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Biol 121 225 Freeman 282-286 Ch. 13 – Extending Mendel’s Rules – From Genotype to Phenotype Complete dominance

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BIOL 121

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Biol 121 225 Freeman 282-286 Feb. 10, 10 Ch. 13 – Extending Mendel’s Rules – From Genotype to Phenotype Complete dominance -alleles are either dominant or recessive, each allele codes for different phenotypes -there is no intermediate phenotype (e.g. black and white only, no grey) -when an individual is heterozygous for a gene (one dominant allele, one recessive), the individual will have the dominant phenotype -the individual will only have the recessive phenotype if the individual has two recessive alleles What happens when ratios are -when ratios are not what is expected, then complete dominance is not the case not as expected based on complete dominance of a single -it will be either incomplete dominance or codominance instead. gene? Incomplete dominance e.g. Four o’ clock flowers P: Purple x White flowers F1: Expect all purple (if purple is dominant), but instead get all lavender -this is the intermediate phenotype (lavender = b/t purple and white) Lavender x Lavender F2: 1 purple : 2 lavender : 1 lavender (RR : 2 Rr : rr – same genotype as in complete dominance inheritance, but different phenotype) Codominance -when codominance occurs, heterozygotes (individuals that are heterozygous for a gene) have the phenotype associated with both alleles present -for example, blood type has 4 phenotypes: Type A, B, AB and O A A IAI – A I i – A I I – AB I i – B B B I I – B ii – O -This is not incomplete dominance because no intermediate property – the phenotype AB is not an intermediate phenotype b/t A and B – it is simply both A and B. -However I and I are completely dominant over ‘i' Pleiotropy -as far as is known, alleles Mendel analyzed affect only a single trait -the gene for seed colour doesn’t seem to affect other characteristics for example -sometimes, a single allele may affect a wide variety of traits -a gene that influences many traits instead of just one trait is pleiotropic -the condition of cystic fibrosis involves a pleiotropic gene that has more than one phenotypic effect in the individual Summary of NON-Mendelian 1) Incomplete dominance: phenotypic ratio = genotypic ratio ratios for one gene 2) Codominance: See product of both alleles in a heterozygote 3) Pleiotropy – multiple phenotypical effects Gene Interactions What happens when the products of different genes interact? -five broad categories of interactions (not mutually exclusive): 1) Both gene products apparent 2) Combinations produce new phenotypes 3) Complementation 4) Epistasis 1 Biol 121 225 Freeman 282-286 Feb. 10, 10 5) Quantitative 1) Both gene products apparent -e.g. coat colour in mice Gene 1: Pigment is black or brown Gene 2: Presence of yellow tip Possible phenotypes are therefore: Black with yellow tip (agouti), Brown with yellow tip (cinnamon), black w/o tip (fully black), and brown w/o tip (fully brown) B = black, b = brown, Y = yellow, y = not yellow P: BBYY (agouti) x bbyy (brown) F1: BbYy (agouti) F2: 9 agouti : 3 black : 3 cinnamon : 1 brown 2) Combinations result in new e.g. Comb type in chickens: Walnut, rose, pea, single combs phenotypes and… 3) Complementation P: RRpp (rose) x rrPP
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