CIE IGCSE Biology

Revision Notes

17.2.2 Codominance & Sex-Linked Characteristics

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Codominance

  • 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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Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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