Mendelian Genetics

Introduction to Mendelian Genetics

Mendelian genetics is the foundation of classical genetics, describing how traits are inherited from one generation to the next. Gregor Mendel's experiments with pea plants led to the discovery of fundamental laws governing heredity.

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

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

• Example: Mendel used pea plants due to their short generation time, large offspring numbers, and controlled mating.

Mendel's Experimental Approach

Mendel tracked characters with two distinct forms and started with true-breeding varieties. He mated contrasting varieties, producing hybrid offspring and observed inheritance patterns across generations.

• P Generation: True-breeding parents.

• F1 Generation: Hybrid offspring of P generation.

• F2 Generation: Offspring from self- or cross-pollination of F1 individuals.

The Law of Segregation

Mendel observed a 3:1 ratio of purple to white flowers in the F2 generation, leading to the law of segregation.

• Law of Segregation: Two alleles for a heritable character separate during gamete formation, ending up in different gametes.

• Gene: What Mendel called a "heritable factor" is now known as a gene.

• Allele: Alternative versions of a gene located at a specific locus on a chromosome.

Genotype and Phenotype

• Genotype: Genetic makeup of an organism (e.g., PP, Pp, pp).

• Phenotype: Physical appearance (e.g., purple or white flowers).

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

• Heterozygous: Two different alleles for a gene (Pp).

Punnett Squares and Testcrosses

Punnett squares are used to predict the outcome of genetic crosses. A testcross helps determine the genotype of an individual with a dominant phenotype.

• Punnett Square: Diagram showing possible combinations of alleles.

• Testcross: Breeding a mystery individual with a homozygous recessive individual to reveal genotype.

The Law of Independent Assortment

Mendel's dihybrid crosses led to the law of independent assortment, stating that alleles of different genes segregate independently during gamete formation.

• Monohybrid Cross: Cross between heterozygotes for one character.

• Dihybrid Cross: Cross between heterozygotes for two characters.

• Law of Independent Assortment: Applies to genes on different chromosomes or far apart on the same chromosome.

Probability in Genetics

Probability rules help predict genetic outcomes.

• Multiplication Rule: Probability of two independent events occurring together is the product of their individual probabilities.

• Addition Rule: Probability of any one of two or more mutually exclusive events is the sum of their individual probabilities.

Complex Patterns of Inheritance

Degrees of Dominance

• Complete Dominance: Heterozygote and dominant homozygote have identical phenotypes.

• Incomplete Dominance: F1 hybrids have intermediate phenotypes.

• Codominance: Both alleles affect the phenotype in distinguishable ways.

Multiple Alleles

Most genes exist in populations in more than two allelic forms.

• Example: ABO blood group in humans is determined by three alleles: IA, IB, and i.

Pleiotropy

Pleiotropy occurs when one gene affects multiple phenotypic traits.

• Example: Sickle-cell disease and cystic fibrosis show pleiotropic effects.

Epistasis

Epistasis is when the expression of one gene alters the phenotypic expression of another gene.

• Example: Labrador retriever coat color depends on two genes: one for pigment color and one for pigment deposition.

Polygenic Inheritance

Polygenic inheritance involves the additive effect of two or more genes on a single phenotype, often resulting in quantitative variation.

• Example: Human height and skin pigmentation are controlled by many genes.

Environmental Impact on Phenotype

Phenotype can be influenced by environmental factors as well as genotype. Multifactorial traits depend on both genetic and environmental influences.

• Example: Hydrangea flower color varies with soil pH.

Human Genetics and Pedigree Analysis

• Pedigree: Family tree describing inheritance of a trait across generations.

• Application: Used to predict probabilities of specific phenotypes in future offspring.

Recessively and Dominantly Inherited Disorders

Recessively Inherited Disorders

• Carriers: Heterozygous individuals who carry the recessive allele but are phenotypically normal.

• Example: Albinism is a recessive condition characterized by lack of pigmentation.

Dominantly Inherited Disorders

• Example: Achondroplasia (dwarfism) and Huntington's disease are caused by dominant alleles.

Multifactorial Disorders and Genetic Counseling

Many diseases have both genetic and environmental components. Genetic counseling and testing help prospective parents assess risks for inherited diseases.

• Probability Calculation: If both parents are carriers, the probability of having a child with a recessive illness is .

Summary Table: Mendelian Principles and Patterns

Key Equations

• Multiplication Rule:

• Addition Rule:

Additional info:

• Pedigree analysis is essential for understanding inheritance in humans, where controlled breeding is not possible.

• Genetic counseling uses Mendelian principles and probability rules to assess risk for inherited diseases.