Predicting Traits
Dominant, Recessive, and Punnett Squares
Learning Objectives
Define and distinguish between alleles, genotypes, and phenotypes
Explain the difference between dominant and recessive alleles
Use Punnett squares to predict offspring trait probabilities
Predicting Traits
~5 minutesPredicting Traits
In Lesson 1, you learned that traits are passed from parents to offspring through genes. But can we actually PREDICT what traits offspring might have before they're born?
The answer is yes—using a simple tool developed over 100 years ago by a scientist named Reginald Punnett. In this lesson, you'll learn to use Punnett squares to calculate the probability of offspring inheriting specific traits.
This same technique is used today by:
• Genetic counselors helping families understand inherited conditions
• Animal breeders selecting for specific traits
• Plant scientists developing new crop varieties
• Medical researchers studying inherited diseases
Essential Question: How can we predict the traits of offspring?
The Language of Genetics
~15 minutesThe Language of Genetics
Alleles: Different Versions of a Gene
Remember that you have two copies of each gene—one from each parent. These copies might be identical, or they might be slightly different. These different versions of a gene are called alleles.
For example, the gene for earlobe attachment has two alleles:
• One allele codes for detached (free-hanging) earlobes
• Another allele codes for attached earlobes
You inherited one allele from your mother and one from your father. The combination you received determines your earlobe type.
An allele is a specific version or form of a gene. Different alleles can produce different expressions of the same trait. For example, the gene for eye color has alleles for brown, blue, green, and other colors. Think of alleles as different 'flavors' of the same gene.
Dominant and Recessive Alleles
When you have two different alleles for the same gene, one allele often 'masks' or hides the other. The allele that shows up in the trait is called dominant. The allele that gets hidden is called recessive.
The Rules:
• If you have at least ONE dominant allele, the dominant trait will show
• The recessive trait ONLY shows when you have TWO recessive alleles
• Having one of each results in the dominant trait appearing
Notation System:
• Dominant alleles = CAPITAL letters (B, T, R)
• Recessive alleles = lowercase letters (b, t, r)
Example: Tongue Rolling
| Genotype | Alleles | Can Roll Tongue? |
|---|---|---|
| RR | Two dominant | Yes |
| Rr | One dominant, one recessive | Yes |
| rr | Two recessive | No |
Dominant alleles (capital letter) only need ONE copy to show the trait.
Recessive alleles (lowercase letter) need TWO copies to show the trait.
Important: Dominant does NOT mean 'better' or 'more common'—it simply means the allele masks the recessive allele when both are present.
Genotype vs. Phenotype
~10 minutesGenotype vs. Phenotype
Two More Essential Terms
Genotype is the actual genetic makeup—the specific alleles an organism has.
• Written as letter combinations like BB, Bb, or bb
• This is the 'behind the scenes' genetic code
Phenotype is the observable trait—what you can see or measure.
• Examples: 'brown eyes,' 'can roll tongue,' 'has dimples'
• This is the visible result of the genotype
The Three Possible Genotypes
| Genotype | Technical Name | Meaning | Phenotype |
|---|---|---|---|
| BB | Homozygous dominant | Two same alleles (both dominant) | Dominant trait shows |
| Bb | Heterozygous | Two different alleles | Dominant trait shows |
| bb | Homozygous recessive | Two same alleles (both recessive) | Recessive trait shows |
• Homozygous = Having TWO of the SAME allele (homo = same)
• Heterozygous = Having TWO DIFFERENT alleles (hetero = different)
Two FlufflePets both have brown fur (same phenotype), but:
• FlufflePet A has genotype BB
• FlufflePet B has genotype Bb
They LOOK identical, but their genetic makeup is different. This matters when they have offspring!
Punnett Squares: The Prediction Tool
~15 minutesPunnett Squares: The Prediction Tool
Meet the FlufflePets
FlufflePets are imaginary creatures perfect for learning genetics! In FlufflePets, fur color is controlled by a single gene:
• B = Brown fur (dominant)
• b = White fur (recessive)
Building a Punnett Square
A Punnett square is a grid that shows all possible offspring genotypes when two parents reproduce. Here's how to use one:
Step 1: Identify the parents' genotypes
• Parent 1: Bb (brown, heterozygous)
• Parent 2: Bb (brown, heterozygous)
Step 2: Write one parent's alleles across the top
Step 3: Write the other parent's alleles down the side
Step 4: Fill in each box by combining the row and column alleles
` Parent 1 B b ┌───────┬───────┐ B │ BB │ Bb │ P2 ├───────┼───────┤ b │ Bb │ bb │ └───────┴───────┘ `
Step 5: Count the results
• BB = 1 box (25%) → Brown fur
• Bb = 2 boxes (50%) → Brown fur
• bb = 1 box (25%) → White fur
Phenotype ratio: 3 brown : 1 white (75% brown, 25% white)
Punnett squares show probability, not certainty!
