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IB DP Biology — Unit D: Continuity and Change | SL | D3.2 Inheritance
📋 What You Will Learn
- Identify the key terminology of genetics — allele, genotype, phenotype, dominant, recessive, homozygous, heterozygous — and apply them precisely in genetic problems
- Explain Mendel's Law of Segregation and Law of Independent Assortment using the behaviour of chromosomes during meiosis
- Construct and interpret Punnett squares for monohybrid and dihybrid crosses, including test crosses
- Analyse inheritance patterns from pedigree charts and predict genotype/phenotype ratios in offspring
- Apply chi-squared analysis to determine whether observed offspring ratios deviate significantly from expected Mendelian ratios
🔍 Guiding Question How do the laws of inheritance explain the transmission of heritable characteristics from parents to offspring, and what are the molecular and cellular mechanisms that underpin these patterns?
🌍 Why This Matters Cystic fibrosis affects approximately 100,000 people worldwide — yet two healthy parents can produce an affected child. Understanding Mendelian inheritance explains exactly how a recessive allele can hide silently in a carrier for generations before reappearing. Genetic counsellors use these same Punnett square principles every day to calculate the probability that a couple will pass on a heritable condition to their children.
The Problem Mendel Solved
Before Gregor Mendel, inheritance was thought to work like mixing paints — blend two colours and you get something in between, permanently. But this "blending hypothesis" made a catastrophic prediction: variation should disappear within a few generations, smoothed into uniformity. It didn't. Variation persisted stubbornly across generations, and nobody could explain why.
Mendel's genius was to treat inheritance as a counting problem. Working with pea plants in the 1860s, he tracked discrete, countable traits across thousands of offspring and found that they appeared in remarkably consistent mathematical ratios. His conclusion was radical: heritable information is carried in discrete units — what we now call genes — that do not blend, but segregate cleanly during reproduction.
We now know the molecular basis for everything Mendel observed. Genes are sequences of DNA on chromosomes. Chromosomes come in homologous pairs. During meiosis, those pairs separate. The ratios Mendel counted are the direct consequence of chromosome behaviour — and that is the story this note tells.
Foundational Vocabulary
Genetics has a precise vocabulary that must be used correctly in exams. Each term below is introduced at the moment it becomes necessary — not as a list to memorise, but as a tool you need to solve a specific problem.
📖 Key Definition — Gene A gene is a heritable factor that controls a specific characteristic. It consists of a sequence of nucleotides that occupies a specific locus (position) on a chromosome.
📖 Key Definition — Allele An allele is one specific form of a gene, occupying the same locus as other alleles of that gene on a homologous chromosome. Different alleles arise