Revision Notes

17.2.2 Codominance & Sex-Linked Characteristics

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  • Inheritance of blood group is an example of codominance
  • There are three alleles of the gene governing this instead of the usual two
  • Alleles IA and IB are codominant, but both are dominant to IO
  • I represents the gene and the superscript A, B and O represent the alleles
  • IA results in the production of antigen A in the blood
  • IB results in the production of antigen B in the blood
  • IO results in no antigens being produced in the blood
  • These three possible alleles can give us the following genotypes and phenotypes:


Inheritance of Blood Group table, IGCSE & GCSE Biology revision notes


  • We can use genetic diagrams to predict the outcome of crosses that involve codominant alleles:

‘Show how a parent with blood group A and a parent with blood group B can produce offspring with blood group O’

Blood group inheritance, IGCSE & GCSE Biology revision notesPunnett square showing the inheritance of Blood Group


  • The parent with blood group A has the genotype IAIO
  • The parent with the blood group B has the genotype IBIO
  • We know these are their genotypes (as opposed to both being homozygous) as they are able to produce a child with blood group O and so the child must have inherited an allele for group O from each parent
  • Parents with these blood types have a 25% chance of producing a child with blood type O
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Sex-Linked Characteristics

  • When alleles that control a particular characteristic are found on the sex chromosomes, we describe the inheritance that results as ‘sex linked’
  • In almost all cases, there are only alleles on the X chromosome as the Y chromosome is much smaller
  • Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia)
  • Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele
  • A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring
  • If that offspring is a male, he will have the disease
  • The results of a cross between a normal male and a female who is a carrier for colourblindness is as follows:

X-linked genetic cross, IGCSE & GCSE Biology revision notes Punnett square showing the inheritance of colourblindness, an X-linked condition


  • In the cross above, there is a 25% chance of producing a male who is colourblind, a 25% chance of producing a female carrier, a 25% chance of producing a normal female and a 25% chance of producing a normal male

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