Interactions Between Unlinked Genes 7
- 1. 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
- 2. 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
- 3. 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
- 4. 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
- 5. 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
- 6. 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
- 7. 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
- 8. 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
- 9. 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