# Inheritance > IB Biology SL · Theme D: Continuity and Change > Source: https://www.owlsprep.com/study/ib-biology-sl-u4-inheritance/ This module covers core principles of Mendelian and non-Mendelian inheritance for IB Biology SL, including key terminology, genetic crosses, Punnett grids, pedigree analysis, sex linkage, and codominance, all frequently tested in IB SL exams. **Prerequisites:** [Chromosomes and alleles](https://www.owlsprep.com/study/ib-biology-sl-u3-chromosomes/); [Meiosis](https://www.owlsprep.com/study/ib-biology-sl-u2-meiosis/) ## Learning objectives - Distinguish between key inheritance terms including genotype, phenotype, dominant and recessive alleles - Construct and interpret Punnett grids for monohybrid genetic crosses - Analyze pedigree charts to identify inheritance patterns of genetic disorders - Predict outcomes of crosses involving codominance and sex linkage ## Key Terms and Core Mendelian Principles **Gene** — A heritable factor made of DNA that influences a specific characteristic, occupying a fixed locus on a chromosome *Notation:* Alleles symbolized $A$, $a$ for different variants *Example:* The gene for pea plant height is found at a fixed locus on pea chromosome 4 **Dominant vs Recessive Alleles** — A dominant allele is expressed in both homozygous and heterozygous genotypes. A recessive allele is only expressed when no dominant allele is present (homozygous recessive). Mendelian inheritance describes the pattern of single-gene traits first documented by Gregor Mendel in pea plants. The core law of segregation states each parent passes one randomly selected allele to their offspring. **Worked example:** A pea plant heterozygous for height ($T$ = tall, $t$ = short) is self-fertilized. What are the expected genotype and phenotype ratios of offspring? 1. Identify parental gametes: both parents are heterozygous $Tt$, so each produces 50% $T$ and 50% $t$ gametes. 2. Construct a Punnett grid to combine gametes: 3. $$\begin{array}{c|c|c} & T & t \\ \hline T & TT & Tt \\ \hline t & Tt & tt \\ \end{array}$$ 4. Count genotypes: 1 $TT$ : 2 $Tt$ : 1 $tt$, giving a genotype ratio of 1:2:1. 5. Since $T$ is dominant, $TT$ and $Tt$ are tall, only $tt$ is short. Phenotype ratio = 3 tall : 1 short. ## Codominance and Multiple Alleles Codominance is a non-Mendelian pattern where both alleles are fully and equally expressed in the heterozygous phenotype. Human ABO blood type is the most common IB example, and it also demonstrates multiple alleles (three alleles for the same gene). **Multiple Alleles** — When a gene exists in more than two allelic forms in a population, leading to more than two possible phenotypes *Example:* The ABO blood group gene has three alleles: $I^A$, $I^B$, $i$ **Worked example:** A father with blood type A (genotype $I^A i$) and a mother with blood type B (genotype $I^B i$) have a child. What is the probability the child has blood type AB? 1. Identify parental gametes: father produces $I^A$ and $i$, mother produces $I^B$ and $i$. 2. Construct Punnett grid: 3. $$\begin{array}{c|c|c} & I^A & i \\ \hline I^B & I^A I^B & I^B i \\ \hline i & I^A i & ii \\ \end{array}$$ 4. Only one of four possible offspring genotypes is $I^A I^B$, which gives blood type AB. 5. Probability = 1/4 or 25% > **Exam tip:** Always write superscripts for codominant alleles (e.g. $I^A$ not just $A$) to earn full marks in IB exams. ## Sex-Linked Inheritance Sex-linked traits are controlled by genes located on the X or Y sex chromosomes. In humans, the X chromosome is much larger than the Y, so most sex-linked traits are X-linked. Males only have one X chromosome, so they express all X-linked alleles regardless of dominance, making them more likely to show recessive X-linked disorders. **X-linked Recessive Inheritance** — Inheritance pattern where a recessive allele on the X chromosome causes the recessive phenotype. Males are affected with one copy; females need two copies to be affected. **Worked example:** A woman who is a carrier for red-green color blindness (X-linked recessive) has a child with a man with normal vision. What is the probability their son is color blind? 