Preview
IB DP Biology — Unit D: Continuity and Change | HL | D3.2 Advanced Genetics
📋 What You Will Learn
- Identify the chromosomal basis of sex determination and explain how sex-linked traits are inherited differently from autosomal traits
- Explain the mechanisms of epistasis, polygenic inheritance, and gene linkage, and how they modify expected Mendelian ratios
- Analyse pedigree charts and genetic cross data to deduce patterns of inheritance including autosomal, sex-linked, dominant, and recessive conditions
- Apply chi-squared analysis to evaluate whether observed genetic ratios deviate significantly from expected Mendelian predictions
- Evaluate how crossing over, recombination, and gene mapping extend our understanding of chromosome structure and inheritance
🔍 Guiding Question How do the molecular mechanisms of inheritance explain both the predictability and the surprising variation we observe in the traits of offspring?
🌍 Why This Matters Colour blindness affects approximately 8% of males but fewer than 0.5% of females worldwide — a disparity that cannot be explained by simple Mendelian inheritance alone. Understanding sex-linkage, epistasis, and polygenic inheritance reveals why human traits from eye colour to disease susceptibility follow patterns far more complex than Mendel's pea plants ever suggested.
📋 What You Will Learn
- Identify the chromosomal basis of sex determination and explain how sex-linked traits are inherited differently from autosomal traits
- Explain the mechanisms of epistasis, polygenic inheritance, and gene linkage, and how they modify expected Mendelian ratios
- Analyse pedigree charts and genetic cross data to deduce patterns of inheritance including autosomal, sex-linked, dominant, and recessive conditions
- Apply chi-squared analysis to evaluate whether observed genetic ratios deviate significantly from expected Mendelian predictions
- Evaluate how crossing over, recombination, and gene mapping extend our understanding of chromosome structure and inheritance
Sex Determination and Sex-Linked Inheritance
Here is the problem the cell had to solve: how do you encode biological sex using the same genome that encodes everything else? The answer, in most animals, is a pair of chromosomes that are not quite a pair. In humans, the X and Y chromosomes determine sex — but they are structurally and genetically very different from each other, and this asymmetry has profound consequences for inheritance.
In humans, females carry two X chromosomes (XX) and males carry one X and one Y (XY). The Y chromosome is much smaller than the X and carries relatively few genes — primarily those involved in male sex determination, including the SRY gene. The X chromosome, by contrast, carries hundreds of genes unrelated to sex determination. Genes located on the X chromosome are said to be X-linked.
📖 Key Definition — Sex-Linked Gene A sex-linked gene is a gene located on a sex chromosome (X or Y). In IB Biology, sex-linked almost always refers to X-linked genes, since the Y chromosome carries very few genes. Males (XY) have only one copy of each X-linked gene and are therefore described as hemizygous for X-linked loci.
Because males have only one X chromosome, they express whatever allele is present on that X — even if it is recessive. A female with