Genes & Linkage (Edexcel A (SNAB) A Level Biology)

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Locations of Genes on a Chromosome

  • Every chromosome consists of a long DNA molecule that contains several hundred or even thousands of different genes coding for 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 known as its locus (plural: loci)
    • Through experiments and genetic mapping techniques, scientists have been able to work out the specific physical locations of the genes on different chromosomes
    • Each gene occupies a specific locus so that the gene for a particular characteristic is always found at the same position on a particular chromosome
  • 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

Five different genes found at five different loci

Autosomal Linkage

  • As its name implies, autosomal linkage only occurs on the autosomes (any chromosome that isn’t a sex chromosome)
  • Two or more genes on the same autosome do not assort independently during meiosis
  • Instead, these genes are linked and they stay together in the original parental combination
  • These linked genes are passed on to offspring all together (through the gametes)

Sex Linkage

  • There are two sex chromosomes: X and Y
  • Females have two copies of the X chromosome (XX), whereas males have one X chromosome and one shorter Y chromosome (XY)
  • Some genes are only present on one sex chromosome and not the other
  • As the inheritance of these genes is dependent on the sex of the individual they are known as sex-linked genes
    • Most often sex-linked genes are found on the longer X chromosome
  • If the gene is on the X chromosome, males (XY) will only have one copy of the gene, whereas females (XX) will have two
    • Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia)
    • Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele

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Alistair

Author: Alistair

Expertise: Biology & Environmental Systems and Societies

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.