AQA A Level Biology

Revision Notes

7.1.1 Key Terms in Genetics

Key Terms in Genetics

Genes & alleles

  • The DNA contained within chromosomes is essential for cell survival
  • Every chromosome consists of a long DNA molecule which codes for several different proteins
  • A length of DNA that codes for a single polypeptide or protein is called a gene
  • The position of a gene on a chromosome is its locus (plural: loci)
  • Each gene can exist in two or more different forms called alleles
  • Different alleles of a gene have slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome

Chromosomes showing gene and loci, downloadable AS & A Level Biology revision notes

Chromosomes showing genes, alleles and loci

Example of alleles

  • One of the genes for coat colour in horses is Agouti
  • This gene for coat colour is found on the same position on the same chromosome for all horses
  • Hypothetically there are two different forms (alleles) of that gene found in horses: A and a
  • Each allele can produce a different coat colour:
    • Allele A → black coat
    • Allele a → chestnut coat

Genotype & phenotype

  • The chromosomes of eukaryotic cells occur in homologous pairs (there are two copies of each chromosome)
  • As a result cells have two copies of every gene
  • As there are two copies of a gene present in an individual that means there can be different allele combinations within an individual
  • The genotype of an organism refers to the alleles of a gene possessed by that individual. The different alleles can be represented by letters
  • When the two allele copies are identical in an individual they are said to be homozygous
  • When the two allele copies are different in an individual they are said to be heterozygous
  • The genotype of an individual affects their phenotype
  • A phenotype is the observable characteristics of an organism

Example of genotype & phenotype

  • Every horse has two copies of the coat colour gene in all of their cells
  • A horse that has two black coat alleles A has the genotype AA and is homozygous. The phenotype of this horse would be a black coat
  • In contrast a horse that has one black coat allele A and one chestnut coat allele a would have the genotype Aa and is heterozygous


  • Not all alleles affect the phenotype in the same way
  • Some alleles are dominant: they are always expressed in the phenotype
    • This means they are expressed in both heterozygous and homozygous individuals
  • Others are recessive: they are only expressed in the phenotype if no dominant allele is present
    • This means that it is only expressed when present in a homozygous individual

Example of dominance

  • If for horses the allele A for a black coat is dominant and the allele a for a chestnut coat is recessive the following genotypes and phenotypes occur:
    • Genotype AA → black coat
    • Genotype Aa → black coat
    • Genotype aa → chestnut coat


  • Sometimes both alleles can be expressed in the phenotype at the same time
  • This is known as codominance
  • When an individual is heterozygous they will express both alleles in their phenotype
  • When writing the genotype for codominance the gene is symbolised as the capital letter and the alleles are represented by different superscript letters, for example IA

Example of codominance

  • A good example of codominance can be seen in human blood types
  • The gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O
  • Allele A results in blood type A (IAIA or IAIO) and allele B results in blood type B (IBIB or IBIO)
  • If both allele A and allele B are present in a heterozygous individual they will have blood type AB (IAIB)
  • Blood type O (IOIO) is recessive to both group A and group B alleles

F1, F2 & test crosses

  • When a homozygous dominant individual is crossed with a homozygous recessive individual the offspring are called the F1 generation
    • All of the F1 generation are heterozygous
  • If two individuals from the F1 generation are then crossed, the offspring they produce are called the F2 generation
  • A test cross can be used to try and deduce the genotype of an unknown individual that is expressing a dominant phenotype
    • The individual in question is crossed with an individual that is expressing the recessive phenotype
    • The resulting phenotypes of the offspring provide sufficient information to suggest the genotype of the unknown individual
    • If there are any offspring expressing the recessive phenotype then the unknown individual must have a heterozygous genotype


  • There are two types of linkage in genetics: sex linkage and autosomal linkage
  • Sex linkage:
    • There are two sex chromosomes: X and Y
    • Women have two copies of the X chromosome (XX) whereas men have one X chromosome and one shorter Y chromosome (XY)
    • Some genes are found on a region of a sex chromosome that is not present on the other sex chromosome
    • As the inheritance of these genes is dependent on the sex of the individual they are called sex-linked genes
    • Most often sex-linked genes are found on the longer X chromsome
    • Haemophilia is well known example of a sex-linked disease
    • Sex-linked genes are represented in the genotype by writing the alleles as superscript next to the sex chromosome. For example a particular gene that is found only on the X chromosome has two alleles G and g. The genotype of a heterozygous female would be written as XGXg. A males genotype would be written as XGY
  • Autosomal linkage:
    • This occurs on the autosomes (any chromosome that isn’t a sex chromosome)
    • Two or more genes on the same chromosome do not assort independently during meiosis
    • These genes are linked and they stay together in the original parental combination

Exam Tip

When referring to the different alleles be careful about your notation. When describing a dominant allele use capitals (for example allele B ) and when describing a recessive allele use lower case ( for example allele b ). Be careful when choosing the letters to represent the alleles when writing the genotype. Use letters that are easy to distinguish between the capital and the lower case (eg. B and b).


Alistair graduated from Oxford University in 2014 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 and Societies.

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