Mendel and the Gene Idea: Principles of Inheritance (Chapter 14 Study Notes)

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Mendel and the Gene Idea

Introduction to Heredity and Genetics

The study of heredity explores how traits are passed from parents to offspring. Gregor Mendel's experiments with pea plants established the foundation for modern genetics, introducing the concept of genes as discrete units of inheritance.

Blending Hypothesis: Early idea that parental traits mix in offspring (e.g., blue and yellow make green).
Particulate Hypothesis: Mendel's idea that parents pass on discrete units (now called genes).

Mendel's Experimental Design

Why Pea Plants?

Many varieties with distinct heritable characters (e.g., flower color).
Each character has different traits (e.g., purple or white flowers), which are related to alleles.
Controlled mating: Pea plants have both male (stamens) and female (carpels) organs.
Cross-pollination: Fertilization between different plants.
Self-pollination: Same plant acts as both parents.

True-Breeding and Binary Characters

Mendel tracked only binary (either-or) characters.
Used true-breeding varieties: Plants that, when self-pollinated, always produce offspring with the same trait for the character studied.

Typical Mendelian Experiment

P generation: True-breeding parents.
F1 generation: Hybrid offspring of the P generation.
F2 generation: Produced when F1 individuals self-pollinate.

Crossing true-breeding purple and white flowers: All F1 hybrids are purple.
F1 x F1 crosses yield a 3:1 ratio of purple to white in F2.

Observed Patterns

Character	Dominant Trait	Recessive Trait	F2 Generation Ratio
Flower color	Purple	White	3.15:1
Seed color	Yellow	Green	2.96:1
Seed shape	Round	Wrinkled	2.96:1
Pod color	Green	Yellow	2.82:1
Pod shape	Inflated	Constricted	3.01:1
Flower position	Axial	Terminal	3.14:1
Stem length	Tall	Dwarf	2.84:1

Mendel's Model of Inheritance

Four Key Concepts

Alternative Versions of Genes: Genes exist in different forms called alleles (e.g., purple vs. white flower color).
Two Alleles per Gene: Each organism inherits two alleles for each gene, one from each parent.
Dominance: If alleles differ, the dominant allele determines the organism's appearance; the recessive allele has no noticeable effect.
Law of Segregation: The two alleles for a gene segregate during meiosis and end up in different gametes.

Genotype and Phenotype

Genotype: The genetic makeup (e.g., PP, Pp, pp).
Phenotype: The physical appearance (e.g., purple or white flowers).
Proteins link genotype (DNA) to phenotype (trait).

Punnett Squares

Used to predict the outcome of genetic crosses.
Capital letter = dominant allele; lowercase = recessive allele.
Shows all possible combinations of gametes and resulting offspring genotypes/phenotypes.

Homozygous vs. Heterozygous

Homozygous: Two identical alleles for a gene (true-breeding).
Heterozygous: Two different alleles for a gene (not true-breeding).

Testcross

Used to determine the genotype of an individual with a dominant phenotype.
Cross the individual with a homozygous recessive; if any offspring show the recessive trait, the parent is heterozygous.

Law of Independent Assortment

Monohybrid and Dihybrid Crosses

Monohybrid cross: F1 offspring heterozygous for one character.
Dihybrid cross: F1 offspring heterozygous for two characters.

Law of Independent Assortment

Each pair of alleles segregates independently during gamete formation.
Applies only to genes on different, nonhomologous chromosomes.
Genes close together on the same chromosome tend to be inherited together (linkage).

Dihybrid Cross Example

Gametes	YR	Yr	yR	yr
YR	YYRR	YYRr	YyRR	YyRr
Yr	YYRr	YYrr	YyRr	Yyrr
yR	YyRR	YyRr	yyRR	yyRr
yr	YyRr	Yyrr	yyRr	yyrr

Additional info: This table illustrates the 9:3:3:1 phenotypic ratio expected in a dihybrid cross with independent assortment.

Probability in Genetics

Rules of Probability

Multiplication Rule: Probability of two independent events occurring together is the product of their probabilities.
Addition Rule: Probability of either of two mutually exclusive events is the sum of their probabilities.

Example Problems

Given BbWw x BBww, what fraction of progeny will have black scales and long wings?
Given parents AABbCc and AaBbCc, what is the probability their child will be AAbbCC?

Extending Mendelian Genetics

Degrees of Dominance

Complete Dominance: Heterozygote and dominant homozygote are indistinguishable in phenotype.
Incomplete Dominance: Heterozygote phenotype is intermediate between the two homozygotes (e.g., red x white snapdragons yield pink).
Codominance: Both alleles affect the phenotype in separate, distinguishable ways (e.g., AB blood type).

Multiple Alleles

More than two alleles exist for some genes (e.g., ABO blood group in humans: IA, IB, i).
Each individual still inherits only two alleles per gene.

Pleiotropy

One gene affects multiple phenotypic traits.
Example: Cystic fibrosis gene affects multiple organs and functions.

Epistasis

One gene at one locus affects the expression of a gene at another locus.
Example: In mice, one gene determines pigment color (B = black, b = brown), another gene (C = color, c = no color) determines if pigment is deposited.

Polygenic Inheritance

Multiple genes contribute additively to a single trait, resulting in continuous variation (e.g., human skin color, height).

Nature and Nurture

Phenotype is influenced by both genotype and environment.
Examples: Hydrangea flower color varies with soil pH; height influenced by nutrition.
Multifactorial characters: Traits influenced by multiple genes and environmental factors.

Human Genetics and Pedigree Analysis

Studying Human Inheritance

Humans have long generation times and few offspring, making controlled crosses impractical.
Pedigree: Family tree showing inheritance of traits across generations.
Used to deduce genotypes and inheritance patterns (dominant, recessive, carriers).

Carriers

Individuals heterozygous for a recessive allele but phenotypically normal.
Can pass the recessive allele to offspring.

Key Terms and Definitions

True breeding: Organisms that produce offspring identical for a trait when self-pollinated.
Hybridization: Mating of two different true-breeding varieties.
Monohybrid cross: Cross involving one character.
P generation: Parental generation.
F1 generation: First filial generation (offspring of P generation).
F2 generation: Second filial generation (offspring of F1 self-pollination).
Dominant vs. Recessive: Dominant allele masks the effect of the recessive allele.
Heterozygous vs. Homozygous: Heterozygous = two different alleles; Homozygous = two identical alleles.
Genotype vs. Phenotype: Genotype = genetic makeup; Phenotype = observable traits.

Practice and Application

Use Punnett squares to predict genotypic and phenotypic ratios.
Analyze pedigrees to determine inheritance patterns and identify carriers.
Apply probability rules to genetic crosses.