IGCSEBiologyCambridge (CIE)Revision Notes17. InheritanceInheritance, Genes & Cell DivisionThe Inheritance of Sex

The Inheritance of Sex (Cambridge (CIE) IGCSE Biology): Revision Note

Exam code: 0610 & 0970

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Jenna Quinn

Updated on

XX & XY chromosomes

  • 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, and of those:

    • Around half will be carrying an X chromosome, and if one of these sperm fertilises the egg, the fetus will be female

    • The other half will be carrying a Y chromosome and if one of these sperm fertilises the egg, the fetus will be male

Diagram comparing male and female karyotypes, showing X and Y chromosomes, sperm and egg cells, and how inheritance of X or Y determines a child’s sex
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

Diagram of a Punnett square showing XX mother and XY father, giving 50% chance of XY male and 50% chance of XX female offspring each fertilisation
Punnett square showing the inheritance of sex

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Author
Reviewer
Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Content Creator

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Jenna Quinn

Reviewer: Jenna Quinn

Expertise: Content Creator

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.