Edexcel International A Level Biology

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3.9 Gene Locus

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Gene Loci

  • 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

Gene Linkage

  • Gene loci are said to be linked if they are on the same chromosome
    • Loci (singular: locus) refers to the specific linear positions on the chromosome that genes occupy
  • Linked genes located on human chromosomes 1 to 22 (i.e. any chromosome that is not a sex chromosome, known as autosomes) are said to be examples of autosomal linkage
  • If genes are located on the same sex chromosome, they are said to be sex-linked

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
    • A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring
    • If that offspring is a male, he will have the disease
  • The presence of sex linkage can be identified using pedigree diagrams and Punnett squares
    • When a gene is sex-linked the phenotypes are not spread evenly across the sexes
    • In the case of a gene that causes a sex-linked disease, one sex will be disproportionately affected
    • The results of a cross between a normal male and a female who is a carrier for colour blindness are shown below. In this cross, there is a 25% chance of producing a male who is colourblind, a 25% chance of producing a female carrier, a 25% chance of producing a normal female and a 25% chance of producing a normal male

X-linked genetic cross

Punnett square showing the inheritance of colourblindness, an X-linked condition

  • Working in the USA in the early 20th century, a scientist called Thomas Hunt Morgan bred fruit flies (Drosophila melanogaster) over successive generations
  • In his cross-breeding experiments, he came across red-eyed wild types and white-eyed mutants
  • He realised there was a distinct sex bias in the phenotypic distribution of the offspring:
    • All-female offspring of a red-eyed male were red-eyed while all male offspring of a white-eyed female were also white-eyed
  • Morgan hypothesised that this occurred because the gene for eye colour was located on a sex chromosome (i.e. it was X-linked)

Sex linkage in drosophila

Sex linkage in Drosophila. A cross between a homozygous white-eyed female and a male with red eyes gives all white-eyed males and red-eyed female offspring

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Author: Alistair

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.