Lesson Notes By Weeks and Term v4 - SHS 3
CELL STRUCTURE AND FUNCTIONS
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CELL STRUCTURE AND FUNCTIONS
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Subject: Biology
Class: SHS 3
Term: 1st Term
Week: 14
Grade code: 3.2.2.LI.2
Strand code: 2
Sub-strand code: 1
Content standard code: 3.2.1.CS.3
Indicator code: 3.2.2.LI.2
Theme: LIFE IN THE FUNDAMENTAL UNIT
Subtheme: CELL STRUCTURE AND FUNCTIONS
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This lesson explores the fascinating concept of human variation. While we are all one species, *Homo sapiens*, a look around our classroom, community, and country shows a beautiful diversity in our appearances and traits. This variation is not random; it arises from differences in our genetic material, the DNA found within the nucleus of our cells. Understanding the principles behind this variation is not just an academic exercise. It is a powerful tool that allows us to make life-saving medical decisions, solve complex crimes, and make informed choices about our health and families.
A. What is Genetic Variation?
Genetic variation refers to the differences in DNA sequences among individuals within a population. These differences in our genes (genotype) lead to the observable differences in our characteristics or traits (phenotype).
Types of Variation: Discontinuous Variation: This is where traits fall into distinct, separate categories with no in-between forms. It is usually controlled by one or a few genes. Environmental factors have little to no effect. Examples: ABO Blood Groups: A person can be blood group A, B, AB, or O. There is no "in-between." Ability to Roll Tongue: You either can or cannot roll your tongue. Albinism: An individual is either albino or not. PTC Tasting: Phenylthiocarbamide (PTC) paper tastes either very bitter or like plain paper. Continuous Variation: This is where a trait shows a continuous range of values from one extreme to another. It is usually controlled by multiple genes (polygenic inheritance). Environmental factors often have a significant influence. Examples: Height: People are not just "tall" or "short." There is a complete range of heights. Skin Colour: There is a wide spectrum of skin tones, not just a few distinct colours. Weight: An individual's weight can fall anywhere along a wide range. Intelligence: Measured by IQ scores, it shows a continuous distribution in the population. B. Sources of Genetic Variation
Where do all these differences come from? They originate at the cellular level during reproduction. Meiosis (Gamete Formation): Crossing Over: During Prophase I, homologous chromosomes exchange segments of DNA. This shuffles alleles, creating new combinations on a single chromosome. Independent Assortment: During Metaphase I, pairs of homologous chromosomes line up at the cell's equator randomly. The way one pair lines up does not affect how other pairs line up. This creates millions of possible combinations of chromosomes in the resulting gametes. Mutation: A mutation is a permanent change in the DNA sequence of a gene. It is the ultimate source of all new alleles (different versions of a gene). While many mutations are harmful or neutral, some can be beneficial and provide a new trait that helps an organism survive. Sexual Reproduction: The random fusion of a male gamete (sperm) and a female gamete (egg) during fertilisation brings together genetic information from two different parents, creating a unique combination of alleles in the offspring. C. Applying Principles of Variation to Improve Life