Mendel and the Gene: Patterns of Inheritance and Extensions

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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.

Gregor Mendel: The Father of Genetics

Genetics is the branch of biology focused on the inheritance of traits. Mendel, an Austrian monk, conducted experiments in the 1800s to understand heredity—the transmission of traits from parent to offspring. He used the garden pea (Pisum sativum) as a model organism due to its ease of cultivation, short reproductive cycle, and ability to produce large numbers of offspring.

Trait: Any inherited characteristic of an individual.
Heredity: Transmission of traits from parent to offspring.

Mendel’s Experimental System

Studied seven traits, each with two distinct phenotypes (observable features).
Used pure (true-breeding) lines to ensure consistent inheritance patterns.
Created hybrids by crossing different strains.

Mendel’s Experiments with a Single Trait (Monohybrid Cross)

A monohybrid cross examines the inheritance of a single trait. For example, crossing round-seeded plants with wrinkled-seeded plants:

P (parental) generation: round seeds × wrinkled seeds
F1 (first filial) generation: all round seeds (dominant trait)
F2 generation: 3:1 ratio of round to wrinkled seeds

Dominant vs. Recessive Traits

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.

The Punnett Square 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.

Experiments with Two Traits (Dihybrid Cross)

A dihybrid cross examines the inheritance of two traits simultaneously.

Principle of Independent Assortment: Alleles of different genes are transmitted independently of each other during gamete formation.

Testcross

Used to determine the genotype of an individual with a dominant phenotype by crossing with a homozygous recessive individual.

Importance of Mendel’s Work

Use of appropriate model organisms and traits with discrete differences.
Large sample sizes for statistical reliability.
Application of probability rules and testcrosses.
Identified two fundamental patterns: segregation of alleles and independent assortment of genes.

The Chromosome Theory of Inheritance

Mendel’s patterns of inheritance were later explained by the behavior of chromosomes during meiosis.

Principle of Segregation: Homologous chromosomes separate during meiosis I.
Principle of Independent Assortment: Chromosomes assort independently during meiosis I.

Testing the Chromosome Theory

Thomas Hunt Morgan used Drosophila melanogaster (fruit fly) as a model organism.
Discovered sex-linked inheritance through eye color experiments.
Nettie Stevens discovered sex chromosomes (XX in females, XY in males).

Sex-Linked Inheritance

X-linked inheritance: Gene located on the X chromosome.
Y-linked inheritance: Gene located on the Y chromosome.
Autosomal inheritance: Gene located on a non-sex chromosome.

Extending Mendel’s Rules

Gene Linkage

Alleles of genes located close together on the same chromosome tend to be inherited together (linkage).
Linked genes do not assort independently unless crossing over occurs.

Linkage and Genetic Mapping

Genes are arranged linearly on chromosomes.
Recombination frequency can be used to estimate distances between genes (linkage mapping).

Map Distance Formula

Genes with cross-over frequencies ≥ 50% are considered unlinked.

Multiple Allelism

More than two alleles exist for a gene, resulting in polymorphism.
Example: ABO blood type system (IA, IB, i alleles; four phenotypes).

Incomplete Dominance

Heterozygotes display 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: Type AB blood (both IA and IB alleles expressed).

Pleiotropy

A single gene influences multiple traits.
Example: Marfan syndrome (affects connective tissue, eyes, skeleton, etc.).

Environmental Effects on Phenotype

Phenotype is influenced by both genotype and environment (e.g., nutrition, sunlight).
Example: Phenylketonuria (PKU) can be managed with a low-phenylalanine diet.

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 degree to which a genotype is expressed in an individual can vary.
Example: Polydactyly (extra digits) varies in expression among individuals.

Quantitative Traits

Traits that vary continuously and are usually influenced by multiple genes (polygenic inheritance).
Often show a bell-shaped distribution in populations.
Example: Human height.

Pedigrees and Human Inheritance

Pedigrees are family trees used to study inheritance patterns in humans.

Inheritance Pattern	Key Features	Examples
Autosomal Recessive	Trait appears only in homozygous recessive individuals; parents are often carriers; can skip generations	Cystic fibrosis, Sickle cell anemia
Autosomal Dominant	Trait appears in every generation; affected individuals have at least one affected parent	Huntington’s disease
X-linked Recessive	More common in males; often skips generations; passed from mother to son	Red-green color blindness
X-linked Dominant	Rare; affected males pass trait to all daughters, not sons	Hypophosphatemia

Pedigrees help identify carriers and predict inheritance patterns for genetic counseling.

Additional info: For practice problems and further examples, refer to the indicated pages in your lab manual.