosome.

• Y-linked inheritance: Gene is on the Y chromosome.

• Autosomal inheritance: Gene is on a non-sex chromosome.

Gene Linkage and Genetic Mapping

• Gene linkage: Tendency of alleles of particular genes to be inherited together because they are on the same

  chromosome.

• Linked genes do not display independent assortment at Metaphase I of meiosis.

• Crossing over can produce recombinant genotypes.

• Genetic mapping: The frequency of recombination between genes can be used to estimate their distance

  apart on a chromosome.

• Genes with cross-over frequencies ≥ 50% are considered unlinked (on different chromosomes or far

  apart on the same chromosome).

Extensions of Mendelian Genetics

Multiple Allelism and Polymorphism

• Multiple allelism: More than two alleles exist for a gene in a population.

• Polymorphism: Multiple phenotypes associated with a single gene.

• Example: ABO blood type system (alleles: IA, IB, i; phenotypes: A, B, AB, O).

Incomplete Dominance

• Heterozygotes have a phenotype intermediate between the two homozygotes.

• Example: Four o’clock flowers (red × white = pink offspring).

Codominance

• Both alleles are fully expressed in heterozygotes.

• Example: AB blood type (both IA and IB alleles expressed).

Pleiotropy

• A single gene influences multiple traits.

• Example: Marfan syndrome affects connective tissue, impacting multiple organ systems.

Environmental Effects on Phenotype

• Phenotype is often influenced by both genotype and environment (e.g., nutrition, sunlight).

• Example: Phenylketonuria (PKU) is a genetic disorder that can be managed by dietary intervention.

Gene-Gene Interactions

• Expression of one gene can be affected by one or more other genes.

• Example: Epistasis, where one gene masks the effect of another.

Expressivity

• The extent to which an allele is expressed in an individual can vary.

• Example: Polydactyly (extra digits) shows variable expressivity.

Quantitative Traits

• Traits that vary continuously and are usually influenced by multiple genes (polygenic inheritance).

• Often display a bell-shaped (normal) distribution in populations.

• Example: Human height.

Pedigree Analysis

A pedigree is a family tree used to study the inheritance of traits in humans.

Pedigree analysis helps identify inheritance patterns and predict the likelihood of traits appearing in future generations.

Additional info: Some examples and definitions were expanded for clarity and completeness. For more practice,

refer to the lab manual pages indicated in the original notes.