Chapter 3: Mendelian Genetics

Introduction to Mendelian Genetics

Mendelian genetics is the foundation of classical genetics, describing how traits are inherited from one generation to the next. Gregor Mendel, an Austrian monk, conducted pioneering experiments using pea plants to elucidate the basic principles of heredity.

• Key Terms: Trait, Gene, Allele, Genotype, Phenotype

• Example: Mendel studied traits such as flower color, seed shape, and plant height.

Experimental Design: Pea Plant Crosses

Mendel used pea plants for their distinct, easily observable traits and ability to self- or cross-fertilize. He carefully controlled pollination to study inheritance patterns.

• Self-fertilization: Pollen fertilizes eggs within the same flower.

• Cross-fertilization: Pollen from one plant fertilizes eggs of another plant with a different phenotype.

Traits Studied by Mendel

Mendel selected seven traits, each with two contrasting variants, to ensure clear inheritance patterns.

• Examples: Seed shape (round/wrinkled), flower color (purple/white), stem height (tall/dwarf).

Mendel’s Experimental Procedure

Mendel cross-fertilized true-breeding lines for each trait, observed the F1 generation, and allowed self-fertilization to produce the F2 generation. He analyzed the ratios of phenotypes in the F2 generation.

• P generation: True-breeding parents

• F1 generation: Offspring of P cross, all showing dominant phenotype

• F2 generation: Offspring of F1 self-fertilization, showing both dominant and recessive phenotypes

Interpretation of Mendel’s Data

Mendel observed that traits did not blend but were inherited as discrete units. He proposed the concept of particulate inheritance, now known as genes.

• Dominant trait: Masks the presence of a recessive trait

• Recessive trait: Only expressed when two recessive alleles are present

• Law of Segregation: Two copies of a gene segregate during gamete formation

Genetic Terminology

Understanding key genetic terms is essential for interpreting Mendelian crosses.

• Homozygous: Two identical alleles for a gene (e.g., TT or tt)

• Heterozygous: Two different alleles for a gene (e.g., Tt)

• Allele: Variant form of a gene

• Testcross: Cross with a homozygous recessive individual to determine genotype

Punnett Squares and Probability

Punnett Squares are used to predict the outcome of genetic crosses. Probability rules help calculate the likelihood of specific genotypes and phenotypes.

• Sum Rule: Probability of mutually exclusive events is the sum of their probabilities

• Product Rule: Probability of independent events occurring together is the product of their probabilities

• Binomial Expansion: Used for calculating probabilities of unordered events

Example Equation:

Where n = total number of offspring, x = number of offspring of one phenotype, p = probability of phenotype x, q = probability of the other phenotype.

Mendel’s Law of Independent Assortment

This law states that alleles of different genes assort independently during gamete formation, leading to genetic variation.

• Dihybrid Cross: Cross involving two traits, resulting in a 9:3:3:1 ratio in the F2 generation

Chromosomal Theory of Inheritance

This theory connects Mendelian inheritance to the behavior of chromosomes during meiosis. Chromosomes carry genes, and their segregation and independent assortment explain Mendel’s laws.

• Meiosis: Explains the law of segregation and independent assortment

• Diploid: Two sets of chromosomes, one from each parent

• Haploid: Gametes contain one set of chromosomes

Statistical Analysis: Chi Square Test

The Chi Square test is used to determine if observed genetic data fit expected ratios. It compares observed and expected values to test hypotheses.

• Formula:

• Degrees of Freedom: Number of phenotypes minus one

• P value: Typically 0.05 is used for significance

Human Genetics and Pedigree Analysis

In humans, controlled crosses are not possible. Pedigree analysis is used to infer inheritance patterns by examining family histories.

• Pedigree: Diagram showing inheritance of traits across generations

• Questions: Are males and females affected equally? Does the trait skip generations?

• Autosomal recessive: Trait may skip generations, affects both sexes equally

• Autosomal dominant: Trait appears in every generation, affects both sexes equally

Summary of Mendelian Principles

Mendelian genetics provides the basis for understanding inheritance. Key principles include the law of segregation, law of independent assortment, and the use of probability and statistical analysis to interpret genetic data. Pedigree analysis extends these principles to human genetics.

• Perform crosses and analyze ratios to determine inheritance patterns

• Use Punnett Squares and probability rules for predictions

• Apply Chi Square analysis to test hypotheses

• Use pedigrees to study human inheritance