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Jeremiah 23:24 24 Can any hide himself in secret places that I shall not see him? saith the LORD. Do not I fill heaven and earth? saith the LORD. ©2000 Timothy G. Standish Beyond Mendel Timothy G. Standish, Ph. D. ©2000 Timothy G. Standish When The Ratios Are Wrong Some traits, when they are tested using Mendel’s techniques, do not produce a 3:1 or 9:3:3:1 ratio Example: When disk shaped and long summer squash are crossed they result in a F2 phenotypic ratio of 9/16 disk, 6/16 sphere and 1/16 long; a 9:6:1 ratio instead of the expected 9:3:3:1 or 3:1 In such cases it is not necessary to abandon Mendel’s basic principle of independent assortment of genes or the chromosome theory that genes occupy specific loci on ©2000 Timothy G. Standish Explainations Exceptions to Mendelian ratios may be accounted for in the following ways: 1 Incomplete or codominance - Two or more alleles exist, but none is dominant to the other/s 2 Multiple alleles for a single gene 3 Epistasis - In which interactions between more than one gene result in a trait 4 X-linkage - In which the locus of a gene is on the X chromosome 5 Sex influenced or limited genes, where expression is influenced or limited by gender or ©2000 Timothy G. Standish 1 Incomplete Or Codominance Incomplete or codominance - Two or more alleles exist, but none are dominant to the other Incomplete dominance results in blending of the parental traits Example: In four O’clock flowers (and others) red crossed with white results in pink F1 progeny X ©2000 Timothy G. Standish 1 Incomplete Or Codominance In the F2 generation a 1:2:1 ratio results of red to pink to white P F1 F2 results show X this is not blended RCR WCW RCW C C C inheritance F2 Generation CR CW CR CR CR CR CW 1: 2: 1 CW CRCW CWCW ©2000 Timothy G. Standish 1 Incomplete Or Codominance Codominant traits show up clearly whether the other allele is present or not Example: MN blood group genes in humans are codominant M phenotype MM genotype Membrane Cytoplasm M Anti M antibodies Erythrocyte M antigen ©2000 Timothy G. Standish 1 Incomplete Or Codominance Codominant traits show up clearly whether the other allele is present or not Example: MN blood group genes in humans are codominant N phenotype NN genotype Membrane N Cytoplasm Erythrocyte N Anti N antibodies N N antigen ©2000 Timothy G. Standish 1 Incomplete Or Codominance Codominant traits show up clearly whether the other allele is present or not Example: MN blood group genes in humans are codominant MN phenotype MN genotype Membrane Cytoplasm Anti M antibodies N Erythrocyte M and N antigens Anti N antibody ©2000 Timothy G. Standish 2 Multiple Alleles Any gene with two or more alleles is said to have multiple alleles Mendel worked with only two allele systems, but variations from the kind of results he obtained occur when more than two alleles are involved Note that while individuals cannot have more than two alleles for a given gene, populations can have many different alleles Human ABO blood types provide an excellent example of multiple alleles in human populations ©2000 Timothy G. Standish 2 Multiple Alleles ABO blood types are determined by the presence of antigens on the surface of erythrocytes in much the same way as MN blood types The antigens are oligoscaccharides presented on the cell surface Almost everyone makes an oligosaccharide called “H substance” which is a chain of sugars joined together in the following order: L-fructose b galactose Nacetylglucosamine Individuals with O type blood only display the H ©2000 Timothy G. Standish 2 Multiple Alleles Type A and B blood result from the presence of enzymes which add a sugar to the H substance Type A individuals produce an enzyme that adds N-acetylgalactosamine to the galactose in the H substance Type B individuals express a very similar enzyme that adds galactose to the same place ©2000 Timothy G. Standish 2 Multiple Alleles If neither enzyme is expressed type O blood results If the N-acetylgalactosamine adding enzyme is present type A blood results If the galactose adding enzyme is present