Chapter 14

Historical Context and Mendel's Background

• Gregor Mendel was an Austrian monk whose background in physics and mathematics influenced his scientific methodology.

• At the time, two main theories of inheritance existed: equal parental contribution and blending inheritance. Mendel's work provided evidence against the blending theory.

Mendel’s Experimental Approach

Key Terminology: Characters and Traits

Mendel distinguished between characters (heritable features, e.g., flower color) and traits (variants of a character, e.g., purple or white flowers).

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white color for flowers).

The Pea Plant as a Model Organism

• Short generation time and large number of offspring make pea plants ideal for genetic studies.

• Controlled mating is possible due to the presence of both male and female reproductive organs in the same flower.

• Pea plants can undergo self-pollination or cross-pollination, allowing for precise experimental manipulation.

Mendel’s Experimental Technique

Mendel began with true-breeding parental plants (P generation) and performed controlled crosses to observe inheritance patterns in subsequent generations (F1 and F2).

• True-breeding: Plants that produce offspring of the same variety when self-pollinated.

• Cross-pollination involved removing anthers from one flower and transferring pollen from another.

Results of Mendel’s Crosses

• F1 generation: All offspring displayed the dominant trait (e.g., purple flowers).

• F2 generation: Both dominant and recessive traits reappeared in a 3:1 ratio (e.g., 705 purple:224 white).

• This disproved the blending theory, as the recessive trait was not lost or diluted.

Mendel’s Model of Inheritance

Four Concepts of Mendel’s Model

1. Alternative versions of genes (alleles) account for variations in inherited characters.

2. Each organism inherits two alleles for each character, one from each parent.

3. If alleles differ, the dominant allele determines appearance; the recessive allele is masked.

4. Law of Segregation: The two alleles for a heritable character segregate during gamete formation and end up in different gametes.

Molecular Basis of Dominance and Recessiveness

• Alleles are different DNA sequences at the same gene locus.

• The dominant allele produces a functional enzyme (e.g., for purple pigment), while the recessive allele does not.

• One functional allele is sufficient for the dominant phenotype.

Punnett Squares and Genetic Predictions

Punnett squares are used to predict the genotypic and phenotypic ratios of offspring from genetic crosses.

• Gametes from each parent are placed on the axes; combinations fill the squares.

• F2 generation from a monohybrid cross yields a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.

Genotype vs. Phenotype

• Genotype: The genetic makeup (e.g., PP, Pp, pp).

• Phenotype: The observable trait (e.g., purple or white flowers).

• Homozygous: Two identical alleles (PP or pp).

• Heterozygous: Two different alleles (Pp).

• Dominant phenotype can result from either homozygous dominant or heterozygous genotype.

The Test Cross

A test cross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

• If all offspring display the dominant trait, the unknown is homozygous dominant.

• If offspring are 1:1 dominant:recessive, the unknown is heterozygous.

The Law of Independent Assortment

Tracking Two Characters: Dihybrid Crosses

Mendel’s second law states that alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes or far apart on the same chromosome.

• Crossing true-breeding parents differing in two characters produces dihybrids in the F1 generation.

• A dihybrid cross (F1 × F1) reveals whether characters are inherited together or independently.

Results of Dihybrid Crosses

• If genes assort independently, the F2 generation shows a 9:3:3:1 phenotypic ratio.

• If genes are dependent, only parental combinations appear (3:1 ratio).

• Independent assortment produces new combinations not seen in the parental generation.

Generating Gametes: Chromosomal Basis and FOIL Method

• Gametes from a dihybrid (YyRr) can be YR, Yr, yR, or yr, reflecting independent assortment.

• The FOIL method (First, Outer, Inner, Last) is a quick way to determine all possible gamete combinations.

• Chromosome alignment during meiosis explains the random assortment of alleles.

Probability and Mendelian Inheritance

Probability Laws in Genetics

• The probability of independent events (e.g., allele segregation) is the product of their individual probabilities.

• Punnett squares and probability calculations are both valid approaches for predicting genetic outcomes.

• For a monohybrid cross (Rr × Rr), the probability of each genotype can be calculated as the product of the probabilities for each allele from each parent.

Summary Table: Key Terms and Ratios in Mendelian Genetics

Key Equations

• Probability of independent events:

• Genotypic ratio (monohybrid cross):

• Phenotypic ratio (monohybrid cross):

• Phenotypic ratio (dihybrid cross):

Additional info: Understanding Mendelian genetics is foundational for later topics such as chromosomal inheritance, gene mapping, and molecular genetics. Practice with Punnett squares and probability calculations is essential for mastering genetic problem-solving.