# Predicting 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

## 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.

## 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 |

# Genotype 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 |

Vocabulary: - Homozygous = Having TWO of the SAME allele (homo = same) - Heterozygous = Having TWO DIFFERENT alleles (hetero = different)

# Punnett 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)

## 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

## 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)

# Summary

## 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!