GEN 5.2 Predicting Inheritance- Pre AP Biology Study Notes - New Syllabus.
GEN 5.2 Predicting Inheritance- Pre AP Biology Study Notes
GEN 5.2 Predicting Inheritance- Pre AP Biology Study Notes – New Syllabus.
LEARNING OBJECTIVE
GEN 5.2(a) Create and/or use models to analyze the probability of the inheritance of traits.
GEN 5.2(b) Predict the inheritance of traits that do not follow Mendelian patterns.
GEN 5.2(c) Use a pedigree to predict the inheritance of a trait within a family.
Key Concepts:
GEN 5.2.1 The inheritance of certain traits from parents to offspring can be predicted using models.
a. Rules of probability can be applied to make predictions about the passage of alleles from parent to offspring using mathematical models (Punnett squares).
b. Pedigrees are useful tools for modeling inheritance patterns to examine and/or make predictions about inheritance of a specific trait from one generation to the next.
Using Models to Analyze Probability of Trait Inheritance
🌿 Introduction
When parents reproduce, they do not “choose” which alleles they pass on.
Allele inheritance follows the rules of:
- Meiosis
- Random fertilization
- Probability
Because gamete formation and fertilization are random events, inheritance can be predicted using:
Mathematical probability
Punnett square models
These tools allow us to calculate the likelihood that offspring will inherit certain traits.
🧠Why Probability Applies to Inheritance
During meiosis:
- Each parent produces gametes
- Each gamete carries one allele for each gene
- Alleles separate randomly
Example:
If a parent has genotype Aa:
- 50% of gametes carry A
- 50% carry a
This is a probability of ½ for each allele.
When two parents reproduce, their allele combinations are random.
So, inheritance is predictable using probability rules.
🎲 Basic Rules of Probability
Two key rules are used.
Multiplication Rule (AND Rule)
Used when calculating probability of two independent events occurring together.
Example:
Parent 1 passes A (½ probability)
Parent 2 passes A (½ probability)
Probability of offspring being AA:
½ × ½ = ¼
This rule applies because each parent’s allele contribution is independent.
Addition Rule (OR Rule)
Used when calculating probability of either of two possible outcomes.
Example:
Probability of Aa = ½
Probability of AA = ¼
Probability of dominant phenotype:
½ + ¼ = ¾
So dominant phenotype probability = ¾
🧬Punnett Squares as Models
Punnett squares organize possible allele combinations.
Example:
Cross Aa × Aa
Gametes:
- Parent 1: A or a
- Parent 2: A or a
Punnett square produces:
AA
Aa
Aa
aa
Genotype ratio:
1 AA : 2 Aa : 1 aa
Phenotype ratio:
3 dominant : 1 recessive
So probability of recessive phenotype:
1/4
Punnett squares visually represent probability.
🧠 Example Analysis
Example Question:
Two heterozygous parents (Aa × Aa)
What is probability of offspring being recessive (aa)?
Step 1: Identify allele probabilities
Each parent gives “a” with probability ½
Step 2: Multiply probabilities
½ × ½ = ¼
So:
25% chance of recessive phenotype.
🧬Multi-Trait Probability
If analyzing two traits at once:
Example:
AaBb × AaBb
Each trait follows independent probability.
Probability of recessive for first trait = ¼
Probability of recessive for second trait = ¼
Probability of both recessive traits:
¼ × ¼ = 1/16
Multiplication rule is applied for independent traits.
🧠 Why Models Are Important
Models allow us to:
- Predict outcomes before offspring are produced
- Estimate trait likelihood
- Understand inheritance risk
- Analyze genetic crosses
They do not guarantee exact outcomes in small families, but they predict probabilities in large populations.
