A Level Biology - How to Solve: Inheritance of Blood Groups and Multiple Alleles

How to Solve: Inheritance of Blood Groups and Multiple Alleles

Complete Guide For GCSE/A-Level Biology (AQA, Edexcel, OCR, WJEC)

Introduction

"Mastering blood group inheritance unlocks 6–8 marks on your GCSE/A-Level exam—enough to boost your grade by a full level. Plus, it’s how doctors predict whether a baby could have haemolytic disease or why some siblings have different blood types."

(On camera: Hold up a blood donation poster or a Punnett square example.) "Today, you’ll learn the exact steps to solve any blood group inheritance question—no guesswork, just full marks."

WHAT YOU NEED TO KNOW FIRST

Before starting, you must understand:
1. Basic genetics: Genes, alleles, dominant/recessive traits (e.g., B for brown eyes > b for blue).
2. Punnett squares: How to cross two parents’ alleles to predict offspring genotypes.
3. Codominance: When two alleles are equally expressed (e.g., AB blood type).

(On camera: Flash a quick Punnett square for Bb × Bb to jog memory.)

KEY TERMS & FORMULAS

Key Terms

Formulas & Rules

1. Blood group alleles:
2. Iᴬ = A antigen (dominant)
3. Iᴮ = B antigen (dominant)
4. i = no antigen (recessive)

5. MEMORISE THIS: Iᴬ and Iᴮ are codominant; i is recessive to both.

6. Possible genotypes & phenotypes: | Genotype | Phenotype (Blood Type) | |----------|------------------------| | IᴬIᴬ or Iᴬi | A | | IᴮIᴮ or Iᴮi | B | | IᴬIᴮ | AB | | ii | O |

7. Punnett square setup:

8. Write one parent’s alleles on top, the other’s on the side.
9. Fill in the boxes to find possible offspring genotypes.

STEP-BY-STEP METHOD

Follow these 5 steps for every blood group inheritance question.

1. Identify the parents’ phenotypes (blood types).

2. Example: Parent 1 = A, Parent 2 = B.

3. Write all possible genotypes for each parent.

4. A blood type → IᴬIᴬ or Iᴬi
5. B blood type → IᴮIᴮ or Iᴮi

6. (If the question gives genotypes, skip this step.)

7. Determine which genotype combinations are possible.

8. If the question says "heterozygous," use Iᴬi or Iᴮi.
9. If it says "homozygous," use IᴬIᴬ or IᴮIᴮ.

10. If unsure, try all possible combinations (exam questions usually specify).

11. Set up a Punnett square for each possible genotype pair.

12. Example: Parent 1 = Iᴬi, Parent 2 = Iᴮi.

13. Calculate the probability of each offspring blood type.

14. Count the genotypes in the Punnett square.
15. Convert to percentages or ratios (e.g., 25% AB, 25% A, 25% B, 25% O).

(On camera: Draw a Punnett square live while explaining.)

WORKED EXAMPLES

Example 1 – Basic

Question: A mother has blood type A (heterozygous) and a father has blood type B (heterozygous). What is the probability their child has blood type O?

Step 1: Parents’ phenotypes → Mother = A, Father = B. Step 2: Possible genotypes → Mother = Iᴬi, Father = Iᴮi. Step 3: Only one possible pair → Iᴬi × Iᴮi. Step 4: Punnett square:

Step 5: Offspring genotypes → 1 IᴬIᴮ (AB), 1 Iᴬi (A), 1 Iᴮi (B), 1 ii (O). Probability of O = 25% (1/4).

What we did and why: - We used the parents’ heterozygous status to pick Iᴬi and Iᴮi. - The Punnett square showed one ii out of four possible offspring.

Example 2 – Medium

Question: A woman with blood type AB marries a man with blood type O. What blood types could their children have, and in what ratio?

Step 1: Parents’ phenotypes → Mother = AB, Father = O. Step 2: Possible genotypes → Mother = IᴬIᴮ (only option), Father = ii (only option). Step 3: Only one possible pair → IᴬIᴮ × ii. Step 4: Punnett square:

Step 5: Offspring genotypes → 2 Iᴬi (A), 2 Iᴮi (B). Ratio = 50% A, 50% B (no AB or O).

What we did and why: - AB parent must be IᴬIᴮ (codominant). - O parent must be ii (recessive). - The Punnett square showed no ii or IᴬIᴮ offspring.

Example 3 – Exam-Style

Question: In a paternity test, the mother has blood type A, the child has blood type B, and the alleged father has blood type AB. Could he be the biological father? Explain.

Step 1: Mother = A, Child = B, Alleged father = AB. Step 2: Mother’s possible genotypes → IᴬIᴬ or Iᴬi. Child’s genotype → Iᴮi or IᴮIᴮ (but IᴮIᴮ would require two Iᴮ alleles, unlikely here). Step 3: If mother is Iᴬi and child is Iᴮi, the father must have passed Iᴮ. Alleged father = IᴬIᴮ → can pass Iᴮ. Step 4: Punnett square for Iᴬi × IᴬIᴮ:

Step 5: Possible offspring → IᴬIᴬ (A), IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B). Child could be Iᴮi (B) → Yes, he could be the father.

What we did and why: - We considered all possible genotypes for the mother. - The child’s Iᴮ allele must come from the father (mother has no Iᴮ). - The alleged father’s IᴬIᴮ genotype can pass Iᴮ.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP

(On camera: Speak naturally, like a last-minute revision.)

"Okay, listen up—this is your 30-second cheat sheet for blood group inheritance.

1. Memorise the alleles: Iᴬ and Iᴮ are codominant; i is recessive.
2. Genotypes → Phenotypes:
3. IᴬIᴬ or Iᴬi = A
4. IᴮIᴮ or Iᴮi = B
5. IᴬIᴮ = AB
6. ii = O
7. Punnett squares: Write parents’ alleles, fill in the boxes, count the outcomes.
8. Watch for heterozygous parents: A or B blood type could be Iᴬi or Iᴮi.
9. If the question doesn’t specify genotypes, try all possible combinations.

Exam tip: If a child has a blood type the parents can’t produce, check for heterozygous parents—it might still be possible!

Now go smash those 6–8 marks. You’ve got this!"