Mendel and Heredity

1. Mendel and Heredity Chapter 8

2. 8.1 The Origins of Genetics • Heredity is the passing of characters from parents to offspring • Used throughout history to alter crops and domestic animals • Gregor Johann Mendel – Austrian Monk • Used pea plants and bred different varieties • Developed rules to accurately predict patterns of heredity

3. Why Peas? • 2 characters have clearly different forms • Character = inherited characteristic (color) • Trait = single form of character (purple) • Male and female reproductive parts are in same flower • Can control fertilization • Flower can fertilize itself (self-fertilization) or can cross pollen from 1 plant to another (cross-pollination) • Peas are small, grow easily, mature quickly, and produces many seeds so results obtained quickly

4. Traits Expressed as Simple Ratios • Mendel started by looking at 1 characteristic (monohybrid), such as color, with 1 pair of contrasting traits, purple or white flowers • Only allowed plants to self-pollinate for many generations • True-breeding – all offspring show only 1 trait • Parental (P) generation • Cross pollinated 2 P generation plants with contrasting traits • Offspring called filial (F1) generation • Counted numbers of each trait

5. Allowed F1 generation to self pollinate • Offspring called F2 generation • Each characterized and counted

6. Mendel’s Results • F1 showed only 1 form of character other had disappeared • When F1 self pollinates other trait reappears in some of F2 • Found ratio of traits to be 3 to 1 • 3 white flowers to 1 purple flower • Same ratio found for any trait he studied

7. 8.2 Mendel’s Theory • We used to think offspring were blend of traits • Tall x short = medium • Mendel’s experiments showed us this is not entirely true

8. Mendel’s Hypothesis • There are 2 copies of a gene, one from each parent, for each inherited characteristic • There are different versions of genes called “alleles” • Tall or short • When both versions are present one may be dominant (completely expressed) and the other may be recessive (not expressed when dominant is present) • When you form gametes, alleles separate independently so only one allele in each gamete

9. Mendel’s Finding in Modern Terms • Use letters to show alleles • Capitol = dominant (T, P, Y, etc…) • Lower case = recessive (t, p, y, etc…) • Homozygous = letters are same • Homozygous dominant = TT, PP • Homozygous recessive = tt, pp • Heterozygous = letters are different • Tt, Pp • Only dominant allele is expressed

10. Genotype = set of alleles • What you actually have • TT, Tt, or tt • Phenotype = what is expressed • How it looks • Tall, Tall, or Short

11. Mendel’s Laws of Heredity • Law of Segregation • 2 alleles for a character segregate when gametes are formed • Behavior of chromosomes during meiosis • Law of Independent Assortment • 1 character does not affect another • Alleles of different genes separate independently of on another • Now know this only applies to genes located on different chromosomes or that are far apart on same chromosome

12. 8.3 Studying Heredity: Punnett Squares • Breeders want certain characteristics when they breed (cross) animals • Horticulturists produce plants with specific characteristics

13. Punnett Square • Used to predict outcomes • Shows all possible combinations of gametes • Put 1st parents genotype on top • Put 2nd parents genotype on side • Do the cross TT Tt tt Tt

14. So in Mendel’s F1 generation, a pure Tall plant bred with a pure short plant can only give 1 kind of offspring due to dominance of tall allele

15. Determining Unknown Genotypes • How do you know if a tall plant is homozygous or heterozygous? They both look tall • Can do a Test Cross • If dominant phenotype is shown with unknown genotype, cross it with homozygous recessive

16. Test Cross Results • If unknown is homozygous dominant, all offspring of test cross will have dominant trait

17. Test Cross Results • If unknown is heterozygous, offspring of test cross will have 2 dominant and 2 recessive phenotypes

18. Can use probability calculations to predict results of genetic crosses • Probability is the likelihood a specific event will occur • Probability = # of 1 kind of possible outcome divided by total number of possible outcomes • We will express these as fractions • Chance a coin will come up heads • 1 head / 2 sides = ½