If a cross predicts 75% brown offspring, it doesn't mean exactly 3 out of 4 babies will be brown. Each offspring has a 75% CHANCE of being brown, determined independently—like flipping a coin for each one.
Think of it this way: if you flip a coin 4 times, you might not get exactly 2 heads. But over many flips, you'll get close to 50%.
Practice Problems
Cross #1: BB × bb
Parent 1: BB (Brown, homozygous dominant) Parent 2: bb (White, homozygous recessive)` B B ┌───────┬───────┐ b │ Bb │ Bb │ ├───────┼───────┤ b │ Bb │ Bb │ └───────┴───────┘ `
Results: 100% Bb (all heterozygous) Phenotype: 100% brown fur
Cross #2: Bb × bb
Parent 1: Bb (Brown, heterozygous) Parent 2: bb (White, homozygous recessive)` B b ┌───────┬───────┐ b │ Bb │ bb │ ├───────┼───────┤ b │ Bb │ bb │ └───────┴───────┘ `
Results: 50% Bb, 50% bb Phenotype: 50% brown, 50% white
Human Trait Examples
~10 minutesHuman Trait Examples
Simple Dominant/Recessive Traits in Humans
Many human traits follow simple dominant/recessive patterns:
| Trait | Dominant Allele | Recessive Allele |
|---|---|---|
| Widow's peak | W = Present | w = Absent (straight hairline) |
| Detached earlobes | E = Detached | e = Attached |
| Dimples | D = Present | d = Absent |
| Tongue rolling | R = Can roll | r = Cannot roll |
| Freckles | F = Freckles | f = No freckles |
Real Example: Widow's Peak
A father with a widow's peak (Ww) and a mother without one (ww) have children.
` W w ┌───────┬───────┐ w │ Ww │ ww │ ├───────┼───────┤ w │ Ww │ ww │ └───────┴───────┘ `
Prediction: 50% chance of widow's peak (Ww), 50% chance of straight hairline (ww)
Not all traits follow simple dominant/recessive patterns!
Many traits (like height, skin color, and eye color) are controlled by multiple genes, making predictions more complex. The traits in this lesson are simplified examples where one gene controls one trait.
Also, environment can affect how genes are expressed.
Summary
~5 minutesSummary
Key Concepts Review
Alleles are different versions of the same gene.
Dominant alleles (capital letters) mask recessive alleles when both are present.
Recessive alleles (lowercase letters) only show when two copies are present.
Genotype is the genetic makeup (BB, Bb, or bb):
• Homozygous = two same alleles (BB or bb)
• Heterozygous = two different alleles (Bb)
Phenotype is the observable trait.
Punnett Square Steps: 1. Write parent genotypes 2. Set up the grid with alleles 3. Fill in each box by combining alleles 4. Count genotypes and calculate percentages
Remember: Punnett squares show probability, not certainty. Each offspring is an independent event!
Check Your Understanding
What is an allele?
A FlufflePet has the genotype Bb for fur color. This genotype is called:
Two FlufflePets have the same phenotype (brown fur) but different genotypes (BB and Bb). Why do they look the same?
For a recessive trait to appear in an organism's phenotype, the organism must:
If 'D' represents the dominant allele for dimples and 'd' represents the recessive allele for no dimples, which genotypes would show dimples?
A Punnett square cross between two Bb FlufflePets produces what genotype ratio?
What is the phenotype ratio when two Bb FlufflePets are crossed? (B = brown, b = white)
A BB FlufflePet is crossed with a bb FlufflePet. What percentage of offspring will be brown?
A mother cannot roll her tongue (rr) and a father can roll his tongue (Rr). What is the probability their child will be able to roll their tongue?
Two parents both have dimples (Dd × Dd). They have 4 children. Based on the Punnett square:
Match each term with its correct definition:
Match each genotype with the correct phenotype for FlufflePet fur color (B = brown, dominant; b = white, recessive):
Match each Punnett square cross with its phenotype ratio for FlufflePets (B = brown, b = white):
Which of the following are TRUE about Punnett squares? (Select ALL that apply)
Select all that apply.