Codominance & Sex-Linked Characteristics: Extended (CIE IGCSE Biology)

Codominance

• Codominance occurs when both alleles in heterozygous organisms contribute to the phenotype
• Inheritance of blood group is an example of codominance

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

  

• 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’

Punnett square showing the inheritance of Blood Group

• The parent with blood group A has the genotype I^AI^O
• The parent with the blood group B has the genotype I^BI^O
• 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

• Alleles on the same chromosome are said to be linked
• 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:

 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