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Chapter 4

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Robert Boulianne

Chapter 4 – Modification of Mendelian Ratios • Wild Type Allele – Allele that occurs most frequently in population – Designated as “+” or “wt” • Mutant Allele – Loss of Function mutation • Null mutation if complete loss of function – Gain of Function mutation • Usually dominant – Neutral mutation • No effect Incomplete Dominance • When one allele is not completely dominant to another then it is said to show incomplete or partial dominance. • The phenotype of the heterozygote is intermediate to those of the homozygotes. • Classic example: – Flower colour in snapdragons: • Red flowered x White flowered – Progeny are all pink flowered Palomino Horses: Incomplete Dominance • Palominos do not breed true when bred together: – Progeny are 1/4 cremellos (extremely light colored), 1/2 palominos and 1/4 light chestnuts. – 1:2:1 ratio is characteristic of incomplete dominacre. cr cr • C/C produces a horse that is light chestnut (or sorrel), C/C palamino, and C /C the cremello. – Other coat color genes are necessary but are at different loci. – It is thought that there up to 7 different loci that contribute to the coat color of horses. • In other words the palamino is a slight dilution of the sorrel, and the cremello is a further dilution to give an even paler colour. Codominance • The heterozygote displays both phenotypes of the two homozygotes. – Incomplete dominance displayed intermediate phenotype between the two homozygotes. • The ABO blood group i and i alleles are codominant alleles. – Individuals can be AB blood type. • Human M-NMbloMd group. M N N N – L /L (homozygote), L L (heterozygote), L L (homozygote). – Antigens present on surface of RBCs. The Nature of the ABO Blood Groups • Red blood cells (RBCs) contain complex polysaccharides linked to the lipids in their membranes. – These polysaccharides are on the outside of the RBCs. – These are antigens, since in organisms where this molecule is foreign, antibodies will be produced against it. • People with type A blood have A antigens on their RBCs, while type B individuals have type B antigens. Type AB individuals have both A and B antigens present on their RBCs. • ABO locus encodes glycosyltransferases, enzymes that add sugar groups to existing polysaccharides. • RBCs have a precursor glycolipid called the H antigen present. – The A allele produces a glycosyltransferase enzyme that adds a -N- acetylgalactosamine to the H antigen. • Produces the A antigen. – The B allele produces a different glycosyltransferase that adds galactose to the H antigen. • Produce the B antigen. – Type O individuals lack these glycosyltransferase enzymes, so the H antigen is unmodified. Antibodies to the ABO Antigens • People with blood type O – Have anti-A and anti-B antibodies • People with blood type A – Have anti-B antibodies • People with blood type B – Have anti-A antibodies • People with blood type AB – Have not antibodies to A or B • What about the H antigen? – It is shared amongst all these individuals so antibodies are not raised against this antigen. • The H antigen is produced by the H allele (FUT1 gene) at a distinct locus from the ABO locus. • People who are homozygous h/h do not make the H antigen. – Regardless of ABO locus, these individuals are similar to blood type O individuals. • They lack the A and B antigens. – Produce anti-O antibodies (antibodies against the H antigen). – These people have the Bombay phenotype. Lethal Alleles • Gene products essential to an organism’s survival need to be present. – One copy may be sufficient, although in some cases it may lead to developmental abnormalities. • Eg. Stunted growth • Two types of lethal alleles: – Recessive lethals -essentially as described above • Eg yellow coat colour allele in mice – Dominant lethal - only need one copy for lethality • Eg Huntington disease Epistasis • Gene interaction where several genes may influence a particular trait. • Epistasis is where one gene or gene pair modifies or masks the expression of another gene. – Three types: • Recessive epistasis eg. Bombay phenotype over ABO locus • Dominant epistasis eg. Fruit color in squash • Duplicate Recessive eg. White flowered sweat peas – Also called, Complementary Gene Action Dominant Epistasis • In dominant epistasis, A_B_ and A_bb individuals have the same phenotype. – Observe 12:3:1 ratio rather than 9:3:3:1 ratio. – One gene when dominant is epistatic to the other gene. • Summer squash example: – Three common fruit colours: white, yellow and green. • White x Yellow gi
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