Interactions Between Unlinked Genes 7
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Interactions Between Unlinked Genes 7

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A2 biology edexcel

A2 biology edexcel

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Interactions Between Unlinked Genes 7 Interactions Between Unlinked Genes 7 Presentation Transcript

  • Interactions between unlinked genes
    • Starter activity:
    • Explain the difference between ‘autosomal linkage’ and ‘sex linkage’
    • Autosomal Linkage refers to genes being on the same chromosome.  These genes tend to show up together in the same combinations in the offspring
    • Sex linkage If a gene is located on the X chromosome, the male has the problem that he does not have a homologue for this chromosome. He has a Y chromosome that doesn't have the same information on it as the X. Therefore, whatever allele he inherits from his mother's X chromosome, he expresses that allele
  • Learning objectives
    • Explain the inheritance of 2 non-interacting unlinked genes
    • Bivalent
    • Pair of two homologous chromosomes formed by synapsis during the early stages of meiosis.
    • Bivalents separate into half bivalents during first meiotic division.
    • Shape of bivalent will depend upon:
    • number of chiasmata
    • position of chiasmata
    • size of chromosome
    •                                                                                                                                              
  • Unlinked genes
    • There are instances when a single character is influenced by 2 or more unlinked genes. Bateson and Punnet did research of unlinked genes such as the ‘combs’ in poultry.
    • There are 2 types of combs (‘pea’ combs and ‘rose’ combs)
    • The F1 were ‘true’ bred
    • Produced ‘walnut’ comb and ‘single’ comb
    • The phenotype occurred in the ratio of 9:3:3:1
  • Cont….
    • They realised that 2 genes were involved
    • 2 dominant alleles ‘ P ’ = pea comb and ‘ R ’ = rose comb
    • So the genotypes of the parents in the original test cross (PPrr (pea comb) and ppRR (rose comb)
    • F1 genotype was PpRr interacting to give a ‘walnut’ comb
    • F2 genotype with no dominant alleles pprr to give a ‘single’ comb
  • Cont…
    • What was realised was that the ratio was the same as ‘Mendel’ dihybrid cross (9:3:3:1)
    • This indicated that the 2 genes were inherited independently
    • ‘walnut’ and ‘single’ combs were new forms of character due to the interaction of the genes and were not considered to be recombinants
    • There are examples gene interaction where the ratio is different
  • Parents Short Long SStt ssTT Gametes St sT F1 X F1 Medium Medium SsTt SsTt Gametes ST St sT st ST St sT st F2
  • Further investigation
    • Bateson and Punnet did a further investigation between 2 ‘pure-breeding’ white-flowered plants sweet pea plants. This produced offspring with ‘purple’ flowers. The purple-flowered plants were allowed to self-pollinate to which the resulting progeny were (purple 9 : white 7) ratio.
    • This suggested that one gene (C) controlled the scent of the pigment and the other gene (P) controlled the conversion to its purple form
    • For the purple colour to develop, both genes must be present ( C, P)
    • For this to occur both white-flowered plants must be homozygous dominant for one of the genes, for the offspring to be purple
  • Cont… Flower scent pigment Purple precursor pigment Controlled by gene P Controlled by gene C Only produced if allele P present Only produced If both alleles P and C present
  • Parents Red rose Red rose SSrr ssRR Gametes Sr sR F1 X F1 Red Red SsRr SsRr Gametes SR Sr sR sr SR Sr sR sr F2