📊 Summary Table
| Tool | Purpose |
|---|---|
| Punnett Square | Visualize allele combinations |
| Multiplication Rule | Calculate combined probabilities |
| Addition Rule | Calculate alternative outcomes |
| Probability Ratios | Predict phenotype likelihood |
📦 Quick Recap
Inheritance follows probability
Gamete formation is random
Multiplication rule → AND events
Addition rule → OR events
Punnett square predicts genotype ratio
3:1 → typical Mendelian phenotype ratio
Predicting Inheritance of Non-Mendelian Traits
🌿 Introduction
Mendelian inheritance works well for traits controlled by:
- One gene
- Two alleles
- One allele completely dominant
But most traits do not follow that simple pattern.
Non-Mendelian inheritance includes cases where:
- Both alleles show in the phenotype
- Heterozygotes show an intermediate phenotype
- Traits depend on sex chromosomes
- Traits involve multiple genes or environment
To predict these traits, we still use genetic models such as Punnett squares, but we change how we interpret results.
🧠 General Strategy to Predict Non-Mendelian Traits
- Identify the inheritance pattern from the description or data
- Write the allele symbols correctly
- List possible gametes
- Use a Punnett square or probability
- Convert genotypes into phenotypes using the correct rule for that pattern
🧬 Pattern 1 – Incomplete Dominance
What It Means
- Neither allele is completely dominant
- Heterozygote shows an intermediate phenotype
Phenotype mapping:
RR = one extreme phenotype
Rr = intermediate phenotype
rr = other extreme phenotype
Prediction Using Punnett Square
If two heterozygotes cross:
Rr × Rr
Genotype ratio:
1 RR : 2 Rr : 1 rr
Phenotype ratio is also:
1 : 2 : 1
Each genotype has a different phenotype.
Key clue in questions:
- Intermediate
- Blended
- Mixed phenotype but not both traits separately
Example:
Red + White → Pink
🧬 Pattern 2 – Codominance
What It Means
- Both alleles are fully expressed in the heterozygote
- No blending occurs
Phenotype mapping:
AA = phenotype A
AB = phenotype A and B together
BB = phenotype B
Prediction Using Punnett Square
AB × AB
Genotype ratio:
1 AA : 2 AB : 1 BB
Phenotype ratio:
1 : 2 : 1
Same numerical ratio as incomplete dominance, but phenotype appearance differs.
Key clue in questions:
- Both expressed
- Both traits visible together
- Patches, stripes, both proteins present
Example:
Red and white both visible on same organism.
🧬 Pattern 3 – Sex-Linked Inheritance
What It Means
- Trait gene located on a sex chromosome
- Males are XY
- Females are XX
Males express whatever allele is on their single X chromosome.
Prediction Strategy:
- Write alleles on X chromosome (example: Xᴬ, Xᵃ)
- Write male as XY and female as XX
- Use Punnett square including X and Y
- Identify affected males, affected females, carriers
Key clue in questions:
- Trait appears more in males
- Trait skips generations through carrier females
- No father-to-son inheritance for X-linked traits
🧬 Pattern 4 – Polygenic Traits
What It Means
- Trait controlled by multiple genes
- Each gene contributes small effect
Results in:
- Continuous variation
- Many possible phenotypes
- No simple 3:1 or 1:2:1 ratio
Prediction becomes probability ranges, not fixed categories.
Key clue in questions:
- Gradual variation
- Bell-shaped distribution
Example:
Height, skin tone
🧬 Pattern 5 – Environmental Influence
Some traits depend on:
- Genotype
- Environment
Prediction:
Same genotype can produce different phenotypes under different conditions.
Clues:
- Temperature effect
- Nutrition effect
- Light exposure effect
📊 Summary Table for Fast Identification
| Pattern | Heterozygote Appearance | Typical Ratio (Hetero × Hetero) |
|---|---|---|
| Incomplete dominance | Intermediate | 1 : 2 : 1 |
| Codominance | Both traits visible | 1 : 2 : 1 |
| Sex-linked | Depends on sex | Often more males affected |
| Polygenic | Continuous range | No simple ratio |
| Environmental influence | Varies with conditions | Depends on environment |
📦 Quick Recap
Non-Mendelian = not simple dominant/recessive
Incomplete dominance → intermediate heterozygote
Codominance → both alleles expressed
Sex-linked → depends on XX vs XY
Polygenic → range, no fixed ratio
Environment can modify phenotype
Using a Pedigree to Predict Inheritance in a Family
🌿 Introduction
A pedigree is a family-tree style model used to track how a trait is inherited across generations.
