Sex Chromosomes (Edexcel IGCSE Biology)

Revision Note

Sex 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

Karyograms

The sex chromosomes that a biological female and male possess.

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Inheritance of Sex

  • Sex is determined by the two sex chromosomes, X and Y

  • Males carry chromosomes XY and females XX

  • Due to this it is the male who determines the sex of offspring as

    • The mother can only pass on an X chromosome

    • The father can pass on either an X or a Y chromosome

  • Therefore the offspring can inherit:

    • An X from the mother and an X from the father: this child will be a female

    • An X from the mother and a Y from the father: this child will be a male

Sex chromosomes, IGCSE & GCSE Biology revision notes

Sperm cells determine the sex of offspring

Genetic diagrams showing determination of sex

  • 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

Inheritance of sex, IGCSE & GCSE Biology revision notes

Punnett square showing the inheritance of sex

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology Lead

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.