The Genetics Casino: Monohybrid Crosses | Grade 11 Life Sciences
★ Grade 11 Life Sciences ★

The Genetics
Casino

Every allele is a card. Every cross is a bet. The house always follows Mendel's rules — and unlike real gambling, you can learn exactly how to predict the outcome.

The House Rules · Punnett Square Builder · Worked Crosses · Quiz

The House Rules

Essential Vocabulary

🎰 Welcome to The Genetics Casino

Every organism is dealt two alleles for every gene — one from each parent. Whether the trait shows up depends on which alleles you're holding. Some alleles dominate. Some stay hidden. And the odds of any combination can be calculated precisely — that's what makes genetics feel like a casino where you can actually know the house odds.

Master the vocabulary first. Everything else builds on these definitions.

Gene
A section of DNA that codes for a specific protein — and therefore a specific trait. Located at a fixed position (locus) on a chromosome.
🎲 The Casino: the game being played (e.g. eye colour, seed colour).
Allele
Different versions of the same gene. For example, the eye colour gene has a brown allele and a blue allele. Each person carries two alleles — one on each homologous chromosome.
🃏 The Casino: the card you're dealt. Two per player.
Dominant Allele
An allele that is expressed whenever it is present — even if only one copy exists. Written as a capital letter (e.g. B). Masks the recessive allele.
♠️ The Casino: the ace — beats everything else in the hand.
Recessive Allele
An allele that is only expressed when two copies are present (homozygous recessive). Written as a lowercase letter (e.g. b). Hidden when paired with a dominant allele.
🂠 The Casino: the hidden card — only wins when you have a pair.
Genotype
The actual allele combination an organism carries (e.g. BB, Bb, or bb). The genotype is the genetic reality — what's actually in the DNA, whether visible or not.
🔒 The Casino: your hand of cards — only you and the dealer know.
Phenotype
The physical expression of the genotype — what you can actually see or measure (e.g. brown eyes, tall stem). Two organisms can have different genotypes but the same phenotype.
👁️ The Casino: what the other players see on the table.
Homozygous
Carrying two identical alleles: either BB (homozygous dominant) or bb (homozygous recessive). Also called true-breeding — homozygous organisms always pass the same allele to offspring.
🎯 The Casino: a matched pair — predictable every time.
Heterozygous
Carrying two different alleles: Bb. Also called a carrier or hybrid. The dominant allele is expressed in the phenotype, but the recessive allele is silently passed to offspring.
🎭 The Casino: hiding a card — looks dominant, carries recessive.

📋 Mendel's Laws — The House Rules That Never Change

Law of Segregation: Each parent has two alleles for each gene. During gamete formation (meiosis), these two alleles separate so that each gamete carries only ONE allele. Which allele ends up in which gamete is random.
Law of Independent Assortment: Genes on different chromosomes are inherited independently of each other (this applies to dihybrid crosses — covered in the next resource). Each gene pair segregates independently during meiosis.
Law of Dominance: When two different alleles are present, the dominant allele is expressed. The recessive allele is masked — but still present in the genotype and can be passed to offspring.
GenotypeNamePhenotype (if B=brown, b=blue)Casino Status
BBHomozygous dominantBrown eyesHolding two aces
BbHeterozygousBrown eyes (B masks b)One ace, one hidden card
bbHomozygous recessiveBlue eyesPair of twos — finally wins!

Punnett Square Builder

Interactive

🃏 Choose Your Cross

Select a cross type to see the Punnett square, ratios, and what it means for offspring. Use the letter B = brown (dominant) and b = blue (recessive) for all examples.

Punnett Square
B B
B BB BB
B BB BB
Offspring Ratios
Homozygous dominant (BB)100%
Heterozygous (Bb)0%
Homozygous recessive (bb)0%

Worked Crosses

Step by Step
🌱

Cross 1 — The Classic Mendel Cross (Bb × Bb)

Two heterozygous parents. The cross that started it all. Mendel used pea plants — we'll use eye colour.

