Understand Inheritance

For centuries, people noticed that children tend to resemble their parents—eye color, hair texture, and even certain diseases seemed to run in families. Yet no one could explain how these traits were transmitted. Early ideas ranged from "blending inheritance," which proposed that parental traits mixed like paint, to the belief that traits could be acquired during a parent's lifetime and passed on. The scientific study of inheritance—the biological process by which traits are transmitted from parents to offspring—began in earnest in a monastery garden in the 1860s, and it has since transformed medicine, agriculture, and our understanding of life itself.

The central question that drove these discoveries remains at the heart of this lesson: How do parents pass specific traits to their offspring, and why do some traits appear to skip generations? To answer this, we need to explore the rules Mendel uncovered and the biological mechanisms that make inheritance possible.

Inheritance operates through a set of core principles that Mendel first described and that modern biology has expanded. Before diving in, it helps to define a few essential terms. A gene is a segment of DNA that codes for a particular protein or trait. Different versions of the same gene are called alleles. Because most organisms inherit one set of chromosomes from each parent, they carry two alleles for each gene—one from mom and one from dad. The combination of alleles an organism carries is its genotype, while the observable characteristic that results is the phenotype.

The most powerful tool for predicting inheritance outcomes is the Punnett square, a grid that maps every possible combination of parental alleles. The diagram below shows a monohybrid cross—a cross examining a single gene—between two heterozygous parents (Aa × Aa). Each parent contributes either an A or an a allele to each offspring, giving four equally likely combinations.

Notice that three out of four offspring show the dominant phenotype, but their genotypes differ: one is homozygous dominant (AA) and two are heterozygous (Aa). Only one out of four is homozygous recessive (aa) and displays the recessive phenotype. This is the classic 3:1 phenotypic ratio and 1:2:1 genotypic ratio that Mendel first observed in his pea plants.

Mendel didn't know about chromosomes or DNA, but today we understand the physical basis for his laws. Genes reside on chromosomes—long strands of DNA found in the nucleus of every cell. Humans have 23 pairs of chromosomes (46 total), with one set of 23 inherited from each parent. During meiosis, the type of cell division that produces gametes (sperm and egg cells), homologous chromosome pairs separate so that each gamete receives just one chromosome from each pair. This physical separation is what Mendel's Law of Segregation describes at the molecular level.

Predicting Probabilities

While Punnett squares help visualize outcomes, we can also express inheritance predictions mathematically. Each gamete has an equal probability of carrying either allele, so the probability of any specific genotype in the offspring can be calculated using the multiplication rule of probability.

Mendel's peas showed neat dominant-recessive patterns, but inheritance in the real world is often more nuanced. Several important patterns extend beyond simple Mendelian dominance, and the IB syllabus expects you to recognize and apply them.

The key distinction between incomplete dominance and codominance often confuses students. In incomplete dominance, the heterozygote's phenotype is an intermediate blend—like mixing red and white flowers to produce pink ones. In codominance, both alleles are fully expressed side by side—like a flower with distinct red and white patches, or human blood type AB where both A and B surface molecules are present on red blood cells.

Let's work through a dihybrid cross—a cross involving two genes simultaneously. In guinea pigs, black coat colour (B) is dominant over brown (b), and short hair (S) is dominant over long hair (s). Both parents are heterozygous for both traits (BbSs). What are the expected phenotypic ratios among their offspring?

A common challenge in genetics is determining the genotype of an organism that shows the dominant phenotype—is it homozygous dominant (AA) or heterozygous (Aa)? A test cross solves this problem. You cross the unknown individual with a homozygous recessive (aa) individual. If any offspring show the recessive phenotype, the unknown parent must be heterozygous.

Pedigree Analysis

In humans, we cannot perform deliberate crosses, so geneticists study inheritance patterns using pedigree charts—family tree diagrams that track traits across generations. In pedigrees, squares represent males, circles represent females, shaded shapes indicate individuals expressing the trait, and horizontal lines connect mating pairs. The IB exam frequently asks you to determine whether a trait is autosomal dominant, autosomal recessive, or sex-linked based on a pedigree.

• Autosomal recessive: Trait can skip generations; unaffected parents can have affected children; males and females affected equally.
• Autosomal dominant: Trait appears in every generation; an affected child must have at least one affected parent; males and females affected equally.
• X-linked recessive: More males affected than females; affected fathers cannot pass the trait to sons; carrier mothers can pass it to sons.

While Mendel's laws form the foundation of genetics, modern biology has revealed that inheritance is far more complex than simple dominant-recessive patterns. Understanding these extensions helps you see where IB Biology connects to HL topics and university-level genetics.

At the HL level and beyond, you will explore gene linkage and recombination, where crossing over during meiosis can create new allele combinations on a chromosome. You'll also encounter epigenetics—heritable changes in gene expression that do not involve changes in the DNA sequence itself. These topics build directly on the Mendelian foundation you've learned in this lesson, so mastering these core concepts now will set you up for success.