Co-dominance and Sex-linked Characteristics

• Codominance - 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 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
• Alleles and the type of antigen they produce:

  • 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:

  A) A

  B) B

  C) AB

  D) O

  
• We can use genetic diagrams to predict the outcome of crosses that involve codominant alleles, e.g. how a parent with blood group A and a parent with blood group B can produce offspring with blood group O:

  • Parent with blood group A has the genotype IAIO
  • Parent with the blood group B has the genotype IBIO
  • We know these are their genotypes 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-
• Genetic diagram of how a parent with blood group A and blood group B can produce a blood group O child

  A) 1/4

  
• Alleles on the same chromosome are said to be linked
• Sex linked - when alleles that control a particular characteristic are found on the sex chromosomes
• In most cases, there are only alleles on the X chromosome as the Y chromosome is much smaller

  • Males only have one X chromosome, they are much more likely to show sex-linked recessive conditions e.g. red-green colour blindness and haemophilia
• Females have two copies of the X chromosome so they are likely to inherit one dominant allele that masks the effect of the recessive allele
• Carrier - a female with one recessive allele that is masked so 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 colour blindness is as follows:

  A) female

  B) normal

  C) female

  D) carrier

  E) male

  F) normal

  G) male

  H) colourblind

  I) 1/4

  J) male

  K) colourblind

  
• Co-dominance - if both alleles are present, both will be expressed in the phenotype
• Sex linkage applies to genes located on the sex chromosomes

  • expression and inheritance is different between males and females
• Genes located on the sex chromosomes are different
• Colour blindness is sex-linked
• Define sex-linked characteristics


  A gene located on a sex chromosome where the characteristics are more common in one sex than another