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How-to-approach-D3.2: Inheritance

Keywords: IB Biology Topic D3.2, Inheritance, Genotype, Phenotype, Alleles, Dominant vs Recessive, Co-dominance, Sex-linkage, Pedigrees, Punnett Squares, Monohybrid and Dihybrid Cross, Polygenic Inheritance.

Welcome to the logic of the gene: Topic D3.2 Inheritance. In the new IB Biology syllabus, this unit moves from the molecular structure of DNA to the mathematical probability of traits. The Bio-Logic focuses on 'Segregation and Assortment'—how alleles are separated during meiosis and recombined during fertilization.

This is arguably the most 'math-heavy' unit in the syllabus. You must be able to perform monohybrid and dihybrid crosses, calculate phenotypic ratios (like the classic 9:3:3:1), and analyze pedigree charts to determine the mode of inheritance. In Paper 1A (MCQs), the IBO frequently tests sex-linked traits (like color blindness and hemophilia) and the blood group system (A, B, O).

Before we draw our squares, remember the Mendelian baseline: Most traits aren't as simple as 'Purple vs. White.' However, by understanding these fundamental rules, we can then layer on the complexities of linked genes and environmental factors. If you can track the movement of a single allele through a family tree, you can master the unit.

1. The Basics: Genotype vs. Phenotype

Inheritance is about the combination of alleles (versions of a gene) that an organism carries.

• Homozygous: Having two of the same allele (e.g., BB or bb).
• Heterozygous: Having two different alleles (e.g., Bb).
• Genotype: The symbolic representation of the alleles (the 'letters').
• Phenotype: The physical characteristic expressed (the 'look').

2. Multiple Alleles and Co-dominance: ABO Blood Groups

Not all alleles are dominant or recessive. Sometimes both are expressed equally.

• Co-dominance: Both alleles contribute to the phenotype (e.g., Type AB blood).
• Blood Types: A and B are co-dominant, while O is recessive.

Take a look at the question below:

3. Sex-Linked Inheritance: The X-Factor

Genes located on the sex chromosomes (usually the X) show different patterns in males and females.

• Females (XX): Can be homozygous dominant, heterozygous (carriers), or homozygous recessive.
• Males (XY): Have only one X. If they inherit the recessive allele, they must express it. There are no male 'carriers' for X-linked traits.
• Examples: Red-green color blindness and Hemophilia.

4. Pedigree Analysis: The Family Tree

Pedigrees track a phenotype through generations. Use these IB rules to identify the mode of inheritance:

• Autosomal Recessive: Two unaffected parents have an affected child (the parents are carriers).
• Autosomal Dominant: Every affected child has at least one affected parent. It doesn't 'skip' generations.
• Sex-Linked Recessive: Far more males are affected than females. Affected mothers will have 100% affected sons.

5. Exam Strategy: The Dihybrid Cross

When dealing with two unlinked genes (e.g., imes AaBb$):

• 1. FOIL the Gametes: First, Outside, Inside, Last. For AaBb, the gametes are AB, Ab, aB, ab.
• 2. Use the Ratio: For a heterozygous cross (AaBb X AaBb), the phenotypic ratio is almost always 9:3:3:1.
• 3. Watch for Linkage: If the question mentions that the genes are on the same chromosome, Mendelian ratios no longer apply!