6 MULTIPLE ALLELES ABO Blood Groups Antibody – protein molecule that recognizes and binds to foreign materialAntigen – molecule that is recognized as foreign and stimulates antibody productionDon’t stimulate antibody formation in organism expressing them (exception: Autoimmune diseases)
7 MULTIPLE ALLELES ABO Blood Groups IA : specifies the ‘A’ antigen; antibodies against ‘B’ and will clump onto the IBIB : specifies the ‘B’ antigen; antibodies against ‘A’ and will clump onto the IAAB : have both antigens, but no “anti-” antibodiesO (ii) : have no antigens and no “anti-” antibodies
8 MULTIPLE ALLELES ABO Blood Groups Safe Transfusions: A (IAIA /IAi) – can receive A or OCan give to A or ABB (IBIB /IBi) – can receive B or OCan give to B or ABAB (IAIB) – can receive AB or OCan give to only ABO (ii) – can receive only OCan give to any blood group, A, B or O
9 Figure 4.4 Antigenic reactions that characterize the human ABO blood types. Blood serum from each of the four blood types was mixed with blood cells from the four types in all possible combinations. In some cases, such as a mix of B serum with A cells, the cells become clumped.
10 MULTIPLE ALLELES What does this have to do with molecular genetics? The base pair sequence of a gene specifies amino acid sequence of a protein…this protein function depends on the sequence of amino acidsSo, a simple change in the base sequence can drastically change the protein function
11 Modifications of Dominance Complete DominanceOne allele is dominant to another, so the heterozygous individual shows the dominant phenotypeIncomplete DominanceOne allele is not completely dominant to another (partial dominance), heterozygous individual shows a new intermediate phenotype
12 Modifications of Dominance Incomplete DominanceEx. Plumage color in chickensCross a true-breeing black (CBCB) with a true-breeding white (CWCW) and the heterozygous F1 offspring (CBCW) exhibits a bluish-grey plumageC : colorB : blackW : whiteCan’t be true-breeders…why?
13 Modifications of Dominance Incomplete DominanceExplanation: believed to occur for this reason…CB : produces color gene expressionCW : produces no gene expressionSo a heterozygous individual produces “half” a dose of gene expressionInsufficientHeterozygotes that produce a “normal” dominant appearance are described as haplosufficient
14 Modifications of Dominance Codominanceone allele is not dominant to another, instead the phenotype produced exhibits both dominant phenotypesEx. ABO blood grouping (AB)Explanation…believed to occur becauseBoth alleles for competing phenotypes are expressed
15 In Review Complete dominance Incomplete dominance & Codominance A/A & A/a produce the same phenotype and can be written as A/- because the second allele does not change the expression of the geneIncomplete dominance & CodominanceA/A & A/a do not produce the same phenotype so they must be written out as they appear
16 Modified Mendelian Ratios Production of NEW PhenotypesEx. Comb Shape in Chickens (may be true-breeders)a) R/- p/pb) R/- P/-c) r/r P/-d) r/r p/pAssuming recessivesdo not take any action,we can assume thesingle comb is aproduct of other geneswhile the others are dueto the activity of theR and P alleles
17 Modified Mendelian Ratios Production of NEW PhenotypesEx. Fruit Shape in Summer Squash – available in long, sphere, and disk-shapedSphere – A dominant allele of either gene and homozygous recessive of the otherDisk-shaped – A dominant allele of both genesLong – double homozygous recessive
18 Modified Mendelian Ratios Produces NO new phenotypeEpistasisInvolves a gene masking or modifying the phenotypic expression of another geneInteraction between 2 or more genes to control a single phenotypeConfined to dihybrid crosses where two pairs of alleles assort independentlyDoes not produce a new phenotype, only masksEpistatic gene – the gene that masks anotherHypostatic gene – the gene that is masked
19 Modified Mendelian Ratios EpistasisRecessive – must be homozygousEx: coat color in rodents – natural coat color in wild rodents is a greyish color (produced by alternating bands of black and yellow – agouti pattern)Aids in camouflageFound in mice, squirrels, etcOther colorations exist, but are recessive to agouti(A/– agouti; a/a nonagouti)(C/– pigment; c/c albino)(B/– black; b/b brown)c is epistatic when homozygous (recessive gene)A is hypostatic