type B blood is made If both the N-acetylgalactosamine and galactose adding enzymes are present, type AB blood results As the enzymes are coded for by genes, blood type is under direct genetic control ©2000 Timothy G. Standish Epistasis When a single trait is controlled by more than one gene epistasis may result The squash example we started with is an example of epistasis Understanding biochemical pathways helps us understand epistasis A 1 B 2 A C C 1 2 B D 3 D ©2000 Timothy G. Standish Epistasis 1 2 A B C Imagine that this pathway produces a red pigment, C, in flowers and that A is a colorless precursor and B is a yellow intermediate A X 1 B 2 C If the gene for enzyme 1 was knocked out, the flower would be colorless ©2000 Timothy G. Standish Epistasis 1 2 A B C Imagine that this pathway produces a red pigment, C, in flowers and that A is a colorless precursor and B is a yellow intermediate A 1 B X 2 C If the gene for enzyme 1 was knocked out, the flower would be colorless If the gene for enzyme 2 was knocked out, the flowers would be yellow ©2000 Timothy G. Standish Epistasis 1 2 A B C If both genes were knocked out, the flowers would be colorless X 1 X 2 A B C Because enzymes can catalyze many reactions in a short period of time, the presence of just one copy of a gene is typically enough to mask the absence of a bad copy Thus an individual heterozygous for enzyme 1 ©2000 Timothy G. Standish Epistasis Consider a cross between a two individuals heterozygous for both enzyme coding genes Lets call the functional enzyme 1 gene 1F and the mutated gene producing nonfunctional enzyme 1, 1n We will use the same convention for enzyme 2 with genes 2F and 2n Our cross would look like this: F n F n 1 12 2 X F n F n 1 12 2 ©2000 Timothy G. Standish F n F n 1 12 2 X F n F n 1 12 2 1F2F 1F2n 1n2F 1n2n F1F2F2F 1F1F2F2n 1F1n2F2F 1F1n2F2n F F 1 1 2 1F2n 1F1F2F2n 1F1F2n2n 1F1n2F2n 1F1n2n2n 1n2F 1F1n2F2F 1F1n2F2n 1n1n2F2F 1n1n2F2n n n 12 1F1n2F2n 1F1n2n2n 1n1n2F2n 1n1n2n2n ©2000 Timothy G. Standish F n F n 1 12 2 X F n F n 1 12 2 1F2F 1F2n 1n2F 1n2n F1F2F2F 1F1F2F2n 1F1n2F2F 1F1n2F2n F F 1 1 2 1F2n 1F1F2F2n 1F1F2n2n 1F1n2F2n 1F1n2n2n 1n2F 1F1n2F2F 1F1n2F2n 1n1n2F2F 1n1n2F2n n n 12 1F1n2F2n 1F1n2n2n 1n1n2F2n 1n1n2n2n ©2000 Timothy G. Standish A 9:4:3 Ratio A biochemical pathway like the one discussed will result in a 9:4:3 ratio as long as there are two alleles each of which behaves in a simple dominant/recessive way The 9:4:3 ratio is really a 9:(3+1):3 ratio Other possible phenotypic ratios for a dihybrid cross involving epistasis include: – 9:7 = 9:(3+3+1) – 12:3:1 = (9+3):3:1 – 12:4 =(9+3):(3+1) – 10:3:3 = (9+1):3:3 – 10:6 = (9+1):(3+3) – 13:3 = (9+1+3):3 A ratio made up of some combination of 9:3:3:1 is generally a good hint that epistasis ©2000 Timothy G. Standish Agouti Mice - A 9:4:3 Ratio Brown mice actually exhibit agouti coloration, a mix of yellow and black with hair strands alternating yellow and black melanin pigment Agouti Yellow Black Albino Mutating a gene coding for an enzyme necessary to make black pigment results in yellow mice Mutating an enzyme for yellow results in black Mice lacking yellow or black are albino ©2000 Timothy G. Standish Agouti Mice - A 9:4:3 Ratio Colorless Y Yellow Precursor Colorless B Precursor Black YB Yb Agouti Two possible explanations of agouti Colorless B Black Y Agouti/ Precursor Yellow yB yb YB YYBB YYBb YyBB YyBb Yb YYBb YYbb YyBb Yybb yB YyBB YyBb yyBB yyBb yb YyBb Yybb yyBb yybb Cross two agouti individuals who are both heterozygous YyBb X YyBb ©2000 Timothy G. Standish Agouti Mice - A 9:4:3 Ratio Colorless Y Yellow Precursor Colorless B Precursor Black YB Yb Agouti Two possible explanations of agouti Colorless B Black Y Agouti/ Precursor Yellow yB yb YB YYBB YYBb YyBB YyBb Yb YYBb YYbb YyBb Yybb yB YyBB YyBb yyBB yyBb yb YyBb Yybb yyBb yybb Cross two agouti individuals who are both heterozygous YyBb X YyBb ©2000 Timothy G. Standish Another Example Of Epistasis: Fruit Shape In Squash, A 9:6:1 Ratio As mentioned earlier, the F2 generation of a cross between a disk shaped and a long squash has a disk:spherical:long