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GENE INTERACTION AND EPISTASIS

GENE INTERACTION AND EPISTASIS. Further information and questions. Epistasis. A form of gene interaction in which a dominant allele at one locus totally inhibits the expression of an allele at a second locus. E.g. pronounced banding in the shell of the snail Cepaea nemoralis.

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GENE INTERACTION AND EPISTASIS

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  1. GENE INTERACTION AND EPISTASIS Further information and questions...

  2. Epistasis • A form of gene interaction in which a dominant allele at one locus totally inhibits the expression of an allele at a second locus. • E.g. pronounced banding in the shell of the snail Cepaea nemoralis.

  3. The banding exists in three common variants and is controlled by different genes on separate chromsomes. • One gene determines whether or not bands are present. The allele for absence of bands (A) is dominant to the allele for presence (a).

  4. A second gene determines the number and position of the bands. A dominant allele codes for a ‘mid-banded’ condition. • A recessive allele codes for a ‘five-banded condition. • The banded characteristic will be expressed when the genotype for banding is aa.

  5. Simple gene interaction • The shape of the comb of farmyard fowl is an example of a single character determined by two different genes. • The product of these interactions are 4 possible phenotypes:- • Pea • Rose • Walnut • single

  6. Rose Pea Simple Walnut

  7. The genes involved, represented by the letters P and R, interact to form phenotypes:- • Pea – PPrr or Pprr • Rose – ppRR or ppRr • Walnut – PPRR or PpRR or PPRr or PpRr • Single - pprr

  8. Conclusion • If the dominant P allele is present in the genotype the phenotype will be pea comb • If the dominant R allele is present in the genotype the phenotype will be Rose comb • If the both dominant P and Ralleles are present in the genotype the phenotype will be walnut comb • If neither of the two dominant alleles are present the phenotype will be single comb

  9. In other words, pea and rose combs are the product of dominant alleles for the character in question combined with a recessive allele for the other. • Walnut is a product of the interaction of dominant alleles. • Single is produced in the absence of dominant alleles.

  10. Do the following cross • PPrr(Pea) x ppRR (Rose) • Find the F1 generation phenotype and genotypes • Cross the F1 generation and determine the F2 genotypes and phenotypes

  11. POLYGENETIC INHERITANCE • Very few characteristics of an organism are controlled by a single gene. • Characteristics are generally controlled by a number of genes known as polygenes. • These genes are often located on different chromosomes.

  12. The effects of any one of these gene on the phenotype is very small or insignificant. • But the combined effect of all the genes is to produce an infinite variety among the offspring. • Many features of humans are controlled by polygenes, including height. • In fact the number of genes controlling a characteristic does not have to be large before the variation of the phenotype becomes more or less continuous.

  13. Environmental Effects • If plants of a tall variety are deprived of nutrients (e.g. nitrates and phosphates) in the growing phase of development then full size may not be reached. • A ‘tall’ plant may appear dwarf. • The same might arise with humans who have been seriously and continuously underfed as children.

  14. Many characteristics are affected by both the environment and their genotype. • Phenotype = genotype + influences of the environment

  15. In colonies of the honey be there are 3 different phenotypes – • Workers, drones and queens • But only 2 different genotypes • The phenotypes are determined by both genotype and diet

  16. The drones are the males and they develop from unfertilised eggs – they are haploid. • The queen and the workers develop from fertilised eggs - they are diploid but both have identical genotypes. • The queen, who is a much larger organism, differs from the workers only by the diet she is fed in the larval stage.

  17. For the first three days after hatching all female larvae receive ‘royal jelly’, a secretion from the glands of workers who rear the young. • A queen is kept on this diet until she pupates • But the workers who make up the majority of the colony are switched to a diet of pollen and nectar after the first three days. • It is this dietary difference that is responsible for the two sharply contrasting phenotypes.

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