Heredity and Evolution

Key Concepts

Introduction to Heredity

Heredity is the process by which traits and characteristics are reliably inherited from parents to offspring. While offspring inherit a common basic body design, subtle variations arise during reproduction, forming the basis for diversity and evolution.

Rules for Inheritance - Mendel’s Contributions

Gregor Johann Mendel worked out the fundamental rules of inheritance using garden peas (Pisum sativum) with contrasting visible characters (e.g., tall/short, round/wrinkled seeds).

1. Law of Dominance:

• In the first generation (F1) of a cross between a pure-bred tall plant (TT) and a pure-bred short plant (tt), all plants are tall (Tt).
• The trait that is expressed (T) is the dominant trait, while the trait that remains hidden (t) is the recessive trait.

2. Law of Segregation:

• When F1 progeny (Tt) are self-pollinated, the second generation (F2) consists of both tall and short plants in a 3:1 ratio (75% tall, 25% short).
• This shows that both traits are inherited, but only one is expressed in the F1 generation. The hidden trait reappears in F2.

3. Independent Inheritance:

• When crossing plants with two different characteristics (e.g., tall/round seeds vs. short/wrinkled seeds), traits like plant height and seed shape are inherited independently. This gives rise to new combinations in the F2 generation.

Mechanism of Heredity

• Genes: Sections of DNA that provide information for one specific protein. Proteins, in turn, control the characteristics or traits of an organism.
• Chromosomes: Gene sets are present as separate independent pieces called chromosomes. Each cell has two copies of each chromosome (one from each parent).
• Meiosis: A process of cell division that ensures each germ-cell (gamete) contains only one chromosome from each pair, restoring the stable DNA content after fertilization.

Sex Determination in Human Beings

In human beings, the sex of an individual is genetically determined.

• Humans have 23 pairs of chromosomes, 22 of which are identical in both males and females.
• The 23rd pair is the sex chromosome.
• Women: Have a perfect pair of XX chromosomes.
• Men: Have a mismatched pair, one normal-sized X and one shorter Y chromosome (XY).
• Determination: Every child inherits an X chromosome from their mother. The sex is determined by whether the child inherits an X or a Y chromosome from their father.
  • X from father = Girl (XX)
  • Y from father = Boy (XY)