1. Write parental genotypes: Mother = $X^A X^a$, Father = $X^A Y$, where $X^a$ is the recessive color blind allele. 2. Sons inherit their Y chromosome from their father, so their X chromosome always comes from their mother. 3. The mother has one $X^A$ (normal) and one $X^a$ (color blind) allele, so there is a 50% chance the son inherits $X^a$. 4. Final probability = 1/2 or 50% ## Pedigree Chart Analysis Pedigree charts show inheritance of a trait across multiple generations of a family. They are used to determine if a trait is dominant/recessive and autosomal/sex-linked, and this is a common IB SL exam question. - Squares = males, circles = females - Shaded symbols = affected individuals, unshaded = unaffected - Horizontal lines = mating pairs, vertical lines connect parents to offspring **Worked example:** A pedigree shows two unaffected parents producing an affected child with an autosomal disorder. Is this possible for a dominant disorder? 1. For a dominant disorder, any affected individual must have at least one affected parent, because the dominant allele causes the trait. 2. If two parents are unaffected, they both have two copies of the recessive non-disease allele, so cannot pass on a dominant disease allele. 3. Therefore, this pattern is only possible for an autosomal recessive disorder, and cannot be dominant. > **Exam tip:** If most affected individuals in the pedigree are male, the trait is almost certainly X-linked recessive, the most common sex-linked pattern tested. ## Common pitfalls - **Wrong:** Writing plain alleles for codominance (e.g. $A$ instead of $I^A$) - Why it fails: IB exam markers require correct notation to award full marks for genetic cross questions - Correct: Always use superscripts to denote codominant alleles, following IB notation conventions - **Wrong:** Calculating probability over all offspring when the question specifies male/female - Why it fails: Questions about sex-linked traits often ask for probability for a child of a given sex, requiring conditional probability - Correct: If the question specifies the child's sex, only count offspring of that sex for your calculation - **Wrong:** Confusing codominance with incomplete dominance - Why it fails: IB exams explicitly test the distinction between these two inheritance patterns - Correct: Remember: codominance = both traits fully expressed; incomplete dominance = blended intermediate phenotype - **Wrong:** Assuming all rare traits in a pedigree are recessive - Why it fails: Dominant disorders can be rare if the disease allele has low frequency in the population - Correct: Use the rule: if an affected child has two unaffected parents, the trait is recessive ## Cheatsheet | Inheritance Pattern | Key Rule | Example | | --- | --- | --- | | Autosomal complete dominance | 3:1 phenotype ratio from heterozygote cross | Pea plant height | | Codominance | Both alleles fully expressed in heterozygotes | AB blood type | | X-linked recessive | More affected males; affected daughters need affected father | Red-green color blindness | | Autosomal recessive | Two unaffected parents can have affected child | Cystic fibrosis | | Autosomal dominant | Every affected person has at least one affected parent | Huntington's disease | ## What's next Understanding inheritance is the foundation for understanding genetic variation, biotechnology, and the genetic basis of disease, all core topics in IB Biology SL. This sub-topic regularly makes up 10-15% of total exam marks for IB Biology SL, so mastering genetic crosses, notation conventions, and pedigree analysis is critical for achieving a high score. The principles you learned here apply to all subsequent genetics topics, from biotechnology to evolution, so it is important to solidify your understanding before moving on to more advanced concepts. - [Natural selection](https://www.owlsprep.com/study/ib-biology-sl-u4-natural-selection/) - [Adaptation and speciation](https://www.owlsprep.com/study/ib-biology-sl-u4-adaptation-and-speciation/) - [Evolutionary history and biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u4-evolutionary-history-and-biodiversity/) --- From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-biology-sl-u4-inheritance/