GCSEBiologyAQARevision NotesInheritance, Variation & EvolutionReproductionSex Determination

Sex Determination (AQA GCSE Biology): Revision Note

Exam code: 8461

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Dr Natalie Lawrence

Updated on

AQA BiologyAQA Combined Science: Trilogy: Biology

Human chromosomes

  • Ordinary human body cells contain 23 pairs of chromosomes

  • Twenty-two pairs control characteristics only, but one of the pairs carries the genes that determine sex

  • In females, the sex chromosomes are the same (XX)

  • In males, the sex chromosomes are different (XY)

Diagram comparing male and female karyotypes and showing the inheritance of sex chromosomes. The “Male karyotype” contains chromosome pairs with maternal chromosomes shown in red and paternal chromosomes in blue. Chromosome pair 1 is labelled, and the male sex chromosomes are labelled “X chromosome” and “Y chromosome”. The “Female karyotype” contains two X chromosomes, labelled “X chromosomes” and “Female sex chromosomes”. Below, sperm and egg cells illustrate sex-chromosome inheritance. Text states, “Half of all sperm will carry an X chromosome” and “Half of all sperm will carry a Y sex chromosome”. Both illustrated egg cells contain an X chromosome, with text stating, “All egg cells will carry an X chromosome”. Further text states, “If a child inherits a Y chromosome, it will be a boy.”
Egg cells always carry an X chromosome, while sperm carry either X or Y, giving an approximately equal chance of XX or XY offspring

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Determining sex

  • The inheritance of sex can be shown using a genetic diagram, known as a Punnett square

  • The X and Y chromosomes taking the place of the alleles usually written in the boxes 

Punnett square showing inheritance of human sex chromosomes. The mother is labelled “Mother (XX)” and produces gametes containing X chromosomes. The father is labelled “Father (XY)” and produces gametes containing either X or Y chromosomes. The possible offspring are XX, XX, XY and XY. Text states: “Usually we only represent alleles in a Punnett square. Inheritance of sex depends upon which sex chromosomes a person has, so this is the only time we use chromosomes, rather than alleles.” The offspring ratio is given as 1:1, with a “50% chance of a boy (XY)” and a “50% chance of a girl (XX)” each time fertilisation occurs.
There is a 50% chance of producing an XX offspring and a 50% chance of producing an XY offspring at each fertilisation

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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.

Dr Natalie Lawrence

Reviewer: Dr Natalie Lawrence

Expertise: Content Writer

Natalie has a MCantab, Masters and PhD from the University of Cambridge and has tutored biosciences for 14 years. She has written two internationally-published nonfiction books, produced articles for academic journals and magazines, and spoken for TEDX and radio.