Preview
IB DP Biology — Unit D: Continuity and Change | SL | D3.2 Inheritance
📋 What You Will Learn
- Identify the patterns of inheritance produced by codominance, incomplete dominance, sex-linkage, and multiple alleles
- Explain how autosomal and sex-linked traits produce different ratios in male and female offspring, and why this matters for pedigree analysis
- Analyse pedigree charts to deduce genotypes, modes of inheritance, and the probability of offspring phenotypes
- Apply chi-squared analysis to test whether observed inheritance ratios match expected Mendelian predictions
- Evaluate how ABO blood groups, sickle cell anaemia, and colour blindness illustrate complex inheritance in human populations
🔍 Guiding Question How do the patterns of inheritance observed in human genetics reflect both the predictable rules of Mendelian genetics and the more complex molecular mechanisms that underlie them?
🌍 Why This Matters When a child is born with sickle cell disease, both parents may be completely healthy — yet each silently carries a single altered allele. Understanding how alleles interact, how sex chromosomes skew inheritance, and how multiple alleles produce a spectrum of blood types is not just academic: it is the foundation of genetic counselling, disease screening, and forensic identification.
The Problem Simple Dominance Cannot Solve
Mendel's original model was elegant: one allele dominates, the other hides, and ratios of 3:1 emerge reliably from monohybrid crosses. But the living world quickly revealed exceptions. Some traits blend. Some alleles share the stage equally. Some genes sit on sex chromosomes and produce strikingly different ratios in sons and daughters. And some genes have not two alleles but many, producing a combinatorial explosion of possible genotypes.
Each of these "exceptions" is not a failure of Mendel's logic — it is an extension of it. The rules of segregation and independent assortment still apply. What changes is the relationship between alleles and between genotype and phenotype. Understanding these patterns transforms you from someone who can predict simple crosses into someone who can read a family pedigree and deduce the invisible.
Codominance and Incomplete Dominance
In simple dominance, one allele completely masks the other. But what happens when neither allele is fully dominant? Two distinct outcomes are possible, and confusing them is one of the most common errors in IB exams.
Before defining these terms, consider the problem the cell faces: a heterozygote carries instructions for two different protein products. Whether both products appear, or whether a blend of both appears, depends entirely on the molecular nature of those proteins.
📖 Key Definition — Codominance Codominance is a pattern of inheritance in which both alleles in a heterozygote are fully and simultaneously expressed, producing a phenotype in which both parental traits are distinctly visible. Neither allele is masked. Both alleles are written as superscripts to a capital letter (e.g. I A , I B ).
📖 Key Definition — Incomplete Dominance Incomplete dominance is a pattern of inheritance in which neither allele is fully dominant over the other, producing a