Interpret genetics and heredity patterns. - GED Science

SECTION 1

Historical Context & Motivation

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.

1866 • Mendel's Experiments

Gregor Mendel published results from years of crossing pea plants, identifying patterns of dominant and recessive traits. His work was largely ignored during his lifetime.

1900 • Mendel Rediscovered

Three European scientists independently rediscovered Mendel's paper and confirmed his findings, launching the modern field of genetics.

1953 • DNA Structure Revealed

Watson and Crick described the double-helix structure of DNA, providing the physical basis for how genetic information is stored and copied.

2003 • Human Genome Project Completed

Scientists mapped the entire human genome — roughly 20,000–25,000 genes — opening the door to personalized medicine and deeper understanding of heredity.

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.

SECTION 2

Core Principles of Genetics

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.

Genes & Alleles

A gene is a segment of DNA that codes for a specific trait (like eye color). Each gene can come in different versions called alleles. You inherit one allele from each parent, giving you two alleles per gene.

Dominant & Recessive

A dominant allele (written as a capital letter, like B) masks the effect of a recessive allele (lowercase, like b). You only see the recessive trait when both alleles are recessive (bb).

Genotype vs. Phenotype

Genotype is the pair of alleles an organism carries (BB, Bb, or bb). Phenotype is the observable trait that results (brown eyes vs. blue eyes). Two organisms can look the same but have different genotypes.

Homozygous & Heterozygous

Homozygous means both alleles are the same (BB or bb). Heterozygous means the two alleles are different (Bb). A heterozygous individual is sometimes called a "carrier" of the recessive allele.

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KEY TAKEAWAY

Think of alleles like a pair of playing cards dealt to you at birth — one from Mom's deck, one from Dad's deck. If one card is a "king" (dominant) and the other is a "two" (recessive), the king wins and determines what shows on the outside (your phenotype). Only when you get two "twos" does the recessive trait actually appear.

SECTION 3

The Punnett Square — A Visual Tool

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.

This Punnett square shows a cross between two heterozygous parents (Bb × Bb). The top row shows alleles from Parent 2; the left column shows alleles from Parent 1. Each inner cell represents one possible offspring genotype. The classic 3:1 phenotype ratio emerges: three out of four offspring show the dominant trait.

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.

SECTION 4

Probability in Genetics

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.

PROBABILITY OF A GENOTYPE

Probability = (Number of favorable outcomes) ÷ (Total possible outcomes)

In a Punnett square with 4 cells, if 1 cell shows the genotype bb, then the probability of bb = 1 ÷ 4 = 0.25 or 25%.

MULTIPLICATION RULE (AND)

P(A and B) = P(A) × P(B)

When two independent events must both happen, multiply their individual probabilities. For example, the chance a child inherits allele b from the father AND allele b from the mother: ½ × ½ = ¼ = 25%.

ADDITION RULE (OR)

P(A or B) = P(A) + P(B)

When either outcome satisfies the question, add probabilities. The chance of being heterozygous (Bb): there are 2 Bb cells out of 4, so P = 2/4 = 50%. This is the same as adding the probability of getting Bb from cell 1 (25%) plus Bb from cell 2 (25%).

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GED TIP

On the GED, probability questions often ask you to express answers as fractions, percentages, or ratios. Practice converting between them: 1/4 = 25% = 0.25. When a question says "what is the chance," it is asking for probability.

SECTION 5

Types of Inheritance Patterns

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.

This diagram compares five inheritance patterns. In complete dominance, heterozygotes look like the dominant parent. In incomplete dominance, the heterozygote shows a blended phenotype. In codominance, both alleles are fully visible (as in AB blood type). Sex-linked traits are carried on the X chromosome, and multiple alleles means more than two versions of a gene exist in a population.

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)

Common Inheritance Patterns Tested on the GED

SECTION 6

Worked Example — Predicting Offspring

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.

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STIMULUS PASSAGE

In a certain plant species, tall stems (T) are dominant over short stems (t). A researcher crosses a heterozygous tall plant (Tt) with a short plant (tt). She observes 200 offspring. Question: What percentage of the offspring would you expect to be short?

