Inheritance and Evolution Exhaustive Revision Notes

The Science of DNA and Genetics

• Definition of DNA: DNA stands for deoxyribonucleic acid. It is the genetic code that makes up genes, which are responsible for giving an organism its specific characteristics. It is a chemical made up of two long strands arranged in a spiral known as a double-helix structure.
• Function of DNA: DNA carries the genetic code—all the instructions that a living organism needs to grow, reproduce, and function. It is passed from parents to offspring during fertilisation.
• Discovery of DNA: The structure of DNA was discovered in $1953$ by scientists James Watson and Francis Crick, with significant assistance from Rosalind Franklin and Maurice Wilkins.
  • Rosalind Franklin produced 'X-ray diffraction' images of DNA.
  • James Watson and Francis Crick used one of Franklin's images to work out their model for the DNA structure.
  • Maurice Wilkins, Franklin's colleague, provided work that supported the Watson and Crick model.
• Chemical Components: DNA is composed of four base pairs formed by the DNA bases: A, T, G, and C.
  • Pairing Rules: A always pairs with T; G always pairs with C.
  • Bonding: The base pairs are held together by weak chemical hydrogen bonds.
• DNA Specifics:
  • There are over $3 \times 10^{9}$ (three billion) different DNA base pairs in each human.
  • DNA is unique to every individual, except for identical twins who share the same DNA.
  • DNA is located in the nucleus of almost every cell in the body.
  • Approximately $2\,m$ of DNA is packed into each body cell nucleus.
  • Sex cells (gametes)—sperm and ova—contain only half of this amount ($1\,m$) because they combine during fertilisation.
  • All life on Earth (bacteria, fungi, animals, plants, and single-celled protists) shares the same genetic code made of these same four letters.

From DNA to the Genome

• Genes: These are small sections of DNA that serve as the code for inherited characteristics, such as ear shape or eye colour. Most characteristics are inherited in pairs (one from each parent).
• Alleles: Alleles are pairs of genes inherited from each parent. There are approximately $23,000$ genes that make up a human.
• Chromosomes: Genes are arranged into larger coiled structures called chromosomes.
  • Humans have a total of $46$ chromosomes (inheriting $23$ from each parent).
  • Other organisms vary: The fruit fly has only $8$ chromosomes, while some ferns have over $100$.
• The Genome: The genome is the total collection of all chromosomes in an organism, representing one copy of all its DNA.
  • Every human has a unique genome, except for identical twins who originate from the same fertilised ovum (egg cell).
  • Non-identical twins come from two separate sperm fertilising two different ova and thus have different genomes.
• Hierarchy Model: A useful model for understanding the scale of genetic structures involves currency: Pennies (DNA base pairs) make up $£1$ coins (Genes), which make up $£5$ notes (Chromosomes), which in turn make up $£20$ notes (the Genome).

Variation in Species

• Definition: Variation refers to the differences in characteristics between individuals of the same species.
• Species Examples: Humans, dogs, and goldfish are distinct species. Different species have different characteristics (e.g., dogs have fur and tails, goldfish have scales).
• Causes of Variation:
  • Inherited Variation: Characteristics passed from parents to offspring via genes during reproduction (e.g., eye colour, blood group).
  • Environmental Variation: Differences resulting from surroundings or an individual's actions (e.g., tattoos, scars, language spoken).
  • Combined Factors: Many characteristics result from both genes and environment (e.g., height, weight, hair colour which can lighten in the sun).
• Variation Classification Table:
  • Genetic Only: Eye colour, blood group, attached ear lobes.
  • Environmental Only: Tattoos, scars, language spoken.
  • Genetic and Environmental: Height, weight, hair colour, animal fur colour, size of a plant, and the colour of hydrangea flowers (which turn blue in acid soil and pink in alkaline soil).
• Benefits of Variation: Genetic diversity prevents a single disease from wiping out an entire species. It allows for 'survival of the fittest,' where better-adapted individuals (e.g., faster runners or better camouflaged) survive and reproduce.

Data Analysis: Continuous and Discontinuous Variation

• Continuous Variation:
  • Definition: Characteristics that change gradually over a range of values.
  • Examples: Height, weight, arm span, head circumference at birth.
  • Data Representation: Usually shown in line graphs or histograms. Results often form a "Normal Distribution" (a bell-shaped graph) where most readings are in the middle and fewer are at the extremes.
• Discontinuous Variation:
  • Definition: Characteristics that fall into distinct groups or categories with no values in between.
  • Examples: Blood group (A, B, AB, or O), eye colour, hand used for writing, ability to roll the tongue.
  • Data Representation: Shown in bar charts. No line of best fit is drawn because the categories (x-axis) can be placed in any order.

