3.22 describe the determination of the sex of offspring at fertilisation, using a genetic diagram

  • Sex is inherited by sex chromosomes.
  • 2 xx chromosomes make a female, one xy chromosome makes a person male
  • Half of the sperm contain x chromosomes and the other half contains a y.
  • The type of chromosome that fuses with the egg determines the gender of the zygote (to be baby) – therefore there is an equal chance for both genders


  • When drawing a punnett square, females are homozygous, as they have XX chromosomes – they  only have one column.  

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