Heredity and Variation

Heredity is the process by which biological characters are transmitted from parents to their offspring. This study is fundamental to Genetics.

1. Heritable and Non-Heritable Characters

Heritable characters are traits passed from parents to offspring via genes. Examples include eye color, skin color, height, blood group, and sickle-cell trait. Non-heritable (acquired) characters are traits developed during an organism's lifetime due to environmental influences, such as scars, muscle build from exercise, or learned skills.

2. Chromosomes: The Basis of Heredity

Chromosomes are thread-like structures in the nucleus that carry genetic information. They are composed of DNA (Deoxyribonucleic Acid) and proteins.

• DNA Structure: A double helix made of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (Adenine, Thymine, Cytosine, Guanine).
• Transmission: During Meiosis, homologous chromosomes segregate (separate) so that each gamete receives only one allele for a trait. This is known as Mendel’s First Law (Law of Segregation).
• Recombination: During fertilization, gametes fuse randomly, restoring the diploid number and creating new genetic combinations.

3. Probability and Genetics

Genetic crosses follow the laws of probability. A Monohybrid Cross between two heterozygotes (e.g., Tt x Tt) typically yields a phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1.

4. Applications of Heredity

In Agriculture

• Cross-breeding: Breeding different varieties to produce hybrids with superior traits (Hybrid Vigor or Heterosis).
• In-breeding: Breeding closely related individuals to preserve desirable traits, though it may lead to inbreeding depression.
• Out-breeding: Breeding unrelated individuals to introduce new genes.
• GMOs: Organisms whose DNA has been altered using genetic engineering to improve yield or resistance.

In Medicine

• Marriage Counselling: Helping couples understand the risks of passing on traits like Sickle Cell Anaemia (AS x AS) or Rhesus incompatibility (Rh- mother and Rh+ father).
• Recombinant DNA: Using bacteria to produce human insulin, interferons, and growth hormones by inserting human genes into bacterial plasmids.

5. Sex-Linked Characters

These are traits controlled by genes located on the sex chromosomes (usually the X chromosome). Examples include Haemophilia (failure of blood to clot), Color Blindness, and Baldness. These conditions are more common in males because they only possess one X chromosome.