A LevelBiologyCambridge (CIE)Revision Notes16. InheritancePassage of Information from Parents to OffspringHomologous Chromosomes

Homologous Chromosomes (Cambridge (CIE) A Level Biology): Revision Note

Exam code: 9700

Cara Head

Written by: Cara Head

Reviewed by: Alistair Marjot

Updated on

Homologous chromosomes

  • In diploid cells there are two complete sets of chromosomes in the nucleus

  • Chromosomes have a characteristic shape

    • They have a fixed length

    • The position of the centromere is in a particular location

  • These characteristic features allow for each chromosome to be identified in a photomicrograph

  • In photomicrographs chromosomes are often grouped into their homologous pairs

  • Homologous chromosomes:

    • Carry the same genes in the same positions (locus)

    • Are the same shape

  • During fertilisation a diploid zygote is formed

    • In a zygote one chromosome of each homologous pair comes from the female gamete and the other comes from the male gamete

  • Having the same genes in the same order helps homologous chromosomes line up alongside each other during meiosis

Diagram showing labelled chromosomes 1 to 22, X and Y; inset highlights gene positions on homologous chromosomes with identical length.
Human karyogram showing homologous chromosomes

Examiner Tips and Tricks

Although homologous pairs of chromosomes contain the same genes in the same order they don’t necessarily carry the same alleles (form) of each gene! i.e. We all have the genes to create eyes, some have the allele to create blue eyes and others the allele for brown eyes, for example. 

(A karyogram, as seen in the image, is an organised picture of an organism’s chromosomes arranged in pairs by size and shape.)

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    Author
    Reviewer
    Cara Head

    Author: Cara Head

    Expertise: Biology & Psychology Content Creator

    Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding

    Alistair Marjot

    Reviewer: Alistair Marjot

    Expertise: Environmental Systems and Societies & Biology Content Creator

    Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.