20 Figure 4.11 Recessive epistasis: generation of an F2 9 agouti : 3 black : 4 white ratio for coat color in rodents.
21 Modified Mendelian Ratios EpistasisRecessive - must be homozygousEx: coat color in labrador retrievers – available in black, yellow, & chocolateOne gene specifies black pigment (B/-) or brown (b/b)An independent gene either allows (E/-) or hides (e/e) the expression of the Black / brown geneBlack: B/- E/-Chocolate: b/b E/-Yellow: -/- e/e (B/- black noses; b/b brown noses)e is epistatic when homozygous (recessive gene)B is hypostatic
23 Modified Mendelian Ratios EpistasisDominantEx: fruit color in summer squash – available in white, yellow, or greenW/-, -/- whitew/w, Y/- yelloww/w, y/y greenW – epistatic (homozygous or heterozygous)y - hypostatic
24 Modified Mendelian Ratios EpistasisDominantEx: Greying in horsesIt doesn’t matter what color the horse’s base is (sorrel, black, bay, etc) over time the Grey gene will mask that phenotypeIt is a progressive processGrey does not affect skin or eye color, only hair
26 Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).
27 Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).
28 Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).
29 Modified Mendelian Ratios EpistasisDuplicate GenesWhen a gene at one locus produces a phenotype identical to that produced at another locusEx: sweet peas flower colorsC: coloredc: no colorP: purplep: whitePurple flowers: C/- P/-White flowers: c/c -/- OR C/- p/p(duplicate recessive epistasis OR complementary gene action) when 1 or both loci are homozygous recessive
30 Modified Mendelian Ratios Essential & Lethal GenesMutations not only change phenotypes, they can also cause death (which I guess technically does change the phenotype)Alleles resulting in death are lethal alleles, caused by essential genes (essential to the normal functioning of the organism)When caused by a dominant lethal allele both the heterozygous and homozygous individuals will show the lethal phenotypeWhen caused by a recessive lethal allele, only the homozygous individual will show the lethal phenotype
31 Modified Mendelian Ratios Essential & Lethal GenesLethal allelesEx: Yellow body color in miceActs dominant in determining body color, but acts recessive in determining lethality (only heterozygotes survive to birth)Ex: Huntington’s disease in humansAutosomal dominant (can’t be studied until reproductive age)Onset doesn’t appear until early-thirties, and death in foritesEx: Hemophilia in humansX-linked recessive
32 Modified Mendelian Ratios Gene ExpressionPenetrance – frequency with which a gene manifests itself in individuals in the populationDepends on genotype and environmentExpressivity – degree to which a gene or phenotype are expressed in an individualEnvironment –Age of onset: creates internal environmental changesGenes are not “on” all the time; genes can be activated or deactivated over timePattern baldnessMuscular Dystrophy
33 Modified Mendelian Ratios Gene ExpressionEnvironment –Sex – expression of genes are influenced by genderSex-limited traits –autosomal genes that affect only 1 gender and not the otherEx: milk productionEx: appearance of horns in some speciesEx: facial hairTemperature – Reactions are catalyzed by enzymes, which function in a certain range.Ex: fur color in Himalayan rabbits>30 C – all white<25 C – typical coloration (black paws, ears, nose tail) and anywhere it is artificially cooled
34 Modified Mendelian Ratios Gene ExpressionEnvironment –Chemical – can have significant effect on an organismEX: Phenylketonuria (PKU): autosomal recessive, trouble metabolizing amino acid phenylalanine, diet determines severity (proteins)Nature vs NurtureWhat are the relative contributions of genes and the environment to the phenotype?Ex: Height – influenced by genes (potential) and environment (diet, overall health, hormones)Ex: Alcoholism – influenced by genes (susceptibility) and environment (choice)Ex: Intelligence – influenced by genes (potential) and environment (learning, challenges)
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