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Understand how traits pass from parents to offspring using Punnett squares and inheritance rules.
For most of human history, people noticed that children tend to resemble their parents, but no one could explain why. Farmers selectively bred plants and animals for thousands of years, yet the underlying rules remained a mystery. It took a quiet monk working in a garden to crack the code. The story of genetics is ultimately the story of discovering the hidden instructions that make each living thing unique.
These breakthroughs raised the central question you will master in this lesson: how can we predict which traits offspring will inherit from their parents? On the GED Science test, you will encounter passages and data tables about crosses, family pedigrees, and genetic probabilities. Building a clear mental model of heredity now will make those questions much more manageable.
Before you can interpret inheritance patterns, you need a working vocabulary. Genetics has its own set of key terms, and understanding them is the foundation for everything else in this lesson. Let's break down the essential ideas.
The Punnett square is the single most important tool for predicting the outcome of a genetic cross. Named after geneticist Reginald Punnett, it is a simple grid that shows every possible combination of alleles that offspring can inherit. On the GED, you may be given a Punnett square in a stimulus passage or asked to interpret one. The diagram below shows a cross between two heterozygous parents (Bb × Bb), where B = dominant brown fur and b = recessive white fur.
Notice that each cell in the grid has an equal probability — 25%, or one in four. This is because each parent has an equal chance of passing either allele to the offspring. The genotype ratio from this cross is 1 BB : 2 Bb : 1 bb. But because BB and Bb both show the dominant phenotype (brown fur), the phenotype ratio is 3 brown : 1 white. This famous 3:1 ratio is exactly what Mendel observed in his pea plant experiments over 150 years ago.
Genetics is fundamentally about probability — the chance that a certain combination of alleles will occur. The GED may ask you to calculate the probability of a specific genotype or phenotype appearing in offspring. The math is straightforward once you understand the basics.
Not every trait follows the simple dominant-recessive pattern Mendel described. The GED may present scenarios involving other types of inheritance. Understanding these variations will help you correctly interpret a wider range of genetics passages and data.
| Pattern | Key Feature | Example |
|---|---|---|
| Complete Dominance | Dominant allele fully masks recessive | Mendel's pea pod shape (round vs. wrinkled) |
| Incomplete Dominance | Heterozygote shows a blended phenotype | Red × White snapdragons → Pink flowers |
| Codominance | Both alleles fully expressed simultaneously | AB blood type (both A and B antigens present) |
| Sex-Linked | Gene is on the X chromosome; males more often affected | Color blindness, hemophilia |
| Multiple Alleles | More than two allele versions exist in the population | ABO blood type (three alleles: Iᴬ, Iᴮ, i) |
Let's walk through a typical GED-style genetics problem step by step. This is the kind of question where a stimulus passage gives you information and asks you to analyze the data.
A pedigree is a family tree diagram used by geneticists to track how a trait passes through generations. The GED frequently includes pedigree diagrams in stimulus materials. Squares represent males, circles represent females, filled-in shapes indicate individuals who show the trait, and half-filled shapes sometimes indicate carriers. Horizontal lines connect mating pairs, and vertical lines lead to offspring.
| Pedigree Clue | What It Tells You |
|---|---|
| Trait skips a generation | Likely autosomal recessive — carriers (heterozygotes) do not show the trait |
| Trait appears in every generation | Likely autosomal dominant — only one dominant allele needed to express the trait |
| More males affected than females | Likely X-linked (sex-linked) recessive — males have only one X, so one recessive allele is enough |
| Affected father, unaffected daughters, affected grandsons | Classic X-linked recessive pattern — father passes X to daughters (carriers), who pass it to some sons |
| Two unaffected parents have an affected child | Both parents must be carriers (heterozygous) for an autosomal recessive trait |
Mendel's laws work beautifully for single-gene traits, but most real-world traits are more complex. The GED may briefly reference these more advanced concepts, and having a basic awareness of them can help you eliminate wrong answer choices.
| Concept | Mendelian (Simple) | Non-Mendelian (Complex) |
|---|---|---|
| Number of genes | One gene controls one trait | Multiple genes influence one trait (polygenic inheritance) |
| Phenotype categories | Distinct categories (tall or short) | Continuous range (human height, skin color) |
| Environment | Minimal environmental influence | Environment significantly affects expression (nutrition affects height) |
| Allele behavior | One dominant, one recessive | Incomplete dominance, codominance, epistasis |
| Example | Pea plant color (green vs. yellow) | Human eye color (influenced by many genes) |
The key takeaway for the GED is this: when a passage describes traits with a continuous range (like height or skin color), that's a signal that polygenic inheritance is involved — multiple genes working together. When the trait falls into clear categories (round vs. wrinkled seeds), Mendel's simple model applies well. You may also encounter questions about mutations — changes in DNA that create new alleles — and their role in creating variation within a population. Mutations can be harmful, neutral, or occasionally beneficial, and they are the ultimate source of genetic diversity.
Genetics is the study of how traits are passed from parents to offspring through genes, which come in different versions called alleles. Each individual inherits two alleles per gene — one from each parent. A dominant allele masks a recessive allele, so the recessive trait only appears when both alleles are recessive (homozygous recessive). The Punnett square is your go-to tool for predicting offspring genotype and phenotype ratios. A cross between two heterozygous parents (Bb × Bb) produces the classic 3:1 phenotype ratio.
Beyond simple dominance, you should recognize incomplete dominance (blended phenotype, 1:2:1 ratio), codominance (both alleles fully expressed, as in AB blood type), and sex-linked inheritance (genes on the X chromosome that affect males more often). Pedigrees are family tree diagrams that help you trace inheritance patterns across generations. When reading GED stimuli about genetics, always identify the allele types, parent genotypes, and inheritance pattern first — then use the Punnett square to find your answer.