Monohybrid Cross - Grade 12

What is a Monohybrid Cross?

Monohybrid cross means a genetic cross between two individuals looking at only one trait (one characteristic).
👉 Example of one trait: Flower color, or Seed shape.

We check how that trait is passed from the parents to the offspring.

Step-by-Step Explanation:

Step 1: Understand Vocabulary

• Gene = A unit that controls a trait (like flower color).
• Allele = Different forms of a gene (for example, "R" for red flower and "r" for white flower).
• Dominant = Stronger allele that will always show up when present (symbol: capital letter, like "R").
• Recessive = Weaker allele that only shows up when two copies are there (symbol: small letter, like "r").
• Homozygous = Two same alleles (RR or rr).
• Heterozygous = Two different alleles (Rr).

💡 Tip:
Always CAPITAL letter = Dominant
Always small letter = Recessive

Step 2: Know the Question

🔎 Read the question carefully.
Ask yourself:

• What trait is involved? (e.g., flower color)
• Which alleles are dominant and recessive?
• What are the parents' genotypes? (RR, Rr, or rr?)

Step 3: Set up the Cross

We use something called a Punnett square (a little 2x2 table).

👉 Let's do an example together:

Example:

"Cross a heterozygous red-flowered plant (Rr) with a homozygous white-flowered plant (rr). What will the offspring look like?"

✅ Step 1: Write Parent Genotypes

• Parent 1: Rr (heterozygous red)
• Parent 2: rr (homozygous white)

✅ Step 2: Write Possible Gametes (Sperm/Eggs)

• Parent 1 can give: R or r
• Parent 2 can give: r or r

✅ Step 3: Draw Punnett Square

	r	r
R	Rr	Rr
r	rr	rr

✅ Step 4: Read the Results

• 2 squares show Rr (heterozygous red)
• 2 squares show rr (homozygous white)

✅ Step 5: Write Final Answer

• 50% Red flowers (Rr)
• 50% White flowers (rr)

💡 Trick to Remember:
First letter = Dad's contribution
Second letter = Mom's contribution

How to ALWAYS Get it Right:

1. Identify Dominant and Recessive

• Ask yourself: Which trait wins when mixed?

2. Write Genotypes Carefully

• Don't mix up homozygous and heterozygous.

3. Break Gametes Properly

• Only ONE letter from each parent for each gamete.

4. Draw the Punnett Square Neatly

• Parents’ gametes go across the top and side.
• Fill in inside by combining one letter from top and one from side.

5. Count Carefully

• How many of each combination?
• How many dominant-looking vs recessive-looking?

Quick Summary Chart for Most Common Crosses

Cross Type	Example	Result (in %)
Homozygous Dominant × Homozygous Recessive	RR × rr	100% heterozygous dominant (Rr)
Heterozygous × Heterozygous	Rr × Rr	75% dominant, 25% recessive
Heterozygous × Homozygous Recessive	Rr × rr	50% dominant, 50% recessive

Bonus Tip 🌟

✅ If you see words like "purebred", it usually means homozygous.
✅ If they say "shows dominant trait but carries recessive", it’s heterozygous.

Exam Type Question:

A butterfly homozygous for brown wing color is crossed with a butterfly homozygous for pink wing color. All the F1 butterflies had brown wings.

4.1 Which is dominant: brown or pink wing color? 
4.2 Give a reason for your answer in Question 4.1.
4.3 Represent a genetic cross to show how you will arrive at the offspring with brown wings in the F1.
4.4 If the F, butterflies were allowed to interbreed, use a genetic cross to show the genotypes and phenotypes of the F2.
4.5 State the genotypic and phenotypic ratio of the F2.

Solution 

First, understand the situation like a story:

• Homozygous means the organism has two of the same allele.
• For brown wing butterfly = BB
• For pink wing butterfly = bb
• (We use capital letters for dominant traits and small letters for recessive traits.)
• When BB and bb cross, the offspring (F₁ generation) all have brown wings.

Now, let's answer the questions one by one carefully:

📍 4.1 Which is dominant: brown or pink wing colour?

Answer:
✅ Brown wing colour is dominant.

📍 4.2 Give a reason for your answer in Question 4.1.

Answer:
✅ Because when a homozygous brown-wing butterfly was crossed with a homozygous pink-wing butterfly, all the offspring (F₁) were brown.
(If pink were dominant, some or all offspring would be pink.)

📍 4.3 Represent a genetic cross to show how you will arrive at the offspring with brown wings in the F₁.

Step-by-step method:

Parents’ genotypes:

• Brown-wing = BB
• Pink-wing = bb

Gametes (eggs/sperms):

• Brown butterfly (BB) gives B gametes.
• Pink butterfly (bb) gives b gametes.

Cross:

• When B meets b:
• B × b = Bb

F₁ Offspring genotype:

• All are Bb.

F₁ Offspring phenotype (appearance):

• All brown wings (because B is dominant).

✅ Genetic Cross Summary:

Parent Genotypes: BB × bb
Gametes: B and b
F₁ Genotypes: All Bb
F₁ Phenotype: All brown wings

📍 4.4 If the F₁ butterflies were allowed to interbreed, use a genetic cross to show the genotypes and phenotypes of the F₂.

Now we cross F₁ individuals (Bb × Bb):

Step-by-step method:

Parents’ genotypes: Bb × Bb

Gametes:

• First Bb: B or b
• Second Bb: B or b

Punnett Square:

	B	b
B	BB	Bb
b	Bb	bb

Offspring Genotypes:

• BB (brown)
• Bb (brown)
• Bb (brown)
• bb (pink)

Offspring Phenotypes:

• 3 brown (BB, Bb, Bb)
• 1 pink (bb)

📍 4.5 State the genotypic and phenotypic ratio of the F₂.

✅ Genotypic Ratio (based on BB, Bb, bb):

• 1 BB : 2 Bb : 1 bb

✅ Phenotypic Ratio (based on appearance):

• 3 brown : 1 pink

🎯 TIPS TO NEVER GET CONFUSED AGAIN:

✨ TIP 1: Always first label who is who!

• Homozygous dominant = two capitals (BB)
• Homozygous recessive = two small letters (bb)

✨ TIP 2: Remember "Capital letter = Dominant"

• If an offspring has one capital (like Bb), it will show the dominant trait (brown wings).

✨ TIP 3: Always do 3 simple steps:

• Write parents' genotypes.
• Write gametes.
• Draw a Punnett square and fill it in neatly.

✨ TIP 4:

• If all F₁ look like one parent, that trait is dominant!
• If F₁ are mixed, it’s either incomplete dominance or codominance (not here though).

🧠 QUICK MEMORY AID:

• Homozygous = "Same Same" (BB or bb)
• Heterozygous = "Different" (Bb)
• Punnett Square = "Cross and check"