Extensions of Mendelian Genetic Principles

1. Extensions of Mendelian Genetic Principles Chapter 4

2. Exceptions and Extensions to Mendel’s Rules • Not every gene conforms • Some are exceptions • Some are extensions • Number of genes • Gene interaction • Modifications of dominance

3. Multiple Alleles

4. ABO Blood Type • Allele that predominates in wild-type • Alternate is mutant • Gene may have several alleles • Multiple allelic series • Single individual still only has two copies • ABO blood groups • Other blood groups • 4 group Phenotypes: O, A, B, AB

5. ABO Blood Type Different combinations of IA, IB, i IA, IB dominant to i Cellular antigens attached to outside of blood cells

6. ABO Blood Type • Antibodies attach to foreign antigens • Clump (agglutinate) any blood cells with anti-b antigens • A have A-antigens and anti-B antibodies • B have B-antigens and anti-A antibodies • AB have both antigens and neither Anti-A or Anti-B antibodies • O have neither antigen and both Anti-A and Anti-B antibodies

7. ABO Blood Type • AB universal recipient • O universal donor • Glucosyltransferases • Enzymes add specific sugar to existing polysaccharide (H-antigen) • Α-N-acetylgalactosamine • Galactose • h/h mutants

8. i-Activity Was she Charlie Chaplin’s Child?

9. Drosophila Eye Color • Eye color • White mutant (w) • Wild type red (w+) • X-linked • Found mutants that did not conform to experiments done before • Eosin eyes (we) – dominant to white, recessive to red • Several different mutations of the white allele

11. Multiple Alleles at a Molecular Level Change of one base pair changes the AA and changes the protein Can change at several places and lead to several different alleles

12. Modifications of Dominance Relationships

13. Dominance Complete Dominance Complete Recessiveness Extremes of a range Many alleles share a different dominance relationship Incomplete Codominance

14. Incomplete Dominance • One allele is not completely dominant • Partial dominance • Heterozygote phenotype is intermediate • F1 cross shows all intermediate color • 1:2:1 ratio re-established in the F2

17. Codominance • Related to complete dominance • Heterozygote exhibits phenotypes of both homozygotes • NOT an intermediate • ABO blood series

18. Incomplete Dominance and Codominance at a Molecular Level • In codominance products of both alleles are expressed • Does not always happen • In Incomplete dominance only enough product for intermediate form is made • In complete dominance the dominant is said to be haplosufficient

19. Gene Interactions and Modified Mendelian Ratios

20. Modified Mendelian Ratios • Phenotype is result of complex gene interactions • Any deviation from the 9:3:3:1 ratio is a result of interaction between two or more genes • Interactions from genes for the same phenotypes • One gene masks or modifies or masks the other • Environmental interactions

21. Gene Interactions that Produce New Phenotypes • If two allelic pairs affect the same phenotype • Interaction gives novel phenotypes • Comb shape in chickens • Rose dominant to single • Pea dominant to single • Rose x pea = walnut • Results from two dominant R and P

22. AHHHHHHH!!!

24. Molecular Basis of Funky Combs Not known Single comb phenotype has nothing to do with rose or pea comb Dominant R and P interacts with single comb gene product P and R together interact together as well to form an entirely new phenotype

25. Gene Interactions that Produce New Phenotypes • Fruit shape in summer squash • Long fruit and sphere-shaped • Long fruit is always true breeding • Interaction between certain varieties of sphere-shaped results in disk shaped fruit

27. Molecular Basis of Squidgy Squash • Modification of 9:3:3:1, so 2 genes involved • A dominant allele of either results in the dominant phenotype • A double dominant of each allele results in the new phenotype • Doubly homozygous recessive results in LONG FRUIT!!???

