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Gene Linkage (HL) (HL IB Biology)

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Autosomal 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
  • If genes are on the sex chromosome, they are said to be sex-linked
    • Sex-linked genes have characteristics that generally only affect one gender of a species
    • These genes are usually on the X chromosome because the Y chromosome contains very few genes
    • In humans, colour-blindness and haemophilia are notable examples of genetic conditions that only affect males
  • Linked genes located on the chromosomes 1-22, or any chromosome that is not a sex chromosome (called autosomes) are said to be examples of autosomal linkage
  • The likelihood of genes being inherited together, or the extent to which they are linked, is measured in units called centimorgans, in honour of Thomas Hunt Morgan's work

Notation for linked genes

  • A common way of denoting linked alleles is to link them with lines, representing the homologous chromosomes
    • E.g. in a cross between a double heterozygous individual where the dominant alleles G and R are linked and the recessive alleles g and r are linked, the notation would be:
linked-genes-option-d

Autosomal linkage

  • Dihybrid crosses and their predictions rely on the assumption that the genes being investigated behave independently of one another during meiosis
  • However, not all genes assort independently during meiosis
  • Some genes which are located on the same chromosome display autosomal linkage and stay together in the original parental combination
  • Linkage between genes affects how parental alleles are passed onto offspring through the gametes

Identifying autosomal linkage from phenotypic ratios

  • In the following theoretical example, a dihybrid cross is used to predict the inheritance of two different characteristics in a species of newt
    • The genes are for tail length and scale colour
  • The gene for tail length has two alleles:
    • Dominant allele T produces a normal length tail
    • Recessive allele t produces a shorter length tail
  • The gene for scale colour has two alleles:
    • Dominant allele G produces green scales
    • Recessive allele g produces white scales

Without linkage

  • The outcomes for this dihybrid cross if the genes are unlinked are as follows

Dihybrid Cross without Linkage Punnett Square Table


Dihybrid cross unlinked genes, downloadable AS Level & A Level Biology revision notes

  • Predicted ratio of phenotypes in offspring =
    • 1 normal tail, green scales : 1 normal tail, white scales : 1 short tail, green scales : 1 short tail, white scales
  • Predicted ratio of genotypes in offspring =
    • 1 TtGg : 1 Ttgg : 1 ttGg : 1 ttgg

With linkage

  • However, if the same dihybrid cross is carried out but this time the genes are linked, we get a different phenotypic ratio
    • There would be a 1 : 1 phenotypic ratio (1 normal tail, green scales : 1 short tail, white scales)
    • This change in the phenotypic ratio occurs because the genes are located on the same chromosome
    • The unexpected phenotypic ratio, therefore, shows us that the genes are linked
  • The explanation for this new phenotypic ratio is given in the worked example below:

Worked example

Worked example: Explaining autosomal linkage

  • In reality, the genes for tail length and scale colour in this particular species of newt show autosomal linkage

Parental phenotypes: normal tail, green scales x short tail, white scales

Parental genotypes:       (TG)(tg)             (tg)(tg)

Parental gametes:       (TG) or (tg)              (tg)

Dihybrid Cross with Linkage Punnett Square Table

Dihybrid Cross with Linkage Punnett Square table, downloadable AS & A Level Biology revision notes

  • Predicted ratio of genotypes in offspring =
    • 1 (TG)(tg) : 1 (tg)(tg)
  • Predicted ratio of phenotypes in offspring =
    • 1 normal tail, green scales : 1 short tail, white scales

Examiner Tip

When you are working through different genetics questions you may notice that test crosses involving autosomal linkage predict solely parental type offspring (offspring that have the same combination of characteristics as their parents). However in reality recombinant offspring (offspring that have a different combination of characteristics to their parents) are often produced.  This is due to the crossing over that occurs during meiosis. The crossing over and exchanging of genetic material breaks the linkage between the genes and recombines the characteristics of the parents. So if a question comes along that asks you why recombinant offspring are present you now know why!

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Emma

Author: Emma

Expertise: Biology

Prior to working at SME, Emma was a Biology teacher for 5 years. During those years she taught three different GCSE exam boards and two A-Level exam boards, gaining a wide range of teaching expertise in the subject. Emma particularly enjoys learning about ecology and conservation. Emma is passionate about making her students achieve the highest possible grades in their exams by creating amazing revision resources!