Mendelian Genetics: Patterns of Inheritance

Historical Context and Gregor Mendel's Contributions

Understanding classical inheritance began with Gregor Mendel, an Austrian monk who conducted pioneering experiments with garden peas (Pisum sativum). Mendel's work laid the foundation for modern genetics, even before the discovery of cells, chromosomes, or genes.

• Gregor Mendel: Known as the father of genetics; studied science and mathematics at the University of Vienna.

• Experimental Organism: Used garden peas due to their ease of cultivation, short generation time, and ability to self- or cross-pollinate.

• Traits Studied: Simple, objective traits such as plant height, flower color, seed shape, and pod color.

• True Breeding: Mendel used plants that consistently produced offspring with the same trait, now known as homozygous plants.

Example: Mendel originally wanted to study mice, but switched to peas due to restrictions and the simplicity of pea traits.

Particulate Theory of Inheritance

Mendel proposed that inheritance is governed by discrete particles (now known as genes), not by blending traits. Each parent passes specific factors to offspring, which remain distinct across generations.

• Factors: Mendel's term for what we now call alleles.

• Reshuffling: Traits are reshuffled from generation to generation, not blended.

Monohybrid Crosses and the Law of Segregation

Mendel performed crosses between plants differing in a single trait (monohybrid crosses) and observed consistent ratios in offspring.

• Parental Generation (P): True breeding (homozygous) plants.

• First Filial Generation (F1): Offspring of P generation; all heterozygous for the trait.

• Second Filial Generation (F2): Offspring of F1 self-cross; showed a 3:1 ratio of dominant to recessive phenotypes.

Law of Segregation: Each individual has two alleles for each trait, which segregate during gamete formation so each gamete receives only one allele. Fertilization restores the pair.

• Four main components:

  1. Each individual has a pair of factors (alleles) for each trait.

  2. Factors segregate during gamete formation.

  3. Each gamete contains only one factor from each pair.

  4. Fertilization gives offspring two factors for each trait.

Example: Crossing tall (TT) and short (tt) pea plants yields F1 (Tt), and F2 shows a 3:1 ratio of tall to short plants.

Genotype, Phenotype, Dominant and Recessive Alleles

Modern genetics distinguishes between genotype (genetic makeup) and phenotype (observable traits). Alleles can be dominant or recessive, affecting trait expression.

• Genotype: The combination of alleles an individual possesses (e.g., TT, Tt, tt).

• Phenotype: The physical expression of the genotype (e.g., tall or short).

• Dominant Allele: Masks the expression of the recessive allele; represented by a capital letter.

• Recessive Allele: Expressed only when both alleles are recessive; represented by a lowercase letter.

• Homozygous: Both alleles are identical (e.g., TT or tt).

• Heterozygous: Alleles are different (e.g., Tt).

• Locus: The physical location of a gene on a chromosome.

Example: Purple flower color is dominant; only homozygous recessive (pp) plants show white flowers.

Meiosis and Mendel's Laws

Meiosis explains Mendel's laws at the cellular level. Homologous chromosomes separate during meiosis, ensuring only one allele per gamete.

• Law of Segregation: Homologous chromosomes separate during meiosis I.

• Law of Independent Assortment: Chromosomes align randomly during meiosis, allowing alleles for different traits to assort independently.

Dihybrid Crosses and the Law of Independent Assortment

Mendel extended his experiments to crosses involving two traits (dihybrid crosses), observing that traits segregate independently.

• Dihybrid Cross: Cross between individuals differing in two traits (e.g., plant height and pod color).

• Phenotypic Ratio: Classic 9:3:3:1 ratio in F2 generation for two traits.

• Law of Independent Assortment: Alleles for different traits segregate independently during gamete formation.

Example: Tall/green crossed with short/yellow yields offspring with all combinations of height and pod color.

Punnett Squares: Predicting Genotypes and Phenotypes

Punnett squares are tables used to predict the probability of offspring genotypes and phenotypes from parental crosses.

• Structure: Possible gametes from each parent are listed along the top and side; squares show possible offspring genotypes.

• Application: Used for monohybrid and dihybrid crosses, as well as for predicting inheritance of traits like albinism.

Punnett Square Example: Monohybrid Cross

Phenotype Ratio: 3 tall : 1 short Genotype Ratio: 1 TT : 2 Tt : 1 tt

Punnett Square Example: Albinism

Phenotype Ratio: 3 normal : 1 albino Genotype Ratio: 1 AA : 2 Aa : 1 aa

Dihybrid Cross Example: Guinea Pig Coat Color and Length

Crossing guinea pigs with different coat colors and lengths demonstrates the complexity of dihybrid crosses.

Phenotype Ratio: 9:3:3:1

Test Crosses: Determining Unknown Genotypes

Test crosses are used to determine whether an individual with a dominant phenotype is homozygous or heterozygous.

• Method: Cross the unknown individual with a homozygous recessive.

• Interpretation: If all offspring show the dominant trait, the unknown is homozygous dominant. If offspring show both traits, the unknown is heterozygous.

Example: Crossing a yellow pea plant (unknown genotype) with a green pea plant (yy) reveals the genotype based on offspring ratios.

Summary Table: Mendelian Laws and Cross Types

Key Equations and Probability

Probability calculations are essential for predicting inheritance patterns.

• Probability of a genotype: Number of desired genotype / Total number of possible genotypes

• Example Equation:

Additional info:

• Modern genetics has confirmed Mendel's laws through molecular biology and chromosome studies.

• Complex traits (polygenic inheritance) involve multiple genes and do not follow simple Mendelian ratios.