Principles of Inheritance and Heredity

• Heredity: The study of inheritance.
• Monohybrid Inheritance: This refers to the inheritance of characteristics controlled by a single pair of genes.
  • Examples include fur in animals and red-green colour blindness in humans.
  • Historically, tongue rolling and ear lobe structure were used as examples, though scientists now suspect these are controlled by multiple genes.
• Genetic Terminology:
  • Allele: An alternative form of a gene.
  • Dominant: An allele that is always expressed if at least one copy is present (represented by a capital letter, e.g., $H$).
  • Recessive: An allele that is only expressed if two copies are present (represented by a lowercase letter, e.g., $h$).
  • Homozygous: Having two of the same alleles (e.g., $HH$ for homozygous dominant or $hh$ for homozygous recessive).
  • Heterozygous: Having two different alleles (e.g., $Hh$).
  • Genotype: The combination of alleles present.
  • Phenotype: The physical characteristic expressed by the genotype.
• Punnett Squares: A tool used to predict the probability of genotypes and phenotypes in offspring.
  • Case Study (Cat Hair Length): Short hair is dominant ($H$), long hair is recessive ($h$).
  • If two long-haired cats ($hh$) mate, all offspring will be $hh$ ($100\%$) and have long hair.
  • If two heterozygous parents ($Hh$) mate, the ratio is typically $25\%$ $HH$, $50\%$ $Hh$, and $25\%$ $hh$.
• Inheritance of Genetic Disorders:
  • Genetic disorders are medical conditions passed from parents to children.
  • Carriers: Individuals who are heterozygous for a recessive disorder ($Cc$). They do not have the disease but can pass the allele to offspring.
  • Cystic Fibrosis: Inherited as a recessive allele ($c$). A child needs two copies ($cc$) to develop the condition. Two carrier parents have a $1$ in $4$ ($25\%$) chance of having a child with cystic fibrosis.

Adaptation and Evolution

• Adaptations: Features that help organisms survive in their habitat.
  • Structural: Physical appearance (e.g., webbed feet).
  • Behavioural: Actions of the organism (e.g., hibernation).
  • Physiological: Internal processes (e.g., producing poison).
• Rainforest Animal Examples:
  • Sloth: Camouflage and slow movement to avoid predators and save energy.
  • Spider Monkey: Long, strong limbs for climbing.
  • Toucan: Large, long bill to reach fruit on weak branches.
  • Flying Frog: Webbed hands/feet and loose skin for gliding.
• Plant Adaptations:
  • Rainforest Trees: Tall, thin trunks to reach light; buttress roots for stability; smooth trunks and drip tips on leaves to shed water.
  • Epiphytes: Grow on high branches to absorb more light, getting water/nutrients from the air.
• Extremophiles: Organisms living in conditions outside the normal range.
  • Desert (Cactus): Water-storing stems; deep or widespread roots; spines (modified leaves) to reduce water loss and prevent being eaten; thick waxy cuticles; reduced stomata.
  • Polar Regions (Polar Bear): Thick white fur (camouflage); greasy fur (water shedding); thick blubber (fat) for warmth; large feet to spread weight on snow.
  • Volcanic Vents: Found on the ocean floor. No light exists, so producers use the chemical energy released from vents to make glucose via chemosynthesis instead of photosynthesis.

The Theory of Evolution by Natural Selection

• Evolution: The process by which small inherited changes in a population over many generations lead to the formation of new species.
• Natural Selection: Often called "survival of the fittest." It is the drive behind evolution.
  1. Variation: Inherited variation exists in every population.
  2. Competition: Individuals compete for food, shelter, and mates.
  3. Selection: Individuals with the best adaptations are more likely to survive and reproduce.
  4. Inheritance: Adaptations are passed to offspring.
  5. Time: Over many generations, these changes result in new species.
• Charles Darwin: An English scientist who proposed the theory in his $1859$ book On the Origin of Species.
  • Conducted a $5$-year voyage on the HMS Beagle.
  • Studied finches on the Galapagos Islands; noted that beak shapes were adapted to specific food sources.
  • Co-published ideas with Alfred Wallace.
• Evidence for Evolution:
  • Fossils: The fossil record shows gradual changes over millions of years. Gaps exist because not all fossils are found, some are destroyed in the rock cycle (magma), and soft parts of organisms do not fossilise.
  • Bacterial Resistance: Bacteria reproduce quickly. Mutations can lead to strains resistant to antibiotics (e.g., penicillin), and these resistant bacteria survive and multiply via natural selection.
• Evolutionary Trees: Diagrams showing how species evolved from common ancestors. For example, vertebrates (animals with backbones) evolved from a common ancestor, with traits like jaws, lungs, and hair appearing at different points in time.

Questions & Discussion

• Q: What pig in the family tree is a male with the genetic disorder?
  • A: Based on diagrammatic keys, a square often represents a male and shading represents the disorder; for pig A, B, C, D, E, F, the student must identify the shaded square.
• Q: How does the sex of offspring depend on chromosomes?
  • A: Females have $XX$ chromosomes; males have $XY$ chromosomes. Female egg cells always carry an $X$. Male sperm cells carry either an $X$ or a $Y$. If a sperm with a $Y$ fertilises the egg, the child is male ($XY$).
• Q: What are the chances of two carrier parents for a recessive disorder having a healthy child?
  • A: There is a $75\%$ chance of the child not having the disorder ($25\%$ homozygous dominant and $50\%$ heterozygous carrier).
• Q: Why was Darwin's theory controversial?
  • A: It conflicted with religious views regarding the creation of the world and its organisms.
• Q: What are the advantages of embryo screening?
  • A: It allows for the identification of inherited disorders before birth and can help parents make informed decisions about the pregnancy or prepare for the child's needs.