Apply Inheritance

Historical Context & Motivation

For thousands of years, farmers and breeders noticed that offspring tend to resemble their parents, but nobody could explain why some traits appeared, disappeared, or skipped generations. Before the science of genetics existed, inheritance was a mystery wrapped in superstition — some people even believed that a mother's thoughts during pregnancy could shape her child's appearance. The quest to understand inheritance — the biological process by which traits are transmitted from parents to offspring — required careful experimentation, mathematics, and a monk with a garden full of pea plants.

Mendel's great insight was that inheritance is not a blending process but rather involves discrete units passed from parent to offspring. The central question he answered — and the one you will learn to solve in this lesson — is: Given the parents' genetic makeup, what are the possible genotypes and phenotypes of their offspring, and in what ratios?

Core Principles of Inheritance

To apply inheritance, you need a solid grasp of the vocabulary and rules that govern how alleles behave during reproduction. Every organism that reproduces sexually receives one allele from each parent for each gene. The way those alleles interact determines the organism's observable traits.

The Monohybrid Cross — Visual Explanation

A monohybrid cross tracks the inheritance of a single gene with two alleles. The Punnett square below shows the classic cross between two heterozygous parents (Bb × Bb) — for example, two brown-eyed parents who each carry a recessive blue-eye allele.

Notice that two of the four boxes produce the genotype Bb, which is why heterozygous offspring appear most frequently. The Punnett square is simply a visual way to apply the law of segregation: each parent contributes exactly one allele per gamete, and every combination of maternal and paternal gametes is equally likely. This makes the Punnett square a powerful prediction tool — you can read off both the genotype and phenotype ratios directly from the grid.

Probability & Predicting Offspring Ratios

Inheritance is fundamentally about probability — each fertilisation event is an independent chance occurrence, much like flipping a coin. Because each parent has two alleles and meiosis randomly separates them, the probability of passing on a specific allele to any one offspring is ½ (or 50%). You can combine these individual probabilities to predict the likelihood of any genotype.

Let's apply these rules to the cross Bb × Bb. The probability that an offspring receives B from the mother is ½ and B from the father is ½. Using the product rule, P(BB) = ½ × ½ = ¼. Similarly, P(bb) = ½ × ½ = ¼. For heterozygous Bb, there are two routes: B from mum and b from dad (½ × ½ = ¼), OR b from mum and B from dad (½ × ½ = ¼). Using the sum rule, P(Bb) = ¼ + ¼ = ½. So the genotype ratio is ¼ BB : ½ Bb : ¼ bb, which simplifies to 1 : 2 : 1.

Types of Genetic Crosses

Beyond the standard monohybrid cross, the IB Biology syllabus expects you to work with several variations. Each type follows the same underlying logic — applying segregation and probability — but introduces new layers of complexity. The diagram below compares the key cross types and their expected ratios.

The test cross is especially useful. If an organism shows a dominant phenotype, you cannot tell from its appearance alone whether it is homozygous dominant (BB) or heterozygous (Bb). By crossing it with a homozygous recessive individual (bb), you can deduce the unknown genotype from the offspring ratios. If any offspring show the recessive phenotype, the unknown parent must be heterozygous.

In a dihybrid cross, you track two independently assorting genes simultaneously. This requires Mendel's law of independent assortment: alleles of different genes are distributed to gametes independently of each other (provided the genes are on different chromosomes). A cross between two doubly heterozygous parents (BbRr × BbRr) uses a 4 × 4 Punnett square with 16 boxes, yielding the classic 9 : 3 : 3 : 1 phenotype ratio.

Worked Example — Predicting Offspring from a Genetic Cross

In guinea pigs, black fur (B) is dominant over white fur (b). A heterozygous black guinea pig is crossed with a white guinea pig. Determine the expected genotype and phenotype ratios of the offspring.

Strengths & Limitations of Mendelian Predictions

Mendelian inheritance provides a powerful framework for predicting offspring traits, but real-world genetics is often more complex. Understanding both the strengths and limitations of simple genetic crosses helps you interpret exam questions accurately and appreciate the richness of biological inheritance.

Connection to Advanced Genetics

Mendelian inheritance is the starting point, but modern genetics extends far beyond simple dominant-recessive patterns. As you progress in biology, you will encounter mechanisms that modify or expand upon Mendel's original framework. The table below contrasts what you learn now with concepts you will explore at higher levels.

For the IB exam, you should be comfortable with Mendelian crosses, codominance, sex linkage, and ABO blood groups. Understanding pedigree analysis — interpreting family trees to deduce inheritance patterns — is also essential. The principles you have learned here form the toolkit that makes all of these analyses possible. As you move into HL topics or university biology, the same probability rules and Punnett square logic extend naturally to more complex inheritance patterns.

Practice Problems

Lesson Summary