IB Biology: Genetics and Inheritance

Genetics forms the cornerstone of modern biology, explaining how traits are passed from one generation to the next and driving the diversity of life. For your IB Biology studies, mastering this unit is non-negotiable; it integrates core principles with analytical skills that are frequently assessed in Papers 2 and 3. A firm grasp of inheritance patterns enables you to understand everything from genetic diseases to evolutionary mechanisms.

From Mendel's Peas to Foundational Laws

The science of genetics began with Gregor Mendel's meticulous experiments on pea plants. His work established two fundamental principles that still underpin the field. The law of segregation states that an organism possesses two alleles for each gene, one inherited from each parent, and these alleles separate (segregate) during gamete formation. This means each sperm or egg carries only one allele for a given trait. The law of independent assortment adds that alleles for different genes segregate independently of one another during gamete formation, provided the genes are located on different chromosomes. This principle explains the tremendous genetic variation in sexually reproducing populations. Think of it like shuffling two separate decks of cards; the way one deck is shuffled doesn't influence the other.

These laws apply to genes located on autosomes, or non-sex chromosomes. Mendel's work focused on simple dominant and recessive alleles, where the dominant allele masks the expression of the recessive allele in a heterozygous individual. For example, in pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). A plant with the genotype Tt will be tall because the dominant T allele is expressed.

Constructing and Interpreting Punnett Squares

A Punnett square is a visual tool used to predict the genotypic and phenotypic outcomes of a genetic cross. It applies Mendel's laws to calculate probabilities. Let's walk through a monohybrid cross, which examines the inheritance of one gene.

Suppose we cross two pea plants that are heterozygous for height (Tt x Tt). First, determine the possible gametes each parent can produce. Due to segregation, each parent can produce T or t gametes. You then place these on the axes of a 2x2 grid.

The interior boxes show the possible genotypes of the offspring: TT, Tt, Tt, and tt. This gives a genotypic ratio of 1 TT : 2 Tt : 1 tt. Since T is dominant, the phenotypic ratio is 3 tall : 1 short. For a dihybrid cross (two genes), such as TtYy x TtYy (where Y=yellow seed, y=green seed), you apply independent assortment. Each parent can produce four gamete types (TY, Ty, tY, ty), leading to a 16-square Punnett square and a classic 9:3:3:1 phenotypic ratio.

Beyond Simple Dominance: Complex Inheritance Patterns

Not all traits follow strict Mendelian dominance. Codominance occurs when both alleles in a heterozygous individual are fully and separately expressed. A classic example is the ABO blood group system in humans, where the $I^A$ and $I^B$ alleles are codominant; an individual with genotype $I^A{I}^B$ has blood type AB, expressing both A and B antigens.

Sex-linked inheritance involves genes located on the sex chromosomes, typically the X chromosome. Males (XY) have only one X chromosome, so they express all alleles on it, even recessive ones. Females (XX) have two copies and can be carriers. Disorders like hemophilia and red-green color blindness are X-linked recessive. For a cross between a carrier female ($X^H{X}^h$) and a normal male ($X^{H}Y$), the Punnett square shows that sons have a 50% chance of being affected ($X^{h}Y$), while daughters have a 50% chance of being carriers.

In contrast, polygenic traits are controlled by two or more genes, often located on different chromosomes, and exhibit a continuous range of variation. Examples include human height, skin pigmentation, and wheat kernel color. These traits typically show a bell-shaped distribution in a population rather than discrete categories, because the combined effect of multiple genes leads to many possible phenotypes.

Pedigree Analysis and Examination Strategy

A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes across generations in a family. It is a critical tool for deducing inheritance patterns, especially for human genetic disorders. In an IB exam, you will be asked to analyze pedigrees to determine if a trait is autosomal dominant, autosomal recessive, or X-linked.

Key clues: Autosomal dominant traits appear in every generation, and affected individuals usually have at least one affected parent. Autosomal recessive traits can skip generations and often appear in offspring of unaffected carriers. X-linked recessive traits are more common in males and are not passed from father to son (since sons inherit the Y chromosome from the father). Practice by tracing the inheritance of shaded symbols representing affected individuals, working systematically to deduce genotypes.

For genetic cross calculations, always show your working. State the parental genotypes, possible gametes, construct the Punnett square, and then state the ratios or probabilities explicitly. Use notations like $P$ for probability; for example, the probability of an offspring being homozygous recessive in a Tt x Tt cross is $P=1/4$.

Common Pitfalls

1. Confusing Genotype and Phenotype: The genotype is the genetic makeup (e.g., Tt), while the phenotype is the observable trait (e.g., tall). Always define these terms clearly in your answers. A common error is stating a phenotypic ratio when the question asks for genotypic.

1. Misapplying Mendel's Laws: Independent assortment only applies to genes on different chromosomes. Genes located close together on the same chromosome are linked and do not assort independently, which can alter expected ratios. If a question mentions linkage, adjust your predictions accordingly.

1. Overlooking Sex-Linked Inheritance: When solving crosses for X-linked traits, students often forget that males pass their X chromosome only to daughters and their Y chromosome to sons. This leads to incorrect gamete determination in Punnett squares. Always denote sex chromosomes explicitly (e.g., $X^{H}Y$).

1. Incorrect Pedigree Interpretation: Assuming a trait is dominant because it appears frequently can be misleading. Look for patterns like skipped generations and gender bias. For instance, if only males are affected and it skips generations, it strongly suggests X-linked recessive inheritance, not autosomal dominant.

Summary

• Mendel's laws of segregation and independent assortment provide the foundation for predicting how alleles are distributed to gametes and offspring.
• Punnett squares are essential tools for visualizing genetic crosses and calculating genotypic and phenotypic probabilities, from simple monohybrid to complex dihybrid crosses.
• Inheritance patterns extend beyond simple dominance to include codominance (both alleles expressed), sex-linked inheritance (genes on X chromosome), and polygenic traits (multiple genes contributing).
• Pedigree chart analysis requires systematic deduction to identify patterns indicative of autosomal or sex-linked inheritance, a key skill for IB Paper 2 structured questions.
• Always practice genetic cross calculations with step-by-step working, clearly stating parental genotypes, gametes, and resulting ratios to secure full examination marks.