Step-by-Step Solution

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Step 1 — Identify Parent Genotypes

The problem states that Parent 1 is heterozygous tall (Tt) and Parent 2 is short (tt). Since short is recessive, the only way to show the short phenotype is to be homozygous recessive (tt).
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Step 2 — Set Up the Punnett Square

Place Parent 1's alleles (T and t) across the top. Place Parent 2's alleles (t and t) along the side. Fill in each cell by combining the column allele with the row allele.
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Step 3 — Fill In the Grid

The four resulting cells are: Tt, Tt, tt, tt. That gives us 2 Tt (tall) and 2 tt (short).

Genotype ratio: 2 Tt : 2 tt (or 1:1)
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Step 4 — Determine the Phenotype Ratio

Tt = tall (dominant phenotype). tt = short (recessive phenotype). So the phenotype ratio is 1 tall : 1 short, or 50% tall and 50% short.

50% of the offspring are expected to be short.
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Step 5 — Apply to the Sample Size

With 200 offspring, we expect 50% × 200 = 100 short plants. Remember that this is a predicted (expected) number — actual results may vary slightly due to chance, just like flipping a coin doesn't always land on heads exactly 50 times out of 100.

Expected: ~100 short plants out of 200

SECTION 7

Reading Pedigrees & Common Patterns

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

Common Pedigree Clues and Their Meanings

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KEY TAKEAWAY

Think of a pedigree like a detective's evidence board. Each generation gives you clues. If two "normal" parents have a child with the trait, that's strong evidence the trait is recessive — because both parents were secretly carrying one copy. If the trait shows up mostly in males, it's probably riding on the X chromosome. Follow the evidence generation by generation.

SECTION 8

Beyond Simple Inheritance

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)

Simple vs. Complex Inheritance

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.

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GED TEST STRATEGY

When you see a GED question about genetics, always start by identifying (1) whether the trait is dominant or recessive, (2) the parents' genotypes, and (3) whether the inheritance pattern is simple or complex. These three steps will help you narrow down the correct answer quickly.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A scientist studying pea plants observes that when she crosses two plants that both have purple flowers, some of the offspring have white flowers. Purple flowers (P) are dominant over white flowers (p). What are the most likely genotypes of the two parent plants?

A. PP × PP
B. PP × Pp
C. Pp × Pp
D. pp × pp

Show answer (C)

The correct answer is C. Both parents must be Pp (heterozygous). Why C is correct: Since white flowers (pp) appeared in the offspring, each parent must have contributed a recessive p allele. Both parents show purple flowers, so they must also carry a dominant P allele. The only genotype that is purple AND carries a recessive allele is Pp. Why A is wrong: PP × PP would produce only PP offspring — all purple, no white flowers possible. Why B is wrong: PP × Pp could produce PP or Pp offspring, but never pp. No white flowers possible. Why D is wrong: pp × pp would produce only white flowers. Both parents show purple, so they cannot be pp.

Question 2: BASIC CALCULATION

In guinea pigs, black fur (B) is dominant over brown fur (b). A heterozygous black guinea pig (Bb) is crossed with a brown guinea pig (bb). What is the probability that an offspring from this cross will have brown fur?

A. 25%
B. 50%
C. 75%
D. 100%

Show answer (B)

The correct answer is B (50% or 1/2). Why B is correct: Setting up the Punnett square for Bb × bb yields: Bb, Bb, bb, bb. Two of four cells are bb (brown fur). Probability = 2/4 = 1/2 = 50%. Why A is wrong: 25% would be the result of a Bb × Bb cross producing bb offspring, not this cross. Why C is wrong: 75% would be the probability of the dominant phenotype in a Bb × Bb cross. Why D is wrong: 100% brown would only occur if both parents were bb.

Question 3: INTERMEDIATE

In a family pedigree, two parents who do not have sickle cell disease have a child who does have it. Sickle cell disease is caused by a recessive allele (s), while the normal allele (S) is dominant. Based on this information, which of the following best describes the parents' genotypes and the probability that their next child will also have sickle cell disease?

A. One parent is SS and one is Ss; probability is 0%
B. Both parents are Ss; probability is 50%
C. Both parents are Ss; probability is 25%
D. Both parents are Ss; probability is 100% because they already had one affected child

Show answer (C)

The correct answer is C. Both parents are Ss; probability of next child having sickle cell disease is 25%. Why C is correct: Since both parents are unaffected but produced a child with ss genotype, both must be carriers (Ss). A Punnett square for Ss × Ss produces: SS, Ss, Ss, ss. Only 1 out of 4 (25%) is ss (affected). Why A is wrong: If one parent were SS, no affected child could be produced because every child would get at least one S allele. Why B is wrong: The genotypes Ss are correct, but the probability is 25%, not 50%. A 50% probability would apply to a Ss × ss cross. Why D is wrong: Each new conception is an independent event. Having already had one affected child does not change the 25% probability for the next child.

