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Jeremiah 23:24

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. Beyond Mendel. Timothy G. Standish, Ph. D. When The Ratios Are Wrong.

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Jeremiah 23:24

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  1. 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.

  2. Beyond Mendel Timothy G. Standish, Ph. D.

  3. 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 chromosomes • However, a more subtle understanding is necessary of genes, gene products and alleles

  4. Explainations • Exceptions to Mendelian ratios may be accounted for in the following ways: • Incomplete or codominance - Two or more alleles exist, but none is dominant to the other’s • Multiple alleles for a single gene • Epistasis - In which interactions between more than one gene result in a trait • X-linkage - In which the locus of a gene is on the X chromosome • Sex influenced or limited genes, where expression is influenced or limited by gender or environment

  5. X 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

  6. CR CW P F1 CR X CW CRCR CWCW CRCW 1: 2: 1 CRCW 1 Incomplete Or Codominance • In the F2 generation a 1:2:1 ratio results of red to pink to white • F2 results show this is not blended inheritance CRCW F2 Generation CRCR CWCW

  7. Membrane M Anti M antibodies Cytoplasm M Erythrocyte M M antigen 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

  8. Membrane N Anti N antibodies N Cytoplasm Erythrocyte N N antigen 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

  9. Membrane Anti M antibodies M N Cytoplasm Anti N antibody Erythrocyte M M and N antigens 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

  10. 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

  11. 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 oligosaccharides 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 ˜ N-acetylglucosamine • Individuals with O type blood only display the H substance on their erythrocytes

  12. 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

  13. 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

  14. 1 A B 1 2 D 3 A B C 2 C D 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

  15. 1 1 2 2 A A B B C C Epistasis • Imagine that this pathway produces a red pigment, C, in flowers and that A is a colorless precursor and B is a yellow intermediate X • If the gene for enzyme 1 was knocked out, the flower would be colorless

  16. 1 1 2 2 A A B B C C Epistasis • Imagine that this pathway produces a red pigment, C, in flowers and that A is a colorless precursor and B is a yellow intermediate X • 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

  17. X X 1 1 2 2 A A B B C C Epistasis • If both genes were knocked out, the flowers would be colorless • 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 could still produce intermediate product B

  18. Epistasis • Consider a cross between two individuals heterozygous for both enzyme-coding genes • Let’s 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: 1F1n2F2nX1F1n2F2n

  19. 1F2F 1F2n 1n2F 1n2n 1F2F 1F2n 1n2F 1n2n 1F1n2F2nX1F1n2F2n 1F1F2F2F 1F1F2F2n 1F1n2F2F 1F1n2F2n 1F1F2F2n 1F1F2n2n 1F1n2F2n 1F1n2n2n 1F1n2F2F 1F1n2F2n 1n1n2F2F 1n1n2F2n 1F1n2F2n 1F1n2n2n 1n1n2F2n 1n1n2n2n

  20. 1F2F 1F2n 1n2F 1n2n 1F2F 1F2n 1n2F 1n2n 1F1n2F2nX1F1n2F2n 1F1F2F2F 1F1F2F2n 1F1n2F2F 1F1n2F2n 1F1F2F2n 1F1F2n2n 1F1n2F2n 1F1n2n2n 1F1n2F2F 1F1n2F2n 1n1n2F2F 1n1n2F2n 1F1n2F2n 1F1n2n2n 1n1n2F2n 1n1n2n2n

  21. 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: A 9:4:3 Ratio • 10:3:3 = (9+1):3:3 • 10:6 = (9+1):(3+3) • 13:3 = (9+1+3):3 • 9:7 = 9:(3+3+1) • 12:3:1 = (9+3):3:1 • 12:4 =(9+3):(3+1) • A ratio made up of some combination of 9:3:3:1 is generally a good hint that epistasis is at work

  22. Agouti Yellow Black Albino • Brown mice actually exhibit agouti coloration, a mix of yellow and black with hair strands alternating yellow and black melanin pigment Agouti Mice - A 9:4:3 Ratio • Mice lacking yellow or black are 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

  23. Yellow Agouti YB Yb yB yb Black Agouti/ Yellow Black YB Yb YYBB YYBb YyBB YyBb YYBb YYbb YyBb Yybb yB YyBB YyBb yyBB yyBb yb YyBb Yybb yyBb yybb Agouti Mice - A 9:4:3 Ratio Y Two possible explanations of agouti Colorless Precursor B Colorless Precursor B Y Colorless Precursor Cross two agouti individuals who are both heterozygous YyBb X YyBb

  24. Y Colorless Precursor Yellow Agouti B Colorless Precursor YB Yb yB yb Black YB Yb yB yb Agouti Mice - A 9:4:3 Ratio Two possible explanations of agouti B Y Agouti/ Yellow Colorless Precursor Black Cross two agouti individuals who are both heterozygous YYBB YYBb YyBB YyBb YYBb YYbb YyBb Yybb YyBb X YyBb YyBB YyBb yyBB yyBb YyBb Yybb yyBb yybb

  25. Disk 9 A_B_ 3 A_bb 3 aaB_ 1 aabb Sphere Long 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

  26. 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

  27. 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)

  28. 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, i.e., bees, haploid males diploid females • ZW - Heterogametic (ZW) females, homogametic (ZZ) males, i.e., birds • XY - Heterogametic (XY) males, homogametic (XX) females, i.e., humans and Drosophila

  29. X+ X+ Xw XwX+ XwX+ X P Y X+Y X+Y X X+ Xw F1 X+ X+ X+ XwX+ 1/4 1/4 1/2 Y X+Y XwY F2 Morgan’s Discovery Of An X-Linked Drosophila Gene A white-eyed male was discovered

  30. 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 Pasteur)

  31. 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

  32. X-linked Recessive GenesRelated 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 deficiency of vitamin A • Microphthalmia - Eyes fail to develop • Optic atrophy - Degeneration of the optic nerves

  33. Victoria (1840-1901) Emperor Frederick III of Germany (1831-1888) Leopold Duke of Albany (1853-1884) Alice (1843-1878) King Edward VII of England (1841-1910) Beatrice (1857-1944) King Alfonso XIII of Spain (1841-1910) Alix (Alexandra) (1872-1918) Tsar Nicholas II of Russia (1868-1918) Irene (1866-1953) Victoria (1866-1953) Olga (1895-1918) Marie (1899-1918) Alexis (1904-1918) Tatiana (1897-1918) Anastasia (1901-1918) Royal Pedigree Edward Duke of Kent (1767-1820) Victoria Princess of Saxe-Coburg (1786-1861) Albert of Saxe-Coburg (1819-1861) Victoria Queen of England (1819-1910)

  34. Genotype Phenotype FemaleMale BBBaldBald Bb Flocculent Bald bb Flocculent Flocculent 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

  35. Being Himalayan gives a whole new meaning to the term “brown nosing.” Environmental Effects • Genes do not work in isolation, but their expression is influenced by their environment • Just as expression of sex-influenced genes is influenced by the hormones in their environment other environmental variables impact expression of most genes

  36. The End

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