Codominance & Sex Determination (Cambridge O Level Biology)

• 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:

Blood types phenotypes and genotypes table

Genotype	Phenotype
IAIA or IAIO	A
IBIB or IBIO	B
IAIB	AB
IOIO	O

  

• 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 Punnett square

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

• Sex is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a number of genes)
• Females have the sex chromosomes XX
• Males have the sex chromosomes XY
• As only a father can pass on a Y chromosome, he is responsible for determining the sex of the child
• He does this because:
  • He produces (ejaculates) around 250 million sperm cells during sexual intercourse
  • Of those, half (125 million sperm) will be carrying his X chromosome
    • If one of these sperm fertilises the egg, the fetus will be female
  • The other 125 million of his sperm will be carrying his Y chromosome
    • Which will result in a male fetus if one of these fertilises the egg

Sex chromosomes in the male and female gametes diagram

Sperm cells determine the sex of offspring

• The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes

Sex determination Punnett square

Punnett square showing the inheritance of sex. There is always a 50% chance of a boy child and a 50% of a girl child.