Genetics: Branch of biology dealing with heredity and variation. Study of how traits are transmitted from parents to offspring.
Heredity: Transmission of characteristics from parents to offspring through genes. Explains why offspring resemble parents.
Variation: Differences among individuals of same species. Can be genetic (inherited) or environmental (acquired).
Father of Genetics: Gregor Johann Mendel (1822-1884), an Austrian monk who conducted experiments on pea plants.
Mendel's Work: Conducted experiments (1856-1863) on garden pea (Pisum sativum). Published findings in 1866, but recognized only in 1900.
Why Pea Plant: (1) Easy to grow, (2) Short life cycle, (3) Produces many offspring, (4) Has distinct contrasting traits, (5) Self-pollination possible, (6) Cross-pollination easy.
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Basic Genetic Terminology
Gene: Unit of heredity. Segment of DNA coding for a specific trait. Located on chromosomes.
Allele: Alternative forms of a gene. Example: Gene for height has alleles for tall (T) and dwarf (t).
Dominant Allele: Allele that expresses itself even in heterozygous condition. Represented by capital letter (T).
Recessive Allele: Allele that expresses only in homozygous condition. Represented by small letter (t).
Genotype: Genetic makeup of an organism. Combination of alleles. Example: TT, Tt, tt.
Phenotype: Physical appearance or expression of trait. Example: Tall or dwarf.
Homozygous: Both alleles same. TT (homozygous dominant) or tt (homozygous recessive).
Law of Independent Assortment: Alleles of different genes assort independently during gamete formation. Inheritance of one trait doesn't affect another.
Explanation: During meiosis, chromosomes (and genes) assort independently. Gene for seed shape doesn't influence gene for seed color.
Gametes from F₁: RrYy produces 4 types - RY, Ry, rY, ry (each 25%).
16-square Punnett Square: Shows all 16 possible F₂ combinations from dihybrid cross.
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Test Cross and Back Cross
Test Cross: Cross between organism with dominant phenotype (unknown genotype) and homozygous recessive organism.
Purpose: To determine if organism is homozygous (TT) or heterozygous (Tt).
Method: Tall plant (T?) × Dwarf plant (tt).
Results: If all offspring tall → parent was TT. If 1:1 ratio (tall:dwarf) → parent was Tt.
Back Cross: Cross between F₁ hybrid and either parent.
Types: (1) Cross with dominant parent - all offspring show dominant trait, (2) Cross with recessive parent - 1:1 ratio (same as test cross).
Importance: Test cross used in agriculture to identify pure-breeding organisms.
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Incomplete Dominance
Definition: Neither allele is completely dominant. Heterozygous shows intermediate phenotype.
Example: Four o'clock plant (Mirabilis jalapa). Red (RR) × White (rr) → Pink (Rr) in F₁.
F₂ Ratio: 1 Red : 2 Pink : 1 White. Phenotypic ratio equals genotypic ratio (1:2:1).
Difference from Complete Dominance: In complete dominance, Tt looks like TT. In incomplete dominance, Rr looks different from both RR and rr.
Other Examples: Snapdragon flower color, human hair texture (straight, wavy, curly).
Blending: Appears like blending of traits, but alleles remain separate (proven in F₂).
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Codominance
Definition: Both alleles express equally in heterozygous condition. No dominance.
Example: Human ABO blood groups. IA and IB are codominant.
Blood Groups: A (IAIA or IAi), B (IBIB or IBi), AB (IAIB), O (ii).
AB Blood Group: Shows both A and B antigens. Both alleles express simultaneously.
Difference from Incomplete Dominance: In codominance, both traits visible separately. In incomplete dominance, intermediate trait appears.
Roan Coat in Cattle: Red and white hairs appear together (not pink). Example of codominance.
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Multiple Alleles
Definition: More than two alleles for a single gene in population. Individual has only two alleles.
Example: Human ABO blood group system has three alleles - IA, IB, i.
Possible Genotypes: IAIA, IAi (A), IBIB, IBi (B), IAIB (AB), ii (O).
Dominance: IA and IB are codominant. Both dominant over i (recessive).
Inheritance: Child inherits one allele from each parent. Example: IAi × IBi can produce A, B, AB, or O children.
Importance: Blood group matching for transfusion. Universal donor (O), Universal recipient (AB).
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Sex Determination
Sex Chromosomes: X and Y chromosomes determine sex. Autosomes control other traits.
Human Sex Chromosomes: Females XX (homogametic), Males XY (heterogametic).
Mechanism: Egg always carries X. Sperm carries X or Y (50% each). XX → Female, XY → Male.
Sex Ratio: Theoretically 1:1 (50% male, 50% female).
Father Determines Sex: Since mother always gives X, father's sperm (X or Y) determines child's sex.
Sex-linked Traits: Genes on sex chromosomes. Example: Hemophilia, color blindness on X chromosome.
Expression: Sex-linked recessive traits more common in males (only one X). Females need two recessive alleles.
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Variations - Types and Importance
Genetic Variation: Inherited differences due to genes. Passed to offspring. Example: Eye color, blood group.
Environmental Variation: Differences due to environment. Not inherited. Example: Scars, body weight, language.
Continuous Variation: Range of phenotypes. Controlled by multiple genes (polygenic). Example: Height, skin color, intelligence.
Discontinuous Variation: Distinct categories. Controlled by single gene. Example: ABO blood groups, attached/free earlobes.
Sources of Variation: (1) Mutation - changes in DNA, (2) Meiosis - crossing over and independent assortment, (3) Random fertilization.
Importance: (1) Evolution - raw material for natural selection, (2) Adaptation to environment, (3) Survival of species, (4) Selective breeding in agriculture.
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Mutations
Mutation: Sudden heritable change in DNA sequence. Source of new alleles.
Types: (1) Gene mutation - change in single gene, (2) Chromosomal mutation - change in chromosome structure or number.
Causes: (1) Spontaneous - errors during DNA replication, (2) Induced - radiation, chemicals (mutagens).