Mendelian and Non-Mendelian Inheritance

Mendelian and Non-Mendelian Inheritance

• Gregor Mendel discovered basic principles of inheritance in 1866.
• His success stemmed from studying seven pea plant characteristics and his experimental methods.
• Mendel used the pea plant Pisum sativum for his studies.
• The seven characteristics Mendel studied in pea plants included height, flower color, flower position, seed color, seed shape, pod color, and pod shape.
• These traits existed in two distinct forms (e.g., tall or dwarf for height).

Learning Objectives

• Explain the principles of heredity.
• Describe how genes are passed from one generation to the next.
• Define how factors like dominance influence inheritance.
• Explain Mendel's principles of segregation and independent assortment.
• Analyze the behavior of chromosomes concerning inheritance.
• Understand probability in genetics.

Genetic Terminology

• Gene: An inherited factor, encoded in DNA, influencing a trait.
• Allele: One of multiple alternative forms of a gene.
• Locus: A specific location on a chromosome occupied by a gene.
• Genotype: The set of alleles an organism possesses.
• Homozygote: An organism with two identical alleles at a locus.
• Heterozygote: An organism with two different alleles at a locus.
• Phenotype: The observable expression of a trait.
• Characteristic/character: A general attribute or feature of an organism.
• Trait/variant: specific expression of a characteristic.

Monohybrid Crosses

• Monohybrid cross: A cross between two individuals differing in a single trait (e.g., AA × aa).
• P generation: The first set of parents in a genetic cross.
• Mendel's monohybrid crosses led to the principle of segregation and dominance.
• Principle of segregation: Each individual has two alleles for each trait, and these alleles separate during gamete formation, with each gamete receiving only one allele.
• Concept of dominance: When two different alleles are present, the dominant allele's trait is observed, while the recessive allele's trait is masked (e.g., in a cross between a homozygous dominant and homozygous recessive plant).
• Mendel observed a 3:1 phenotypic ratio in the F2 generation.

True-Breeding Lines

• Character:The type of characteristic that can vary (e.g., height).
• Trait/variant: The different version of a character (e.g., tall or dwarf).
• True-breeding lines: Plants that consistently produce offspring with the same traits when self-fertilized.
• Mendel started his experiments using true-breeding plants.

Mendel's Law of Segregation

• Mendel's experiments with single-factor crosses revealed this law.
• The law of segregation describes how alleles separate during gamete formation and fertilization.
• The predicting outcomes of single-factor crosses use Punnett square.

Mendel's Approach

• Mendel used a quantitative analysis of crosses to reveal mathematical relationships.
• This approach is called an empirical approach, and general findings from it are called empirical laws.

Mendel's Crosses

• Mendel mated plants with different traits to produce hybrids.
• Mating looking at only one trait is considered a single-factor cross.
• Characteristics with different traits are crossed; offspring are called monohybrids.
• Mendel studied seven traits with two variants each. He first crossed only two variants of one trait at a time.

Dihybrid Crosses

• In addition to monohybrid crosses, Mendel did dihybrid crosses.
• He crossed plants with two different pairs of traits (e.g., seed shape and seed color).
• Mendel's dihybrid crosses revealed the principle of independent assortment.
• The principle of independent assortment states that genes encoding different traits separate independently during gamete formation.

Punnett Squares

• A Punnett square is a grid used to predict the outcome of simple genetic crosses.
• It requires knowing the genotypes of the parents.
• Using the Punnett square, you determine and analyze the proportions of possible genotypes and phenotypes.

Non-Mendelian Patterns

• Non-Mendelian inheritance includes incomplete dominance, codominance, and multiple alleles.
• These patterns show that traits can have complex expressions beyond simple dominant-recessive inheritance.