Mendelian Genetics: Principles, Analysis, and Applications

Mendelian Genetics

Introduction

Mendelian genetics is the foundation of classical genetics, describing how traits are inherited through discrete units called genes. Gregor Mendel's experiments with pea plants established the basic principles of heredity, which remain central to genetic analysis today.

Gregor Mendel: Pioneer of Modern Genetics

Factors Contributing to Mendel's Success

• Model Organism: Mendel used the garden pea plant (Pisum sativum), which was ideal due to its easily distinguishable traits, controlled pollination, and rapid generation time.

• Meticulous Quantification: Mendel carefully counted and recorded the outcomes of his crosses, allowing for statistical analysis.

• Controlled Crosses: He used true-breeding plants to ensure consistent results.

Mendel's Experimental Design

Model Organism: Garden Pea Plant

• First model organism for genetics: Pea plants have many distinct, heritable traits.

• Advantages:

  • Easy to grow and cross-pollinate

  • Short generation time

  • Large number of offspring per cross

Meticulous Quantification of Experimental Results

Character	Contrasting Traits	F1 Results	F2 Results	F2 Ratio
Seed shape	round/wrinkled	all round	5474 round, 1850 wrinkled	2.96:1
Seed color	yellow/green	all yellow	6022 yellow, 2001 green	3.01:1
Pod shape	full/constricted	all full	882 full, 299 constricted	2.95:1
Pod color	green/yellow	all green	428 green, 152 yellow	2.82:1
Flower color	violet/white	all violet	705 violet, 224 white	3.15:1
Flower position	axial/terminal	all axial	651 axial, 207 terminal	3.14:1
Stem height	tall/dwarf	all tall	787 tall, 277 dwarf	2.84:1

Mendel's Postulates or Principles

1. Unit Factors Exist in Pairs

• Genetic characters are controlled by unit factors (genes) that exist in pairs in individual organisms.

• Three possible combinations: homozygous dominant, homozygous recessive, heterozygous.

2. Dominant vs. Recessive

• One factor (allele) may mask the expression of another (dominant vs. recessive).

• Example: Tall (dominant) vs. dwarf (recessive) pea plants.

3. Segregation

• During gamete formation, the paired unit factors separate randomly so that each gamete receives one or the other with equal likelihood.

4. Independent Assortment

• Genes for different traits assort independently of one another in gamete formation.

• All possible combinations of alleles can occur in gametes.

Genetics Terminology

• Phenotype: The observable physical or biochemical characteristics of an organism as determined by its genotype (e.g., flower color, blood type).

• Genotype: The genetic constitution of an organism; the specific alleles present.

• Alleles: Different forms of a gene found at the same locus (e.g., tall and dwarf alleles for stem height).

• Gene Nomenclature: The first letter of the recessive trait is used to represent alleles; lowercase for recessive (e.g., 'd'), uppercase for dominant ('D').

• Homozygous: Both alleles for a gene are the same (e.g., DD or dd).

• Heterozygous: The two alleles for a gene are different (e.g., Dd).

Mendel's First Experiments: Monohybrid Crosses

Monohybrid Cross

• Cross between two individuals with contrasting traits for a single characteristic.

• Parents (P1) are true-breeding for the trait.

• F1 generation: All offspring show the dominant trait.

• F2 generation: 3:1 ratio of dominant to recessive phenotype.

Analysis of a Monohybrid Cross

• Genotypes: DD (tall), dd (dwarf), Dd (heterozygous tall).

• Gamete formation and fertilization lead to predictable genotype and phenotype ratios.

Punnett Squares for Determining Offspring Ratios

• Punnett squares are used to visualize the possible combinations of alleles from parental gametes.

• Monohybrid cross results:

  • Genotype ratio: 1 DD : 2 Dd : 1 dd

  • Phenotype ratio: 3 tall : 1 dwarf

Mendel's Test Cross

Purpose and Method

• Used to determine the genotype of an organism expressing a dominant trait.

• Cross the organism with a homozygous recessive individual.

• If all offspring show the dominant phenotype, the tested parent is homozygous dominant.

• If offspring show a 1:1 ratio of dominant to recessive phenotypes, the tested parent is heterozygous.

Additional Experiments: Dihybrid Crosses

Dihybrid Cross

• Cross between individuals differing in two traits.

• F1 generation: All show dominant traits.

• F2 generation: 9:3:3:1 ratio of phenotypes (yellow round, yellow wrinkled, green round, green wrinkled).

Two Dihybrid Cross Variations

• Crosses with different combinations of parental traits yield the same F2 ratios.

Using the Laws of Probabilities in Genetics

Product Law of Probabilities

• When two independent events occur, the combined probability is the product of their individual probabilities.

• Used to predict F2 phenotype frequencies in dihybrid crosses.

• Equation: $P(A ext{ and } B) = P(A) imes P(B)$

Prediction of Dihybrid Cross Phenotypes

Forked-Line Analysis

• Multiply probabilities for each trait to predict combined phenotype frequencies.

• Example: (3/4 yellow) × (3/4 round) = 9/16 yellow, round

Punnett Squares

• Visualize all possible genotype and phenotype combinations for two traits.

• Genotypic and phenotypic ratios are revealed in the squares.

Mendel's Fourth Postulate: Independent Assortment

• Genes for different traits assort independently during gamete formation.

• Each gamete receives one member of every pair of unit factors.

• Segregation of one pair does not affect segregation of another.

• All possible combinations of gametes are formed with equal frequency.

Dihybrid Test Crosses

• Test crosses can be performed to examine the genotypes of organisms heterozygous for two genes.

• Plants with yellow round seeds can have four possible genotypes: GGWW, GGWw, GgWW, GgWw.

• Expected phenotype ratios depend on the genotype of the tested parent.

Additional Experiments: Trihybrid Crosses

Trihybrid Cross

• Cross involving three pairs of contrasting traits.

• Each triple heterozygote produces eight types of gametes.

• Punnett square would have 64 boxes; probabilities are easier to use.

Prediction of Trihybrid Cross Phenotypes: Forked-Line Analysis

• Multiply probabilities for each trait to predict combined phenotype frequencies.

• Predicted trihybrid ratio: 27:9:9:3:9:3:3:1

Human Genetic Analysis: Pedigree Analysis

Pedigree Symbols and Interpretation

• Pedigrees use standardized symbols to represent individuals, relationships, and traits.

• Squares represent males, circles represent females, shaded symbols indicate affected individuals.

• Pedigree analysis helps determine inheritance patterns (autosomal dominant, autosomal recessive, etc.).

Symbol	Meaning
Square	Male
Circle	Female
Shaded	Affected individual
Half-shaded	Carrier
Horizontal line	Mating
Vertical line	Offspring

Pedigree Analysis Examples

• Autosomal recessive trait: Trait appears only when both alleles are recessive; often skips generations.

• Autosomal dominant trait: Trait appears in every generation; affected individuals have at least one affected parent.

Additional info: These notes expand on the provided slides and fill in missing context for definitions, examples, and genetic principles. All key terms and concepts are explained for clarity and completeness.