1

Identify the parents and their genotypes

Both parents have brown eyes but are carriers of the blue allele. Their genotype is heterozygous dominant.

Parent 1: Bb    Parent 2: Bb
2

Write the gametes each parent can produce

Each parent separates their two alleles during meiosis. Each gamete gets ONE allele (Law of Segregation).

Parent 1 gametes: B or b    Parent 2 gametes: B or b
3

Complete the Punnett square

Place Parent 1's gametes along the top, Parent 2's down the side. Fill each box by combining the row and column alleles.

Bb
BBBBb
bBbbb
4

Read the genotypic ratio

Count the genotype combinations from the 4 boxes.

1 BB : 2 Bb : 1 bb  →  Genotypic ratio 1:2:1
5

Read the phenotypic ratio

BB and Bb both show the dominant phenotype (brown eyes). Only bb shows recessive (blue eyes).

3 Brown : 1 Blue  →  Phenotypic ratio 3:1
🎰 Dealer's Tip
The 3:1 phenotypic ratio from a Bb × Bb cross is the most famous result in genetics. Mendel got this ratio from thousands of pea plant crosses before anyone knew what DNA or genes were. The ratio only holds perfectly in large populations — with just 4 offspring, you might get any combination by chance.
Genotypic ratio 1:2:1 Phenotypic ratio 3:1 25% chance bb offspring
🔍

Cross 2 — The Test Cross (? × bb)

You have an organism showing the dominant phenotype — but you don't know if it's BB or Bb. The test cross reveals the hidden genotype.

1

The problem: unknown genotype

A brown-eyed individual (B?) is crossed with a blue-eyed individual (bb). The blue-eyed parent can only contribute b alleles — so all variation in offspring comes from the unknown parent.

Unknown parent: B?    Known parent: bb
2

If the unknown is BB

All offspring receive one B from BB and one b from bb → all offspring are Bb (heterozygous). ALL offspring show the dominant phenotype (brown eyes). No blue-eyed offspring at all.

BB
bBbBb
bBbBb
Result: 100% Bb → 100% brown eyes
3

If the unknown is Bb

Offspring can be Bb (brown) or bb (blue) in a 1:1 ratio. Blue-eyed offspring appear — revealing the parent was heterozygous.

Bb
bBbbb
bBbbb
Result: 1 Bb : 1 bb → 1:1 phenotypic ratio
🎰 Dealer's Tip
The test cross is one of the most powerful tools in genetics. Any time an exam gives you an organism with a dominant phenotype and asks you to determine its genotype — you cross it with a homozygous recessive (bb). If any recessive offspring appear, the parent must have been heterozygous.
🏛️ House Rule — Always Remember
A recessive phenotype (blue eyes, short stem) can ONLY appear if the organism is homozygous recessive (bb). If you see the recessive phenotype, you immediately know the genotype — no test cross needed for that individual.
Test cross: unknown × bb No recessive offspring → parent is BB 1:1 ratio → parent is Bb
🌸

Cross 3 — Incomplete Dominance (R¹R¹ × R²R²)

Neither allele is fully dominant. The heterozygote shows a blended phenotype — the casino pays out a mixed prize.

1

New notation for incomplete dominance

Because neither allele is fully dominant, we use superscript notation instead of upper/lowercase. For snapdragon flower colour: R¹ = red, R² = white. Neither dominates the other.

Red parent: R¹R¹    White parent: R²R²
2

Complete the Punnett square

R¹R¹
R²R¹R²R¹R²
R²R¹R²R¹R²
3

Read the result

All offspring are R¹R² — but in incomplete dominance, this heterozygote shows a BLENDED phenotype: pink flowers. The red and white pigments are both partially expressed.

