Revision Notes

17.1.1 Definitions

Defining Inheritance & Related Terms

  •  Inheritance is the transmission of genetic information from generation to generation
  • Chromosomes are located in the nucleus of cells
  • They are thread-like structures of DNA, carrying genetic information in the form of genes
  • A gene is a short length of DNA found on a chromosome that codes for a specific protein
  • This could be a structural protein such as collagen found in skin cells, an enzyme or a hormone
  • Genes control our characteristics as they code for proteins that play important roles in what our cells do


DNA, genes and chromosomes, IGCSE & GCSE Biology revision notes Genes are short lengths of DNA that code for a protein. They are found on chromosomes


  • Alleles are different versions of a particular gene. The ABO gene for blood group type has three alleles, IA, IB and IO
Extended Only

Diploid & Haploid Nuclei

  • All humans have 23 different chromosomes in each cell
  • In most body cells, not including the gametes (sex cells), we have 2 copies of each chromosome, leading to a total of 46 chromosomes
  • Nuclei with two sets of chromosomes are known as diploid nuclei
  • The gametes (egg and sperm cells) only have one copy of each chromosome, meaning they have a total of 23 chromosomes in each cell
  • Nuclei with one set of unpaired chromosomes are known as haploid nuclei

Exam Tip

An easy way to remember the difference between haploid and diploid is to remember:

Haploid = Half the normal number of chromosomes

It’s worth noting that the human diploid chromosome number is 46. In an exam, you may be given information about a different species, with a different number of chromosomes. Make sure you read exam questions carefully.

Author: Jenna

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.

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