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319 GENE ACTION - GENE ACTION PHENOTYPE = the type(form or...

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GENE ACTION PHENOTYPE = the type (form or appearance) and performance (function) of an individual that can be measured. Genes express themselves phenotypically in two general ways (1) non-additive and (2) additive. Genes may express themselves differently when alone or when they are in different combinations in the individual. NON-ADDITIVE GENE ACTION Different combinations of genes result in different phenotypes. However, the effects of the alleles result in modification . (1) DOMINANCE-RECESSIVENESS – one gene overcomes the influence of the second gene Genotype Phenotype BB black Bb black bb red ___________________________________________ MATINGS: Parents Offspring BB (black) X BB (black) All BB Bb X BB 1 BB; 1 Bb Bb X Bb 1 BB; 2 Bb; 1 bb (red) Bb X bb 1 BB; 1 bb bb X BB All Bb bb X bb All bb Dominant genes are USUALLY desirable in their effects, while recessive genes are USUALLY detrimental.
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VARIABLE EXPRESSIVITY Genes don’t always express themselves identically in two different individuals. Shades of black, shades of red, etc. Two general factors may be responsible for variable expressivity: 1. EXTERNAL FACTORS: environment – sunlight, climate, nutrition, health, etc. 2. INTERNAL FACTORS: hormones, Modifying Genes – those genes which modify (change) the expression of other genes in the same individual. P = G + E + (GXE) G = same E = external factors (GXE) = internal factors
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(2) PARTIAL DOMINANCE (no dominance; lack of dominance): one gene is not completely dominant to its allele. Both genes are expressed in the phenotype. The heterozygote is phenotypically different from either homozygote.
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