Codominance

Codominance is where both alleles for the same characteristic are simultaneously expressed in the heterozygote. This can influence the outcome of monohybrid and dihybrid crosses.

Codominance

Codominant alleles are both expressed in a heterozygote.
Neither of the alleles are recessive.
Codominance influences the phenotypic ratios of monohybrid and dihybrid crosses.

E.g. sickle-cell anemia

An example of codominance is sickle-cell anemia.
There are two alleles for sickle-cell anemia:
- H^N - normal hemoglobin.
- H^S - sickle hemoglobin.

Sickle-cell phenotypes

People who have two copies of the H^N allele (homozygotes) do not have sickle-cell anemia.
People who have two copies of the H^S alleles (homozygotes) do have sickle-cell anemia.
People who have one H^N allele and H^S allele (heterozygotes) have both normal hemoglobin and sickled hemoglobin.
- H^N and H^S are codominant.

Phenotypic ratios

Codominance affects the phenotypic ratios of monohybrid and dihybrid crosses.
E.g:
- If two heterozygous (H^NH^S) breed, the ratio becomes 1:2:1 instead of the normal 3:1 that is expected in a monohybrid cross.

1Cell Biology

2Molecular Biology

2.1Water

2.1.1Water & Hydrogen Bonding

2.1.2IB Multiple Choice - Water & Hydrogen Bonding

2.1.3Extended Response - Water

2.2Carbohydrates & Lipids

2.2.1Monosaccharides

2.2.2Disaccharides & Polysaccharides

2.2.3Triglycerides & Phospholipids

2.2.4Cellulose

2.2.5Starch & Glycogen

2.3Proteins

2.3.1Structure & Functions of Proteins

2.4Enzymes

2.4.1Enzymes

2.4.2Factors Affecting Enzyme Activity

2.4.3Enzyme-Controlled Reactions

2.4.4End of Topic Test - Enzymes

2.5Structure of DNA & RNA

2.5.1DNA & RNA

2.6DNA Replication, Transcription & Translation

2.6.1Protein Synthesis

2.6.2Transcription & Translation

2.6.3End of Topic Test - DNA, Genes & Protein Synthesis

2.6.4Exam-Style Question - Protein Synthesis

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2.6.6IB Multiple Choice - Transcription

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2.7Cell Respiration

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2.7.2Anaerobic Respiration

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2.7.4Respiration Experiments

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2.8Photosynthesis

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3Genetics

4Ecology

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6Human Physiology

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