ratio of 9:6:1 How can this ratio be explained? In the F2 generation the following genotypes must result in the indicated phenotypes 9 3 3 1 A_B_ A_bb aaB_ aabb Disk Sphere Long ©2000 Timothy G. Standish 4 X-Linkage Thomas Hunt Morgan was the first to associate a trait (gene) with a chromosome. Worked with fruit flies (Drosophila melanogaster) In humans and Drosophila, males are XY Thus males are haploid for the X chromosome Because of this, recessive genes on the X chromosome show up far more commonly in male than female phenotypes ©2000 Timothy G. Standish Drosophila Nomenclature + = Wild type, phenotype in nature (i.e. red eyes and round wings) Mutants are alternatives to the wild type Fruit fly genes are named after the mutant Dominant mutations are capitalized (i.e. Hairless or H and Bar or B) Recessive mutants are named using lower case letters (i.e. black or b and white or w) ©2000 Timothy G. Standish Sex Determination Two ways in which sex can be determined: Environment: Turtles - Temperature of development Some fish - Social structure Chromosomes - Three methods: XO - Haploid/diploid ie bees, haploid males diploid females ZW - Heterogametic (ZW) females, homogametic (ZZ) males, ie birds XY - Heterogametic (XY) males, homogametic (XX) females, ie humans and Drosophila ©2000 Timothy G. Standish Morgan’s Discovery Of An XLinked Drosophila Gene X+ X+ A white-eyed male was discovered P X 1/4 Xw X+ Xw X+ Y X+Y X+Y F1 X 1/4 Xw 1/2 X+ Xw X+ X+ X+ Xw X+ Y X+Y XwY F2 ©2000 Timothy G. Standish The Key To Morgan’s Discovery The key to Morgan’s discovery was the observation that all the white eyed individuals in the F2 generation were males Without this vital data on the association of white eyes with being male, the gene for white eyes could have been seen as a simple recessive trait on an autosome This illustrates the importance of recording all the data possible and being alert to the possibility of interesting things being present in the data “Fate favors the prepared mind” (Louis Pasture) ©2000 Timothy G. Standish Human X-linked Recessive Genes Brown enamel - Tooth enamel appears brown rather than white Hemophilia - Two types: – A - Classic hemophilia, deficiency of blood clotting factor VIII – B - Christmas disease, deficiency of blood clotting factor IX ©2000 Timothy G. Standish X-linked Recessive Genes Related to sight Coloboma iridis - A fissure in the eye’s iris Color Blindness - Two types: – Deutan - Decreased sensitivity to green light – Protan - Decreased sensitivity to red light Congenital night blindness - Not due to a deficency of vitimin A Mocrophthalmia - Eyes fail to develop Optic atrophy - Degeneration of the optic nerves ©2000 Timothy G. Standish Royal Pedigree Edward Duke of Kent (1767-1820) Victoria Princess of Saxe-Coburg (1786-1861) Albert of Saxe-Coburg (18XX-18XX) Victoria Queen of England (1819-1910) Victoria (1840-1901) Leopold Duke of Albany (1853-1884) Alice (1843-1878) Alix (Alexandra) (1872-1918) King Edward VII of England (1841-1910) Tsar Nicholas II of Russia (1868-1918) Olga Marie (1895-1918) (1899-1918) Tatiana (1897-1918) Emperor Frederick III of Germany (1831-1888) Beatrice King Alfonso XIII (1857-1944) of Spain (1841-1910) Irene (1866-1953) Victoria (1866-1953) Alexis (1904-1918) Anastasia (1901-1918) ©2000 Timothy G. Standish 5 Sex-Influenced Or Limited Genes Expression of genes that are not necessarily on the X chromosome may be influenced by the gender of the individual One major reason for this is the impact that steroid sex hormones have on the expression of genes Male pattern baldness is the classic example of a sex influenced gene in humans Genotype BB Bb bb Phenotype Female Male Bald Bald Flocculent Bald Flocculent Flocculent ©2000 Timothy G. Standish Environmental Effects Genes do not work in isolation, but their expression is influenced Being Himalayan by their environment gives a whole new Just meaning to the termas expression of “brown nosing.” sex influenced genes are influenced by the hormones in their environment other environmental variables impact expresson of most genes ©2000 Timothy G. Standish