Inheritance

Inheritance: Transmission of characteristics from parents to offspring.

Genetics: Branch of biology studying inheritance.

Trait: A characteristic (e.g., eye color).

Chromosomes: Thread-like structures in nucleus, made of DNA and Histone proteins (Chromatin). Visible during cell division.

• Chromatid: One of the two identical halves of a chromosome.
• Centromere: Point where two chromatids are attached.

Genes: Unit of inheritance. A segment of DNA located on chromosomes that code for specific proteins (traits). Locus is the position of a gene on a chromosome.

Alleles: Alternate forms of a gene (e.g., 'R' and 'r').

Genotype: Genetic makeup (e.g., Homozygous 'TT', Heterozygous 'Tt').

Phenotype: Physical expression of the genotype (e.g., Tall, Short).

Definition: Mechanism of working of genes.

Process: DNA -> (Transcription) -> mRNA -> (Translation) -> Protein.

Gregor Mendel (1822-1884), the father of genetics, chose Pisum sativum (Pea plant) for his experiments because:

• Easy to grow.
• Short life cycle.
• Has distinct traits (7 pairs of contrasting traits e.g., Round/Wrinkled Seeds, Yellow/Green Colour).
• Flowers are hermaphrodite (self-pollinating), but can be cross-pollinated.

1. Law of Segregation: Based on Monohybrid Cross (studying one trait). States that 'In an individual, genes (alleles) normally occur in pairs. These pairs segregate (separate) during gamete formation, so that each gamete receives only one allele.'

2. Law of Independent Assortment: Based on Dihybrid Cross (studying two traits). States that 'Alleles of different genes segregate independently of one another during gamete formation.'

Complete Dominance: One allele completely masks the other (e.g., Mendel's traits).

Incomplete Dominance: Heterozygous phenotype is intermediate between parents (e.g., 4 O'Clock Plant: Red x White = Pink).

Co-dominance: Both alleles are fully expressed in heterozygous condition (e.g., AB Blood Group).

Controlled by gene 'I' with three alleles: I^A, I^B, and i.

• Blood Group A: Genotype I^AI^A or I^Ai. Antigen A.
• Blood Group B: Genotype I^BI^B or I^Bi. Antigen B.
• Blood Group AB: Genotype I^AI^B. Antigens A & B (Co-dominance).
• Blood Group O: Genotype ii. No Antigen (Recessive).

Definition: Differences among members of the same species.

• Continuous Variation: Small differences, range of values (e.g., Height, Skin Color).
• Discontinuous Variation: Distinct categories, no intermediates (e.g., Blood Group, Tongue Rolling).

Sources: Mutation, Crossing over (Meiosis), Random fertilization.

Definition: Gradual change in characteristics of populations over time.

Theory of Natural Selection (Charles Darwin):

• Organisms produce more offspring than can survive.
• Variations exist in population.
• Competition for resources leads to 'Survival of the Fittest'.
• Fitter individuals reproduce and pass traits to next generation.

Artificial Selection: Humans breed plants/animals for desired traits (e.g., high milk yield cows, disease-resistant crops). Originated breeds and cultivars.