It helps us:
- Identify whether a trait is dominant or recessive
- Predict who is likely a carrier
- Predict the chance future children will show the trait
So, pedigrees are used for prediction, not just recording.
🧠 Pedigree Symbols 
Basic symbols:
- Square = male
- Circle = female
- Shaded = has the trait (affected)
- Unshaded = does not have the trait (unaffected)
Connections:
- Horizontal line between a male and female = mating
- Vertical line down = offspring
- Siblings share the same horizontal line
These symbols help you quickly map trait inheritance.
🧬 How to Use a Pedigree to Predict Inheritance
Step 1 – Look Across Generations
Ask:
- Does the trait appear in every generation?
- Or does it skip generations?
This gives your first major clue.
Step 2 – Check Parents and Children
Key questions:
- Can two unaffected parents produce an affected child?
- Does every affected person have an affected parent?
This helps decide dominant vs recessive.
Step 3 – Compare Males vs Females
Ask:
- Are males and females affected equally?
- Is it mostly males?
This helps identify if it could be sex-linked.
Step 4 – Decide the Pattern
Based on the clues, classify as:
- Autosomal dominant
- Autosomal recessive
- Sex-linked (commonly X-linked recessive)
Then predict genotypes and probabilities.
🧠 Recognizing Common Pedigree Patterns
A) Autosomal Dominant Pattern
Key pedigree clues:
- Trait appears in every generation
- Affected individuals usually have an affected parent
- Both males and females affected equally
- Unaffected parents usually do not produce affected children
Genotype idea:
- Affected individual could be Aa (most common)
- Unaffected individual is aa
Prediction use:
If affected parent is Aa and unaffected parent is aa:
Chance child affected = 1/2
B) Autosomal Recessive Pattern
Key pedigree clues:
- Trait can skip generations
- Two unaffected parents can produce an affected child
- Both sexes affected equally
Genotype idea:
- Affected individual is aa
- Unaffected individuals may be AA or Aa
- Carrier = Aa
Prediction use:
If both parents are carriers (Aa × Aa):
Chance affected child (aa) = 1/4
Chance carrier (Aa) = 1/2
Chance non-carrier (AA) = 1/4
C) Sex-Linked Pattern (X-Linked Recessive Commonly)
Key pedigree clues:
- More males affected than females
- Trait may skip generations
- Affected sons often have carrier mothers
- No father-to-son transmission
Reason:
Father gives Y to sons, not X.
Genotype idea:
- Male affected: XᵃY
- Female carrier: XᴬXᵃ
- Female affected: XᵃXᵃ (less common)
Prediction use:
Carrier mother (XᴬXᵃ) + normal father (XᴬY)
50% sons affected
50% sons normal
50% daughters carriers
50% daughters normal
🧬 Using a Pedigree to Predict Future Offspring
Once you identify the inheritance pattern:
- Assign possible genotypes to key individuals
- Identify carriers if recessive inheritance is suspected
- Use Punnett square or probability rules
- Predict likelihood in future children
This is the main skill being tested.
📊 Quick Identification Table
| Clue in Pedigree | Likely Pattern |
|---|---|
| Trait in every generation | Autosomal dominant |
| Trait skips generations | Autosomal recessive |
| Two unaffected parents produce affected child | Autosomal recessive |
| Mostly males affected | X-linked recessive likely |
| No father-to-son inheritance | X-linked |
📦 Quick Recap
Pedigree = inheritance family model
Check generations: every generation or skipping
Unaffected parents → affected child = recessive clue
Compare males vs females for sex-linkage
Assign genotypes → use probability to predict offspring