Genetics and Natural Selection: Grade 8 Study Guide
Offspring resemble their parents because of genes. Over generations, populations change.
Genes are sections of DNA that carry the instructions for traits. They are arranged on chromosomes in the nucleus of cells. Humans have 23 pairs of chromosomes; one of each pair comes from each parent. Different versions of a gene are alleles. A dominant allele (written with a capital letter, B) shows even if only one copy is present; a recessive allele (b) shows only if two copies are present.
Mutations are changes in DNA. Most are harmless, some are harmful and a few are helpful. Sexual reproduction mixes genes, so offspring differ from parents and from each other.
Punnett squares
A cross of two heterozygous parents (Bb × Bb) gives: BB, Bb, Bb and bb. That is 3 with the dominant trait and 1 with the recessive trait, a 3 : 1 ratio, or a 25% chance of bb for each child.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Natural selection
Individuals in a population vary. Some variations help them survive and reproduce in their environment. Those with helpful traits leave more offspring, so the trait becomes more common over generations. Example: peppered moths. When trees were darkened by soot, dark moths were harder for birds to spot and became more common. Natural selection acts on existing variation; it does not mean organisms 'try' to change.
Individuals differ.
Not all survive.
Helpful traits help.
Traits pass to offspring.
Evidence for evolution
Scientists use fossils (older layers hold simpler or different forms), anatomy (a human arm, a whale flipper and a bat wing share the same bones), embryology and DNA comparisons (closely related species share more DNA). Antibiotic resistance in bacteria shows natural selection happening today. Humans also use artificial selection, breeding plants and animals for chosen traits.
Worked example and common misconceptions
Example: If both parents carry one recessive allele for a trait, each child has a 1-in-4 chance of showing it. Common misconceptions: thinking evolution is 'only a theory' in the everyday sense (in science, a theory is a well-tested explanation), that individuals evolve during their lifetime and that dominant means common. Keep scientific language precise.
Check your understanding: model answers
1. Offspring: Bb, Bb, bb, bb, so 1/2 show the recessive trait. 2. A different version of a gene. 3. For example, fossils or similar bone structures in different animals.
✨ Key Takeaways
- Genes on chromosomes carry traits.
- Dominant alleles mask recessive ones.
- Natural selection favors helpful variations.
- Evidence comes from fossils, anatomy and DNA.
💬 Questions for Discussion & Reflection
- Complete Bb × bb: what fraction show the recessive trait?
- What is an allele?
- Give one piece of evidence for evolution.
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