Mendelian Law of Inheritance

• Mendelian Law of Inheritance encompasses the principles governing how traits are passed from parents to offspring.
• These principles were first developed by Gregor Johann Mendel.

Gregor Johann Mendel

• Gregor Johann Mendel was an Austrian monk.
• He was born in Czech Republic on July 20, 1822.
• He studied inheritance using sweet pea plants (Pisum sativum).
• Mendel's work led to the development of the laws of inheritance, now known as Mendel's Principles.
• His work was not recognized until the turn of the 20th century.
• He died on January 6, 1884.

Terminology

• Trait: Any characteristic passed from parent to offspring.
• Heredity: The passing of traits from parent to offspring.
• Genetics: The study of heredity.
• Genotype: The genetic makeup of an organism, including its complete set of genes.
• Phenotype: The physical appearance of an organism with respect to a trait, for example, yellow seeds.
• Dominant Trait: A trait expressed preferentially over another, represented by a capital letter.
• Recessive Trait: The opposite of dominant; a trait that is preferentially masked, represented by a lowercase letter.
• Test Cross: A cross between an individual with a dominant phenotype but unknown genotype and a homozygous recessive individual.
• Reciprocal Cross: The use of male and female gametes for two different traits, alternating the source of gametes.

Genes and Related Terms

• Genes: Basic units of heredity; sequences of nucleotides in DNA that encode RNA or protein.
• Alleles: Two forms of a gene (dominant and recessive).
• Dominant Genes: Stronger of two genes expressed in a hybrid, represented by a capital letter (e.g., 'T' for Tall).
• Recessive Genes: Genes that appear less often in a cross, represented by a lowercase letter (e.g., 't' for tall).
• Genotype: A gene combination for a trait (e.g., TT, Tt, tt, RR, Rr, rr).
• Phenotype: A physical feature resulting from a genotype (e.g., Tall, Dwarf, Red, White).

Genotypes

• Homozygous Genotype: A gene combination involving two dominant or two recessive genes (e.g., TT, RR, tt, rr); also called 'pure'.
• Heterozygous Genotype: A gene combination of one dominant and one recessive gene (e.g., Tt, Rr); also called hybrid.

Types of Genetic Crosses

• Monohybrid Cross: Involves a single trait (e.g., flower color, seed shape, eye color).
• Dihybrid Cross: Involves two traits (e.g., flower color and plant height, eye color and abdomen color in Drosophila melanogaster, eye color and hair color in humans).
• Trihybrid Cross: Involves three traits or alleles belonging to three different genes (e.g., plant size, pod color, and seed shape of a Sweet pea plant).

Generation in a Crossing

• Parental P1 Generation: The parental generation in a breeding experiment.
• F1 Generation: The first-generation offspring in a breeding experiment; also called the 1st filial generation.
• F2 Generation: The second-generation offspring in a breeding experiment; also called the 2nd filial generation.

Punnett Square

• Punnett Square: A square diagram used to predict the genotypes of a particular cross or breeding experiment.
• It was named after Reginald C. Punnett, who devised the method in 1905.

Why Mendel Selected Sweet Pea

• They can be grown in a small area.
• They produce lots of offspring.
• They produce pure plants when allowed to self-pollinate for several generations.
• They can be artificially crossed.

Mendel’s Selection of Traits

• Plant Height: Tall (T) is dominant, Dwarf (t) is recessive.
• Flower Color: Purple (P) is dominant, White (p) is recessive.
• Flower Position: Axial (A) is dominant, Terminal (a) is recessive.
• Pod Shape: Smooth (S) is dominant, Wrinkled (s) is recessive.
• Pod Color: Green (G) is dominant, Yellow (g) is recessive.
• Seed Shape: Round (R) is dominant, Wrinkled (r) is recessive.
• Seed Color: Yellow (Y) is dominant, Green (y) is recessive.

Results of Mendel’s Experiments

• The theoretical or expected ratio is 3:1.
• Mendel's observed ratio was 2.96:1.

Mendel’s Law

• Law of Dominance: Hybrid offspring inherit only the dominant trait in the phenotype; suppressed alleles are recessive traits.
• Law of Independent Assortment: Pairs of traits segregate independently during gamete formation, ensuring equal opportunity for different traits.
• Law of Segregation: Two copies of hereditary factors segregate during gamete production, so offspring acquire one factor from each parent.

Monohybrid Cross

• Involves one trait with contrasting alleles like seed shape (R-Round, r-Wrinkled).
• P1 Monohybrid Cross involves homozygous parents (RR x rr) leading to F1 generation.
• F1 Generation (Rr) - all heterozygous offspring with round phenotype exhibit all alike genotypic and phenotypic ratios.
• F1 Monohybrid Cross involves crossing heterozygous offspring (Rr x Rr) leading to F2 generation.
• F2 Generation has a 1:2:1 genotypic ratio (RR, Rr, rr) and 3:1 phenotypic ratio (Round & Wrinkled).
• F2 Monohybrid Cross or Test Cross (Pure X Hybrid) can involve either cross of homozygous dominant with hybrid offspring (RR x Rr) or cross of homozygous recessive with hybrid offspring(rr X Rr)

Dihybrid Cross

• Dihybrid Cross involves two traits with contrasting alleles, such as seed shape (R-Round, r-Wrinkled) and seed color (Y-Yellow, y-green).
• Gametogenesis (Segregation): Number of resulting gametes is equal to 2^n , where n is the number of heterozygotes.

Incomplete Dominance

• In incomplete dominance, both alleles are partially expressed, leading to an intermediate phenotype.
• F1 hybrids have an appearance in between the two parental varieties.
• An example is flower color in Snapdragon: Red (RR) x White (rr) results in all Pink (Rr) offspring.

Codominance

• In codominance, two versions (alleles) of the same gene are expressed separately to yield different traits in an individual.
• Codominance involves two alleles expressed (multiple alleles too) in heterozygous individuals, such as blood type.
• Example is Type B male (IBIB) x Type A female (IAi) -> 50% Type C (IAIB) and 50% Type B (IBi) offspring.

Explore Mendelian Laws of Inheritance, the principles governing trait inheritance from parents to offspring, developed by Gregor Johann Mendel. Learn about Mendel's life, key terminology such as traits, heredity, genetics, genotype, phenotype, and dominant traits.