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Meiosis and variation Lesson 6 Interactions between gene loci part 1 Learning objectives You should be able to: • Describe the interactions between loci (epistasis). (Production of genetic diagrams is not required) • Predict phenotypic ratios in problems involving epistasis Interaction between alleles for different genes It is possible for different genes at different loci to interact to affect the phenotype. This can work in two ways 1) The two genes may be antagonistic which means they work against each other. If one gene masks the effect of the other this is called epistasis 2) They may work in complementary fashion 1) Working antagonistically Epistasis may be dominant or recessive. In other words, the alleles at the first locus that masks the action of the second may be dominant or recessive. The alleles at the first locus are said to be epistatic The alleles at the second locus, the action of which is being masked, are referred to as hypostatic Recessive epistasis Flower colour in Salvia (the herb sage) is a good example Two gene loci are involved. These are described as A/a and B/b. In other words, both loci may have a dominant or a recessive allele At the second locus the B allele produces purple flowers and the b allele pink if present in homozygous form BUT………….. If both recessive alleles are present at the first locus (aa) then neither allele at the second locus can be expressed (bb, Bb or BB) and the flowers will be white Dominant epistasis This is where the dominant allele at one locus masks the expression of the alleles at the second locus Summer squash (looks like a marrow) is a good example Two gene loci are involved, D/d and E/e The E allele produces yellow fruits and the e allele green fruits BUT……….. If the D allele is present at the first locus then the alleles at the second locus are not expressed and the fruit is white More dominant epistasis Feather colour in some chickens is another example This is controlled by alleles at two gene loci, I/i and C/c The dominant allele C codes for coloured feathers and the recessive allele c for white BUT………….. If the dominant allele I is present at the first locus then the alleles at the second locus are never expressed and the feathers are always white Chicken breeding Pure breeding White Leghorn chickens have the genotype IICC Pure breeding white Wyanodotte chickens have the genotype iicc If these two breeds are mated then all progeny will be white with genotype IiCc If these progeny interbreed they will produce white and coloured feather chickens in a 13:3 ratio 2) Genes at different loci working in complementary fashion Sweet peas may have white and purple flowers This is controlled by alleles at two loci, C/c and R/r Flowers are always white unless one dominant allele is present at both loci This is because the first gene codes for an intermediate colourless pigment, if the dominant C allele is present The second gene codes for an enzyme that converts the intermediate compound to the purple pigment, if the dominant R allele is present Summary of mechanism by which primrose flower colour is determined Gene C/c Precursor substance (colourless) Gene R/r Intermediate compound (colourless) Final pigment (purple