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17.6Inheritance

Codominance — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

5 min read 8 parts Lesson 7 of 9 1 quiz

📚 Cambridge IGCSE Biology 0610 Hub / Topical Revision
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Lesson summary

17.6 Codominance — IGCSE BIOLOGY notes: Describe codominance and multiple alleles, and explain the inheritance of ABO blood groups.Codominance = a situation in which both alleles in a heterozygous organism contribute to the phenotype.Both alleles are fully expressed and neither h…

  • Other organisms
  • Three alleles
  • Two per person
  • Relationships
  • Group A (IAIO) × group B (IBIO)
  • Group O child

Part 1 of 6Codominance

Codominance
Codominance a situation in which both alleles in a heterozygous organism contribute to the phenotype.
  • Both alleles are fully expressed and neither hides the other.
Other organisms
  • Other organisms a plant with one red and one white petal allele may show both colours.
Clue
  • Clue a 1 : 2 : 1 phenotype ratio from two heterozygotes suggests codominance.

Part 2 of 6Multiple alleles of the ABO gene

Three alleles
  • IA, IB and IO, called multiple alleles.
Two per person
  • Two per person each person still inherits only two, one from each parent.
Relationships
  • IA and IB are codominant, and both are dominant to IO.
Notation
  • I represents the gene and the superscripts A, B and O the alleles.

Part 3 of 6What the alleles do

What the alleles do
IA
  • IA produces antigen A on the surface of red blood cells.
IB
  • IB produces antigen B.
IO
  • IO produces no antigen.
Group AB
  • Group AB a person with IAIB makes both antigens, which is why both alleles show.

Part 4 of 6ABO genotypes and phenotypes

GenotypeBlood group
IAIA or IAIOA
IBIB or IBIOB
IAIBAB
IOIOO
  • Six genotypes give four phenotypes.

Part 5 of 6Predicting ABO crosses

1
Write the parental phenotypes and genotypes.
2
Write the gametes.
3
Combine them in a Punnett square.
4
List the offspring genotypes.
5
Translate to blood groups and give the ratio.
Predicting ABO crosses
Group A (IAIO) × group B (IBIO)
  • Group A (IAIO) × group B (IBIO) gives AB, A, B and O in a 1 : 1 : 1 : 1 ratio.
Group O child
  • Group O child both parents must carry an IO allele, giving a 25% chance when both are heterozygous.

Part 6 of 6More from the specification

1
Codominance and Phenotypic Expression
  • In codominance, both alleles are fully expressed in the phenotype of a heterozygous individual without blending or dominance, exemplified by IA and IB alleles forming blood group AB.
2
Sex Chromosomes and Hemizygous Inheritance
  • Human biological sex is determined by X and Y chromosomes;
  • because the Y chromosome is significantly shorter and lacks many loci present on the X chromosome, recessive sex-linked alleles are expressed in males.
3
Mitosis versus Meiosis in Heredity
  • Mitosis produces two genetically identical diploid cells for growth and repair,
  • whereas meiosis involves two nuclear divisions to generate four genetically variable haploid gametes.
4
Gene Structure and Protein Synthesis
  • A gene is a distinct sequence of nucleotides on a chromosome that codes for a specific polypeptide chain, dictating the tertiary structure and function of synthesized proteins.
5
Probability
  • The chance that an offspring has a given genotype or phenotype.
6
Daughter cell
  • Cell produced by cell division.
7
Protein synthesis
  • Making proteins from amino acids using the DNA code.
8
Monohybrid inheritance
  • Inheritance of one gene.
9
Chromosome
  • A thread-like structure of DNA carrying genes.
10
Mitosis
  • Cell division producing genetically identical cells.
11
Recessive
  • Expressed only when no dominant allele is present.
12
Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
13
Meiosis
  • Reduction division giving four genetically different haploid cells.
14
Homozygous
  • Having two identical alleles.
15
Sequence
  • The order of bases in DNA or amino acids in a protein.
16
Specific
  • Able to act on only one type of substrate.

Key terms

TermMeaning
CodominanceBoth alleles in a heterozygote contribute to the phenotype.
Multiple allelesMore than two alleles of one gene in a population.
AntigenA molecule on the surface of a cell, such as a red blood cell.
GenotypeThe alleles an individual has.
PhenotypeThe observable characteristic, such as blood group.

Avoid these mistakes

1
Saying codominant alleles blend — both are fully expressed.
2
Saying a person carries three ABO alleles — each person has two.
3
Saying IO is dominant — it is recessive.
4
Saying group O can be IAIO — group O is always IOIO.

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