Principles of Inheritance and Variation

Genetics Overview

• Genetics: Branch of biology focused on inheritance and variation from parents to offspring.

Inheritance

• Process by which traits are transmitted from parents to progeny.

Variation

• The degree of difference between progeny and parents.

Mendel's Experiments

• Gregor Johann Mendel: Father of genetics, proposed laws of inheritance.
• Experimented with garden peas, using large samples for reliable results.
• Garden peas had contrasting traits (e.g., tall vs. dwarf).
• Seven traits studied, including stem height, flower color, and seed shape.

True Breeding Lines

• Obtained through continuous self-pollination, resulting in pairs with contrasting traits.

Hybridization

• Artificial cross-pollination leading to the first filial generation (F1).
• F1 generation resembles one parent trait (e.g., all tall).
• F2 generation reveals unexpressed traits (e.g., dwarf), showing a 3:1 ratio.

Inheritance of One Gene

• Factors: Elements passed unchanged across generations, now known as genes.
• Alleles: Different forms of a factor, e.g., tall (T) and dwarf (t).

Monohybrid Cross

• Focuses on a single character (e.g., plant height).
• Genotypes: TT (homozygous tall), tt (homozygous dwarf), Tt (heterozygous).
• Phenotypes: Tall (dominant), Dwarf (recessive).
• Punnett Square: Tool to predict offspring genotype probabilities.

Laws of Inheritance

• Law of Dominance: One factor in a pair may mask the other.
• Law of Segregation: Alleles separate during gamete formation.
• Law of Independent Assortment: Traits are inherited independently of each other.

Incomplete Dominance

• F1 phenotype is a blend of parental traits (e.g., pink flowers from red and white).
• Phenotypic Ratio: 1:2:1 (Red: Pink: White).

Co-Dominance

• F1 progeny resembles both parents (e.g., ABO blood groups).
• Multiple Alleles: More than two alleles for a trait (e.g., IA, IB, i in blood groups).

Inheritance of Two Genes (Dihybrid Cross)

• Involves two characters (e.g., seed color and shape).
• Phenotypic Ratio: 9:3:3:1.

Chromosomal Theory of Inheritance

• Rediscovery of Mendel's work attributed to lack of acceptance and communication.
• Chromosomes discovered, linking genetics with chromosomal behavior.

Linkage and Recombination

• Linkage: Genes located on the same chromosome, leading to higher parental trait frequency.
• Recombination: Non-parental gene combinations.

Sex Determination

• Male Heterogamety: XY type (humans, Drosophila), XO type (insects).
• Female Heterogamety: ZW type (certain birds, reptiles).

Mutation and Genetic Disorders

• Mutation: Changes in DNA that alter genotype and phenotype.
• Types: Point mutation, frameshift mutation.

Pedigree Analysis

• Tracing traits across generations to understand inheritance patterns.

Genetic Disorders

• Mendelian Disorders: Caused by single gene mutations, following Mendelian genetics.
• Chromosomal Disorders: Due to changes in chromosome number or structure.

Examples

• Down Syndrome: Trisomy of chromosome 21.
• Klinefelter Syndrome: Additional X chromosome (XXY).
• Turner's Syndrome: Missing one X chromosome (XO).