Chapter 7: Genes and Inheritance Study Notes on Genes and Inheritance

Fundamental Biological Terminology

• Chromosomes: These are structures located inside the nucleus of a cell. A chromosome consists of proteins and Deoxyribonucleic Acid (DNA) organized into specific units called genes.
• Genes: This is the functional and physical unit of heredity that is passed from a parent to their offspring. Physically, genes are segments or pieces of DNA molecules.
• Deoxyribonucleic Acid (DNA): This is the molecule responsible for carrying genetic information required for the development and functioning of any organism.
• Gametes: These are the reproductive cells used by animals and plants.

7.1 Chromosomes and Cell Structure

• Cells are the building blocks of all living organisms. There are two primary types of basic cells:     * Animal Cells: These contain the Mitochondria, Nucleus, Cell Membrane, and Cytoplasm.     * Plant Cells: These contain the Mitochondria, Nucleus, Cytoplasm, Cell Wall, Vacuole, Cell Membrane, and Chloroplasts.
• The Role of the Nucleus: While cell structures vary depending on who they belong to and their specific functions, most cells contain a nucleus. The nucleus contains DNA and acts as the control center for cell activities.
• Visualizing Chromosomes:     * Chromosomes are located within the nucleus and can be viewed under a microscope.     * They are generally not viewable unless the cells are currently in the process of dividing or they have been stained with special coloring.     * Fruit flies possess the most easily viewable chromosomes. A "Fun Fact" noted is that their probable cousin has one of the shortest lifespans of almost any organism.
• Chromosome Counts across Species:     * Humans: Have $23$ pairs of chromosomes, totaling $46$. Deviations from this number (more or less) lead to disorders, such as Down Syndrome.     * Cats: Have $19$ pairs, totaling $38$ chromosomes.     * Dogs: Have $39$ pairs, totaling $78$ chromosomes.     * Durians: Have $28$ pairs of chromosomes.
• Chromosome Structure: Chromosomes have a cross-like shape. During cell division, each chromosome makes a copy of itself, and the copies remain joined part-way along their length.

7.1 Genes and Genetic Characteristics

• Gene Arrangement: Chromosomes are composed of hundreds of genes arranged in a specific, particular sequence.
• Heredity: Characteristics such as eye color, hair color, and height are determined by these genes.
• Variation by Chromosome:     * Each of the $23$ human chromosomes contains a different number of genes.     * Chromosome 1: Contains approximately $2000$ different genes.     * Chromosome 15: Contains approximately $600$ genes.     * Specific characteristics are determined by genes located in the same area on these chromosomes. For instance, a gene on Chromosome $15$ determines eye color.
• Eye Color Rarity: Brown is the most common eye color globally, followed by blue. Other colors are considered rare, although rarity depends on local genetics and geography.

7.1 Deoxyribonucleic Acid (DNA) Properties

• Molecular Structure: Each chromosome is essentially one enormously long molecule of DNA. Therefore, because genes are on chromosomes, genes are also made of DNA.
• The Double Helix: This is the name for the twisted ladder-like structure of the DNA molecule.
• Scale: A single gene can consist of a length of DNA with roughly $2500$ twists.
• Packaging: DNA is very tightly wound. It wraps around proteins called Histones. Observing the double helix requires many levels of unwinding.
• Chemical Components (Not examinable): DNA consists of a Phosphate backbone and four nitrogenous bases:     * Adenine     * Thymine     * Cytosine     * Guanine
• Red Blood Cell Exception: While DNA determines cell function in most instances, mature red blood cells and platelets (fragments of red blood cells) lack a nucleus and, for the most part, lack DNA.

7.2 Gametes and the Beginning of Life

• Biological Life Start: From a scientific perspective, life begins during conception. This is the moment a new DNA code is written for a unique person.
• Conception Process: This occurs when the $23$ chromosomes from a male gamete (sperm) combine with the $23$ chromosomes from a female gamete (ovum).
• Gender Determination: The $23^{rd}$ chromosome pair determines the gender of the offspring.
• Sperm (Male Gamete):     * The smallest cell in the human body.     * Contains a head, nucleus, midpiece, and mitochondria.     * Features a tail known as a Flagellum, which allows movement (swimming).     * Sperm cells are produced starting at puberty and released by the millions.     * Can carry either an $X$ or a $Y$ chromosome.
• Ovum (Female Gamete):     * The largest cell in the human body (roughly the size of a full-stop).     * Contains a nucleus, cell surface membrane, and cytoplasm.     * Lacks a flagellum and is stationary.     * Contains large food reserves in its cytoplasm to support the embryo until it can feed via the umbilical cord and placenta.     * All ova are present since birth, and usually, only one is released per month.     * Always carries an $X$ chromosome.

