Genetics – Some Basic Fundamentals

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.

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).

Heterozygous: Alleles different. Tt (shows dominant trait).

Hybrid: Offspring of cross between parents with different traits. F₁ generation.

True-breeding/Pure Line: Organisms that produce offspring identical to themselves when self-pollinated.

Monohybrid Cross: Cross between parents differing in one trait. Example: Tall × Dwarf pea plants.

Parental Generation (P): Pure tall (TT) × Pure dwarf (tt).

F₁ Generation: All offspring tall (Tt). Shows dominance of tall trait.

F₂ Generation: F₁ × F₁ cross produces 3 tall : 1 dwarf ratio. Genotypic ratio: 1 TT : 2 Tt : 1 tt.

Observations: (1) F₁ all tall - dominance, (2) F₂ shows 3:1 ratio - segregation of alleles, (3) Dwarf trait reappears in F₂.

Punnett Square: Grid method to predict offspring genotypes and phenotypes. Shows all possible combinations of gametes.

Statement: When two contrasting traits are crossed, only one (dominant) appears in F₁ generation. The other (recessive) remains hidden.

Explanation: In heterozygous condition (Tt), dominant allele (T) masks recessive allele (t).

Example: Tall (T) is dominant over dwarf (t). TT and Tt both appear tall; only tt appears dwarf.

Factors: Mendel called genes 'factors'. Each trait controlled by pair of factors (alleles).

Application: Explains why F₁ hybrids show only one parental trait.

Statement: Alleles of a gene separate during gamete formation. Each gamete receives only one allele of each gene.

Explanation: During meiosis, homologous chromosomes (carrying alleles) separate. Each gamete gets one chromosome, hence one allele.

Example: Tt plant produces two types of gametes - 50% with T, 50% with t.

Reunion: During fertilization, alleles reunite randomly, producing different combinations in offspring.

Evidence: Reappearance of recessive trait in F₂ generation proves alleles separate and recombine.

Significance: Explains 3:1 phenotypic ratio in F₂ generation of monohybrid cross.

Dihybrid Cross: Cross between parents differing in two traits. Example: Round-Yellow × Wrinkled-Green seeds.

Parental Cross: RRYY (Round-Yellow) × rryy (Wrinkled-Green).

F₁ Generation: All RrYy (Round-Yellow). Shows dominance of both traits.

F₂ Generation: 9:3:3:1 ratio. 9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green.

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.

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.

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).

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.

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).

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.

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.

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).

Effects: (1) Harmful - diseases (sickle cell anemia, cancer), (2) Neutral - no effect, (3) Beneficial - antibiotic resistance in bacteria, evolution.

Sickle Cell Anemia: Gene mutation causing abnormal hemoglobin. RBCs become sickle-shaped.

Importance: Mutations provide genetic diversity essential for evolution and adaptation.