28. Epistasis

29. Epistasis • Interaction between two or more genes to control a single phenotype • Masks or modifies the phenotype expression • No new phenotype is produced • Masker is called epistatic • Maskee is called hypostatic • May be caused by double recessive allele or the presence of a dominant allele

30. Recessive Epistasis • A/_ b/b and a/a b/b have the same genotype • Phenotypic ratio is 9:3:4 • Coat color in mice • Wild mice have grey color (alternating black and yellow bands) called agouti pattern • Albinos are totally white without any pigment • Recessive to everything • Black rodents do not have the agouti gene and have no yellow bands • Recessive to agouti

31. Recessive Epistasis • 9:3:4 agouti:black: albino • Albino shows recessive epistasis over agouti • White hairs are produced regardless of other gene

32. Molecular Basis of Ti-Dyed Mice • Three alleles involved • C- allows pigment formation • cc prevents pigment formation regardless of color • A- dominant for agouti • B- codes for black coat color • bb codes for brown • All of the mice in figure 4.11 have B otherwise some brown mice would be present

33. Molecular Basis of Yoohoo Labs • Two genes involved • B codes for formation of black pigment • bb codes for formation of brown pigment • E allows expression of the B gene • ee does not allow expression of the B gene • B/_E/_ produces a black lab • b/b E/_ produces a chocolate lab • _/_ e/e produces a yellow lab

34. Molecular Basis of Yellow Labs • Variation of yellow labs • B/_ e/e genotype have dark noses and lips • b/b e/e genotype have pale noses and lips

35. Dominant Epistasis • A/_ B/_ and A/_ b/b have the same phenotype • 12:3:1 phenotypic ratio rather than 9:3:3:1 • Dominant gene is epistatic to other gene • Three common colors: white, yellow and green • White x yellow = white • White x green = white • Yellow x green = yellow

37. Molecular Basis of Garish Squash • Y is needed to convert white to yellow with a green intermediate • W allele codes for a product that inhibits white to green transition • Plants that have at least one W are white no matter the Y • White is W/_ Y/_ and W/_ y/y • Yellow is w/w Y/_ • green is w/w y/y

38. Dominant Epistasis • Two genes: W/w and Y/y • W/_ white no matter what • w/w Y/_ is yellow • w/w y/y is green • Progeny of F1 are 12 white: 3 yellow: 1 green

39. Dominant Epistasis • Greying in horses • GG or Gg progressive silvering of the coat • gg do not go gray • G allele affects the gene that grays the animal • Lipizzaner • Lipizzaner Clip

40. Epistasis Involving Duplicate Genes • Genes on different loci that produce identical phenotypes • Can be dominant or recessive epistasis • Example sweet pea flower color (recessive) • Crosses between true-breeding whites produce all purple • F1 produces 9/16 purple and 7/16white • All white progeny breed true when selfed • 1/9 of purple progeny breed true when selfed

41. Epistasis Involving Duplicate Genes • Pea plants, two genes • Colored flowers only appear when at least on of each of the dominant alleles are present • White only appears when homozygous recessive for either gene • C/c flower will be colored • P/p flower will be purple • Called duplicate because either gene being recessive makes the phenotype

43. Essential Genes and Lethal Genes

44. Essential Genes and Lethal Genes • Lethal allele – allele that results in the death of an organism • Gene that is involved in the death is called the essential gene • Dominant lethal allele • Recessive lethal allele • Recessive Lethal allele • Yellow body color in mice • Hetero-cross exhibited only hetero yellow mice

46. Mystery of The Mustard Mice • 2:1 ratio of hetero yellow and non-yellow • Yellow (AY) is dominant over non-yellow (A) • AY/ AY = lethal • AY/ A= yellow • A/ AY= non-yellow

47. Essential Genes and Lethal Genes • Recessive lethal genes • Tay-Sachs – enzyme deficiency that prevents nerve function • X-linked lethal genes • Hemophilia • Huntington’s Disease

48. Gene Expression and the Environment

49. Penetrance and Expressivity • Not all individuals with a genotype show expected phenotype • Frequency with which it manifests is called penetrance • Depends on genotype of environment • Complete penetrance • Incomplete penetrance • Brachdactyly

50. Expressivity • Degree to which a penetrant gene or genotype is phenotypically expressed • Depends on gene and environment • Different degrees of function in the gene • Osteogenesis imperfecta • Blue sclera • Brittle bones • Deafness