Question 4: APPLIED

2 pts

A researcher crosses red snapdragon flowers (RR) with white snapdragon flowers (rr). All of the F₁ (first generation) offspring have pink flowers. The researcher then crosses two F₁ pink plants with each other. In 3–5 sentences, explain what type of inheritance pattern this represents, predict the phenotype ratio of the F₂ (second generation) offspring, and describe how this result differs from what you would expect under complete dominance.

Show answer

RUBRIC (2 points): Point 1 (1 pt): Correctly identifies the inheritance pattern as incomplete dominance and predicts the F₂ phenotype ratio of 1 red : 2 pink : 1 white. Must state that the heterozygote (Rr) shows a blended phenotype (pink) rather than one allele masking the other. Point 2 (1 pt): Correctly contrasts incomplete dominance with complete dominance. Under complete dominance, the F₁ generation would all look like the dominant parent (e.g., all red), and the F₂ would show a 3:1 phenotype ratio instead of the 1:2:1 ratio seen here. The key distinction is that under incomplete dominance, the heterozygote has its own distinct phenotype. Sample response: This is an example of incomplete dominance, because the heterozygous F₁ offspring show a phenotype (pink) that is intermediate between the two parents. When two pink (Rr) plants are crossed, the Punnett square yields 1 RR (red) : 2 Rr (pink) : 1 rr (white), giving a 1:2:1 phenotype ratio. Under complete dominance, the F₁ plants would all appear red (the dominant phenotype), and the F₂ would show a 3:1 ratio of red to white instead. The difference is that in incomplete dominance, heterozygous individuals have a distinct blended appearance.

Question 5: CRITICAL THINKING

3 pts

A biology class conducted an experiment on fruit fly (Drosophila) wing shape. Normal wings (N) are dominant over vestigial (short) wings (n). They crossed two heterozygous flies (Nn × Nn) and recorded the following data from 400 offspring: • Normal wings: 312 • Vestigial wings: 88 The expected Mendelian ratio for this cross is 3:1. Using the data, complete the following: (a) Calculate the expected number of normal-winged and vestigial-winged offspring out of 400. (b) Compare the observed results to the expected results. (c) Propose one scientific reason why the observed results might differ from the expected ratio. (d) Evaluate whether this experiment provides strong evidence for Mendelian inheritance of wing shape.

Show answer

RUBRIC (3 points): Point 1 (1 pt): Correctly calculates expected values. Expected normal wings = 3/4 × 400 = 300. Expected vestigial wings = 1/4 × 400 = 100. Must show both calculations. Point 2 (1 pt): Accurately compares observed vs. expected. Observed normal (312) is 12 more than expected (300). Observed vestigial (88) is 12 fewer than expected (100). Must note both the direction and approximate size of the difference. Point 3 (1 pt): Provides a valid scientific explanation for the deviation AND evaluates the overall evidence. Valid reasons for deviation include: random chance (sampling variation), differential survival (some vestigial flies may not survive to counting), small sample size, or environmental factors. For evaluation, must state that the results are reasonably close to the 3:1 ratio and therefore support Mendelian inheritance, even though they are not an exact match. Sample response: (a) The expected ratio is 3:1. For 400 offspring: expected normal = 3/4 × 400 = 300; expected vestigial = 1/4 × 400 = 100. (b) The observed results (312 normal, 88 vestigial) are close to but not exactly the expected values. There are 12 more normal-winged flies and 12 fewer vestigial-winged flies than predicted. (c) One reason for this difference is random chance — probability predicts ratios over many trials, but any individual experiment can show natural variation, much like flipping a coin 100 times may not give exactly 50 heads. Another possibility is that some vestigial-winged flies had lower survival rates, reducing their observed count. (d) Despite the small deviation, the observed ratio (approximately 3.55:1) is close to the expected 3:1, providing solid evidence that wing shape in fruit flies follows Mendelian inheritance.

SUMMARY

Lesson Summary

Summary

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.