7.2 Fertilization, Reproduction, and Inheritance

• The Fertilization Process:     * Meiosis creates gametes with $23$ chromosomes.     * Sperm ($23$ chromosomes) + Egg ($23$ chromosomes) = Zygote ($46$ chromosomes in $23$ pairs).     * The zygote undergoes differentiation and multiplication to become an Embryo ($46$ chromosomes).     * The embryo develops into a human fetus over a $9$-month period.
• Sex Inheritance:     * Female: A double $X$ chromosome pair ($XX$).     * Male: An $XY$ chromosome pair.     * The father determines the gender because the $Y$ chromosome comes exclusively from the sperm cell. There is an ideal $50\%$ chance for either gender.
• Types of Twins:     * Fraternal Twins: Two sperm fertilize two separate eggs simultaneously. They share $50\%$ maternal and $50\%$ paternal DNA.     * Identical Twins: A single sperm fertilizes a single egg, which then divides. They share $100\%$ maternal and $100\%$ paternal DNA.     * Semi-identical Twins: Two sperm fertilize one egg, which then divides. This is extremely rare, with $100\%$ maternal and $78\%$ paternal DNA; embryos rarely survive.
• Scope of Inheritance: Inheritance is the process of passing DNA from parent to offspring. While physical features are the focus, inheritance can also include behaviors and fears.

7.3 Variation within Species

• Genetic Variation: Differences between animals of the same species (e.g., different horses) result from different versions of genes on specific chromosomes.
• Equine Example Characteristics:     * Horse: Tallest, slender, solid hooves, fastest.     * Quagga (Extinct): Distinct half-striped pattern, less compact.     * Donkey: Stockiest, sure-footed, fluffier, and comparatively the strongest.     * Zebra: Wild, resilient, full striped pattern.     * Pony: Shorter and more compact than horses, described as mischievous.
• Environmental Variation: Characteristics are also shaped by environmental factors, including habitat, diet, and behavior.

7.4 Natural Selection

• Survival of the Fittest: This is the rule where the fittest organisms (possessing the most ideal versions of genes) survive to pass those genes to offspring, ensuring species survival.
• Modern Examples:     * Bacteria and Viruses: They develop resistance to medicines or antibiotics via natural selection.     * Cockroaches: They develop variants immune to lethal chemicals in sprays. The resistant variants survive and pass immunity to their offspring.
• Charles Darwin Quote (1809): "It is not the strongest of the species that survives, nor the most intelligent, but the one most responsive to change."
• The Role of Competition: Predators pick off weaker prey or stragglers. Animals with deformities or less effective genes are less likely to win resources, preventing the passage of defective genes.
• Human Exception: Humans are noted as a "somewhat" exception to these strict natural selection rules.
• Mechanism Summary: Genetic Variation $\rightarrow$ Competition $\rightarrow$ Selection (Survivors with helpful variations) $\rightarrow$ Inheritance (Passing down variations).

7.4 Case Studies in Natural Selection

• The Peppered Moth:     * Originally, most were light-colored to camouflage against pale tree barks/lichens.     * Industrial Revolution ($1760$): Soot killed lichens and darkened tree trunks.     * Effect: By the late $1900$s, dark moths were prominent because light moths were easily spotted and eaten by birds on dark trees. In unpolluted areas, light moths continued to thrive.
• Lizard Evolution:     * Desert Lizards: Have evolved to be limbless because claws find it difficult to gain traction on slippery sand. No limbs allow better maneuvering.     * Tree Lizards: Retain clawed limbs to grip tree bark for climbing to reach food and habitat.
• Tiger Camouflage:     * Orange/Brown tigers are more successful than white ones because their prey (deer, boars) are typically red-green colorblind. The tigers blend into the environment from the prey's perspective.

Questions and Discussion

• Difference between Male and Female Gametes:     * Sperm is the smallest cell; Ovum is the largest.     * Sperm is mobile/has a flagellum; Ovum is stationary/lacks a flagellum.     * Sperm has less cytoplasm/food reserves compared to the Ovum.
• Antibiotic Effectiveness:     * Option A: Making the strongest antibiotic possible is more effective as it completely destroys bacteria, leaving no survivors to evolve.     * Option B: Gradually increasing strength is less effective for eradication as it allows bacteria time to adapt and evolve resistance.
• Genes vs. Chromosomes: A chromosome is a long strand of DNA, while genes are specific sections contained within that chromosome.