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ch 13 extensions of and deviations from mendelian genetic principles.docx

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University of Toronto Mississauga
Karen Williams

Ch. 13: Extensions of and deviations from Mendelian genetic principles -mendels principle apply to all diploid eukaryotic organisms and form the foundation for predicting the outcome of crosses in which segregation and independent assortment occur Multiple Alleles -in a population of individuals, a given gene may have several alleles (often one wild type and the rest mutant). Such genes are said to have multiple alleles, and the alleles are said to constitute a multiple allelic series Number of alleles kinds of genotypes kinds of homozygotes kinds of heterozygotes n n(n+1)/2 n n(n-1)/2 ex. 5 15 5 10 ABO Blood groups -an example of multiple alleles of a gene is found in the human ABO blood group series. -O, A, B, and AB are the four blood group phenotypes in the ABO system -different combinations of three alleles of the ABO blood group gene I , I , and i. give rise to the four pehnotypes, -O i/i -A I /I or I /i B B B -B IA/BI or I /i -AB I /I -with blood transfusions, the blood types of donors and recipient must be carefully matched, because the blood group alleles specify molecular groups, called cellular antigens, that are attached to the outsides of red blood cells. -antigen: is any molecule that is recognized as foreign by an organism and that therefore sitmulates the production of specific protein molecules called antibofies, which bind to the antigen -antibody: a protein molecule that recognizes and binds to the foreign substance (antigen) introduces into the orgniasm as part of the immune response tor emove the foreign antigen from the body. -except in autoimmune diseases, antigens are not recognized as foreign by the organism expressing them -the I allele gene encodes a product that is needed for the biosynthesis of the A antigen but that is not involved in the biosynthesis of the B antigen. People of the blood type A have only the A antigen on their red blood cells and therefore the B antigen is foreign to them. Blood serum prepared from them contains naturally occurring antibodies against the B antigen (called anti-B antibodies), but none against the A antigen. Antibodies against the B antigen agglutinate, or clump, any red blood cells that have the B antigen on them. Since clumped cells cannot move through the fine capillaries, agglutination may lead to organ failure and, possibly, death -The I allele of the ABO blood group gene encodes a product needed for biosynthesis of the B antigen, but it is not involved in biosynthesis of the A antigen. Therefore, people of blood type B have the B antigen on their red blood cells, and their blood serum contains naturally occurring anti-A antibodies but no anti-B antibodies, People of AB blood type have both A and B antigens on the blood cells and neither anti-A nor Anti-B antibodies in their blood stream -the I allele encodes no functional products involved in the synthesis of either the A or the B antigen. Therefore people with blood type O, the red blood cells have neither A nor B antigen and their blood serum contains both anti-A and anti-B antibodies. Figure 13.2 clumping of blood cultures -if something has an antibody for it, then it will clump if that antigen is introduced -for O, it has both anti-A and anti-B antibodies, thus when mixed with anything with A or B antigens (A, B, and AB blood) then the blood will clump -for A, it has anti-B antibodies, thus when mixed with anything with B antigens (B or AB) then the blood will clump -for B, it has anti-A antibodies, thus when mixed with anything with A antigens (A or AB) then the blood will clump -for Ab, it has no antibodies thus it will never clump with any mixture -what transfusions are safe? 1. people with blood type A produce the A antigen so blood can be transfused only into recpeinct who dont have the anti-A antibody: A or AB 2. people with the blood type B produce the B antigen so blood can be transfused into a recipient who doesnt have the anti-B antibody: B or AB 3. people with blood type AB produce both A and B antigens, so their blood can be transfused only into recipient who do not have either the anti-A antibody or the anti-B antibody, that is only AB 4. people with the blood type O produce neither antigen so their blood can be transfused into any recipient A, B, AB, or O -think of it like this that letter creates those antigens. If it has those antigens then it produces its opposite anit-body. The blood can only be transfused with that dont carry an antibody for their antigen. -blood A has an A antigen. Therefore B-antibody. Therefore can transfuse their blood to a recipient without anti-A antibodies: A and AB (AB has both anitgens therefore neither anitbodies). Blood a can only receive non B antigen blood thus A or O (AB has B anitgens) -A donates to A and AB -B donates to B and AB -AB donates to only AB -O donates to ALL -The ABO blood group encodes glycosyltransferase -most people produce a glycolipid called the H antigen. -the I allele encodes a glycotransferase enzyme that adds a particular type of sugar to the H antigen to the A antigen. B -the I allele encodes a glycotransferase enzyme that adds a particular type of sugar to the H antigen to produce the B antigen -different glycotransferases -I /I encodes both enzymes therefore some H antigen is converted to the A antigen and some is converted to the B antigen -people who are homozygous for the i allele produce no enzymes to convert the H antigen glycoplipid. -this antigen does not elicit an antibody response in people of other blood groups because its polysaccharide componenet is also the basic component of the A and B antigens and therefore it not detected as a foreign substance -people who are homozygous for the recessive mutant allele, h, do not make antigen. Called Bombay blood. Very rare. Like O except that it also produces anti-O antibodies. Drosophila eye color -another example of multiple alleles concern the white locus of drosophila -hundreds of alleles all when homozygous give a recessive color ranging from the wild type red to white. Red is the wild type dominant. One example of this is eosin. Eosin is recessive to the wild type red but dominant to white.Relating multiple alleles to molecular genetics -the base-pair sequence of a gene specifies the amino acid sequence of a protein, and the function of a protein depends on its amino acid sequence -amino acid change at one of mny places in the protein could affect its function adversely and the position and type of change would determine the extent of loss of function of the protein. -two practical consequences of multiple alleles in the case of human genetic diseases are that the symptoms of a disease may vary with the allele and therefore it is important to determine the exact alleles of the patient in such cases Modification of dominance relationships -Complete dominance: phenomenon in which one allele is dominant to another, so that the phenotype of the heterozygote is the same as that of the homozygous dominant. With complete recessiveness, the recessive allele is phenotypically expressed only when it is homozygous Incomplete Dominance -incomplete dominance (semidominance or partial dominance): when one allele of a gene is not completely dominant to another allele of the same gene -with incomplete dominance, the phenotype of the heterozygote lies in the range between the phenotypes of individuals that are homozygous for either allele -the phenotype of the heterozygote is typically referred to as an intermediate phenotype, even though it may not be exactly in the middle between the phenotypes of the homozygotes. -ex. Palomino (color) horse. 1:2:1 ratio when interbred. The 1:2:1 ratio resulting from interbreeding is characteristic of incomplete dominance (heterozygote selfing
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