Inheritance » Codominance in Inheritance

Introduction to Codominance

In most genetics you've studied so far, one allele is dominant over another - like brown eyes being dominant over blue eyes. But nature isn't always that simple! Sometimes both alleles can express themselves at the same time, creating a fascinating pattern called codominance.

Codominance is like having two equally strong voices in a conversation - neither one gets silenced and you can hear both clearly. This creates some amazing patterns in living things, from blood types in humans to flower colours in plants.

Key Definitions:

• Codominance: When both alleles in a heterozygote are fully expressed at the same time, neither being dominant over the other.
• Multiple alleles: When a gene has more than two possible allele forms in a population.
• Phenotype: The observable characteristics of an organism.
• Genotype: The genetic makeup of an organism.

Understanding ABO Blood Groups

The best example of codominance in humans is our ABO blood group system. This system involves three alleles: I^A, I^B and i. The I^A and I^B alleles are codominant to each other, whilst both are dominant over the recessive i allele.

How Blood Types Work

Your blood type depends on which proteins (called antigens) are found on your red blood cells. The I^A allele produces A antigens, I^B produces B antigens and i produces no antigens.

💉 Type A Blood

Genotypes: I^AI^A or I^Ai

Antigens: A antigens only

💉 Type B Blood

Genotypes: I^BI^B or I^Bi

Antigens: B antigens only

💉 Type AB Blood

Genotype: I^AI^B

Antigens: Both A and B antigens

Genetic Crosses with Codominance

When working out genetic crosses involving codominance, we follow the same basic rules as other inheritance patterns, but we need to remember that both alleles in a heterozygote will be expressed.

Example Cross: Type A × Type B Parents

Let's work through a cross between a person with Type A blood (I^Ai) and a person with Type B blood (I^Bi):

📈 The Cross

Parents: I^Ai × I^Bi

Gametes:

Parent 1: I^A or i

Parent 2: I^B or i

Possible offspring:

• I^AI^B (Type AB blood) - 25%
• I^Ai (Type A blood) - 25%
• I^Bi (Type B blood) - 25%
• ii (Type O blood) - 25%

Other Examples of Codominance

Codominance isn't just found in human blood types. It appears throughout the natural world in many fascinating ways.

Codominance vs Incomplete Dominance

It's important not to confuse codominance with incomplete dominance. They're different patterns:

✅ Codominance

Both alleles are fully expressed simultaneously. You can see both traits clearly at the same time (like AB blood type having both A and B antigens).

❌ Incomplete Dominance

Neither allele is fully expressed, creating a blended phenotype. The result is something in between the two parent traits (like pink flowers from red and white parents).

Solving Codominance Problems

When tackling genetics problems involving codominance, follow these key steps:

1. Identify the alleles: Work out which alleles are codominant and which (if any) are recessive
2. Determine parent genotypes: Use the phenotypes given to work out possible genotypes
3. Set up your cross: Use a Punnett square just like with other inheritance patterns
4. Work out offspring: Remember that heterozygotes with codominant alleles show both traits
5. Calculate ratios: Express your results as percentages or ratios

Practice Makes Perfect

The key to mastering codominance is practice. Try working through different scenarios:

• What happens when two Type AB people have children?
• Can two Type A parents have a Type B child?
• What blood types are possible from a Type O and Type AB cross?

Remember, in codominance problems, you're looking for situations where both alleles in a heterozygote are expressed equally. This creates unique phenotypes that wouldn't exist with simple dominant-recessive inheritance.

Summary

Codominance shows us that genetics isn't always about one trait winning over another. Sometimes nature finds ways for multiple traits to coexist, creating fascinating patterns and important biological functions. From the blood flowing through your veins to the coat colours of farm animals, codominance plays a crucial role in the diversity of life around us.

Understanding codominance helps us appreciate the complexity and elegance of genetic inheritance and provides essential knowledge for fields like medicine, agriculture and conservation biology.