Mendelian Genetics: Patterns of Inheritance

Introduction to Classical Inheritance

Classical inheritance describes the patterns by which traits are passed from parents to offspring, first studied in detail by Gregor Mendel in the 19th century. Mendel's experiments with pea plants laid the foundation for modern genetics, even before the discovery of cells, chromosomes, or genes.

• Gregor Mendel: Austrian monk, considered the father of genetics.

• Experimental Organism: Pisum sativum (garden pea).

• Key Approach: Used mathematical analysis to deduce inheritance patterns.

• Particulate Theory of Inheritance: Traits are inherited as discrete units (now known as genes), not blended.

Why Pea Plants?

Mendel chose pea plants for their practical and scientific advantages:

• Easy to cultivate and grow in controlled environments.

• Short generation time: Multiple generations can be observed quickly.

• Self-pollinating: Allows for controlled crosses by hand.

• Simple, objective traits: Traits such as flower color, seed shape, and plant height are easily distinguishable.

Key Terms in Genetics

• Allele: Alternate form of a gene found at a specific locus on a chromosome.

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

• Recessive allele: Expressed only when two copies are present; denoted by a lowercase letter.

• Homozygous: Two identical alleles at a locus (e.g., AA or aa).

• Heterozygous: Two different alleles at a locus (e.g., Aa).

• Genotype: The genetic makeup (allele combination) of an individual.

• Phenotype: The observable physical traits of an individual.

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

Fundamental Laws of Heredity

Law of Segregation

The law of segregation states that each individual has two alleles for each trait, which segregate during gamete formation so that each gamete receives only one allele. Fertilization restores the pair in the offspring.

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

• Factors segregate during gamete formation.

• Each gamete contains only one factor from each pair.

• Fertilization restores the pair of factors in the offspring.

Example: Crossing two heterozygous pea plants (Tt x Tt) for plant height yields a 3:1 ratio of tall to short plants in the F2 generation.

Law of Independent Assortment

The law of independent assortment states that alleles for different traits segregate independently of one another during gamete formation, leading to new combinations of traits in the offspring.

• Traits are inherited independently.

• All possible combinations of alleles can occur in gametes.

Example: Dihybrid cross of pea plants differing in height and pod color produces offspring with all possible combinations of these traits.

Experimental Crosses and Ratios

Monohybrid Crosses

A monohybrid cross involves parents differing in one trait. Mendel used true-breeding (homozygous) plants for these crosses.

• Parental generation (P): Homozygous for trait (e.g., RR x rr).

• First filial generation (F1): All heterozygous (Rr).

• Second filial generation (F2): Crossing F1 individuals yields a 3:1 phenotype ratio and a 1:2:1 genotype ratio.

Dihybrid Crosses

Dihybrid crosses involve parents differing in two traits. The classic result is a 9:3:3:1 phenotype ratio in the F2 generation.

• Example: Crossing plants with tall/green pods and short/yellow pods.

• Result: Offspring show all combinations: tall/green, tall/yellow, short/green, short/yellow.

Punnett Squares

Punnett squares are tables used to predict the probability of genotypes and phenotypes among offspring from a genetic cross.

• Rows: Possible gametes from one parent.

• Columns: Possible gametes from the other parent.

• Cells: Resulting genotypes from each cross.

Example: In a monohybrid cross, the probability of a recessive phenotype (e.g., short plant) is 25%.

Genotype and Phenotype Ratios

• Monohybrid cross: Genotype ratio 1:2:1 (homozygous dominant : heterozygous : homozygous recessive).

• Phenotype ratio: 3:1 (dominant : recessive).

• Dihybrid cross: Phenotype ratio 9:3:3:1.

Test Crosses

Test crosses are used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

• If all offspring show the dominant phenotype: The unknown parent is homozygous dominant.

• If offspring show a mix of dominant and recessive phenotypes: The unknown parent is heterozygous.

Modern Context and Molecular Basis

Dominant and Recessive Alleles

Dominant alleles typically code for functional proteins, while recessive alleles often represent a loss of function or altered protein.

• Dominant allele: Produces normal protein, masks recessive allele.

• Recessive allele: May produce nonfunctional or altered protein; trait only visible when two copies are present.

Meiosis and Mendel's Laws

The process of meiosis explains Mendel's laws:

• Law of Segregation: Homologous chromosomes separate during meiosis I, ensuring each gamete receives one allele.

• Law of Independent Assortment: Chromosomes align randomly during metaphase I, leading to independent assortment of alleles.

Summary Table: Mendelian Ratios

Key Equations

Probability of genotype in monohybrid cross:

Phenotype ratio:

Examples and Applications

• Albinism: A recessive trait; parents who are heterozygous can produce offspring with a 25% chance of being albino.

• Guinea pig coat color and length: Dihybrid crosses show the 9:3:3:1 ratio for combinations of color and length.

Additional info: Modern genetics has expanded on Mendel's work, identifying exceptions such as incomplete dominance, codominance, and polygenic inheritance, but Mendel's laws remain foundational for understanding basic inheritance patterns.