100% R¹R² → 100% pink flowers
4

Cross the F1 generation (pink × pink)

R¹R²
R¹R¹R¹R¹R²
R²R¹R²R²R²
1 Red : 2 Pink : 1 White
🏛️ House Rule — Incomplete Dominance
In incomplete dominance: genotypic ratio = phenotypic ratio (both are 1:2:1 in F2). This is different from complete dominance where phenotypic ratio is 3:1. Also note: in incomplete dominance, you CAN determine genotype from phenotype alone — there are three distinct phenotypes for three genotypes.
Neither allele fully dominant Heterozygote = blended phenotype F2 ratio: 1:2:1 phenotype AND genotype Use R¹R² notation

When The House Changes The Rules

Exceptions to Dominance

⚡ Beyond Simple Dominance

Mendel's original laws assume complete dominance — one allele fully masks the other. But genetics is more complex than that. IEB and CAPS both test these exceptions. Know them, and you'll never be caught out by a "trick" question again.

🌸 Incomplete Dominance

Neither allele is fully dominant. The heterozygote shows an intermediate (blended) phenotype. Example: red × white snapdragons → pink offspring. Notation uses superscripts (R¹, R²).

Key indicator: Three distinct phenotypes exist. Genotypic ratio = phenotypic ratio in F2 (1:2:1). You can always tell genotype from phenotype.

🩸 Codominance

Both alleles are fully expressed simultaneously — neither masks the other and there is no blending. Example: ABO blood groups (IA and IB are codominant — giving blood type AB where both antigens are present). Also: roan coat colour in cattle (red + white hairs both visible).

Key difference from incomplete dominance: Codominance = BOTH phenotypes fully expressed together. Incomplete dominance = a NEW blended phenotype. Roan cattle show both red and white hairs. Pink snapdragons show neither red nor white — they show pink.

♂️ Sex-Linked Inheritance

Genes located on the X chromosome are inherited differently by males and females. Males (XY) only have one X chromosome — so a single recessive allele on the X will be expressed (no second allele to mask it). This is why conditions like colour blindness and haemophilia affect males far more often than females.

Notation: Write sex-linked alleles as superscripts on X: X^B (normal) and X^b (colour blind). Males are X^B Y or X^b Y. Females are X^B X^B, X^B X^b (carrier), or X^b X^b (affected).

🎭 Multiple Alleles

While each individual carries only 2 alleles per gene, some genes have more than 2 versions in the population. The ABO blood group system has three alleles: I^A, I^B, and i. Each individual still carries only 2, but the population contains all three. I^A and I^B are codominant; both are dominant over i.

Blood groups: Type A = I^A I^A or I^A i  |  Type B = I^B I^B or I^B i  |  Type AB = I^A I^B  |  Type O = ii

💡 Quick Comparison — The Four Inheritance Patterns

PatternHeterozygote PhenotypeF2 Phenotypic RatioExample
Complete dominanceDominant phenotype only3:1Brown vs blue eyes
Incomplete dominanceBlended / intermediate1:2:1Red × white → pink flowers
CodominanceBoth phenotypes expressed1:2:1Roan cattle, AB blood type
Sex-linkedFemales can be carriers; males express recessiveVaries by sexColour blindness, haemophilia

🎯 Casino Floor Quiz

Place your bets. Show your working. Eight questions — no luck required.

Question 1 of 8
An organism has genotype Bb. Its phenotype shows the dominant trait. What can you conclude about the recessive allele b?
Question 2 of 8
Two heterozygous brown-eyed parents (Bb × Bb) have 4 children. What is the probability that ALL 4 children have blue eyes?
Question 3 of 8
A tall pea plant (T?) is crossed with a short plant (tt). Half the offspring are tall and half are short. What is the genotype of the tall parent?
Question 4 of 8
Red and white snapdragons are crossed. All F1 offspring are pink. When pink × pink are crossed, what phenotypic ratio appears in F2?
Question 5 of 8
What is the key difference between incomplete dominance and codominance?
Question 6 of 8
A woman with normal vision (carrier) has children with a colour-blind man. What proportion of their SONS will be colour-blind? (X^B = normal, X^b = colour blind)
Question 7 of 8
An individual has blood type O (genotype ii). Both parents must have at least one i allele. Which blood types are POSSIBLE for each parent?
Question 8 of 8
What is the purpose of a test cross?

Grade 11 Life Sciences — IEB & CAPS aligned

Dr Tracey Classens — Study Resources
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