19. DD = ¼ • Dd = 2/4 or ½ • dd = ¼

20. Dihybrid Cross • Uses a Punnett Square to determine outcomes of 2 traits at one time • Example: Surface and Color • Surface: RR, Rr, or rr round or wrinkled • Color: YY, Yy, yy yellow or green • What are the possible combinations? • RY, Ry, rY, ry

21. So if you have 2 purebred homozygous parents RRYY and rryy and you mate them, what do you get? • All offspring will be RrYy • What if you have F1 breed? • Make a Punnett Square of possible gametes for each parent • What possible combos can parents offer? • Do you remember FOIL? • RY, Ry, rY, ry

23. You never have to count the results of a dihybrid cross between heterozygotes! • 9 with both dominant traits • 3 with first dominant and second recessive • 3 with first recessive and second dominant • 1 with both recessive traits • So 9:3:3:1

24. Inheritance of Traits • Pedigree • Family history that shows how a trait is inherited over several generations • Helpful in tracking genetic disorders • Carrier – have allele for trait but show no symptoms

26. Things You Can Find From A Pedigree • Autosomal or Sex-Linked? • If autosomal it will be equal in both sexes • If sex linked generally only found in males • Y linked • Hairy ear rims • X linked • Color-blindness • Hemophilia

27. Dominant or recessive • Autosomal Dominant – every individual with condition will have parent with condition • Achondroplasia – type of dwarfism • Huntington’s Disease – brain degenerates • Autosomal Recessive – 1, 2, or no parents with condition • Cystic fibrosis • Sickle cell anemia • Albinism

28. Heterozygous or Homozygous • Autosomal homozygous dominant or heterozygous phenotype will show dominant allele • Homozygous recessive will show recessive allele • 2 heterozygous of recessive allele don’t show condition but can have children that do

29. 8.4 Complex Patterns of Heredity • Complex Control • Most of the time characters display much more complex patterns than simple dominant-recessive patterns • Characters can be influenced by several genes

30. Polygenic Inheritance • Several genes affect a character • These genes may be scattered along same chromosome or on different chromosomes • Determining the effect of any one gene is difficult • Crossing over and independent assortment create many different offspring combos • Eye color, height, weight, hair, intelligence, and skin color • Usually gives a range of expression

31. Polygenic Inheritance

34. Intermediate Characters • Incomplete dominance • Phenotype that is intermediate between 2 parents, neither is completely dominant • White x red = pink • Straight hair x curly hair = wavy hair

35. Multiple Alleles • Characters controlled by genes with 3+ alleles • Humans have ABO blood types • IA, IB, i • Letters A and B refer to carbohydrates on surface of red blood cells • i has neither carbohydrate • IA and IB are dominant over I, but not over each other (codominant) • Still only 2 possibilities in a person

36. Blood Types • 2 forms are displayed at the same time • Codominance – both expressed, not blended • IAIB both expressed • ii = Type O

37. Characters Influenced by Environment • Plants may change color based on pH of soil • Arctic fox • Summer – enzymes produce pigments for darker fur • Winter – no enzymes, no pigments to darken fur • Siamese cats • Dark fur in cooler parts • Humans • Height related to nutrition • Skin color based on sun exposure • Twins are genetically identical, any difference is due to environment

38. Genetic Disorders • Proteins encoded by genes must function precisely for normal development and function • Genes may be damaged or copied wrong causing faulty proteins • Mutation = changes in genetic material • Rare because cells try to correct errors • Harmful effects produced by inherited mutations • Many carried by recessive alleles

39. Sickle Cell Anemia • Recessive genetic disorder • Mutated allele produces defective form of hemoglobin causing red blood cells (rbc) to be misshapen • These rupture easily causing less O2 to be carried and may get stuck and cut off blood supply • Recessive allele protects heterozygous individuals from malaria • Parasites in sickle rbc die • Normal rbc still transport oxygen

40. Cystic Fibrosis • Most common fatal, hereditary, recessive disorder in Caucasions • 1 in 25 has at least 1 copy of defective gene that makes a protein needed to move chloride in and out of cells • Mucus clogs organs • 1 in 2,500 homozygous for cystic fibrosis • No cure

41. Hemophilia • Impairs bloods ability to clot • Sex-linked • Dozen+ genes code for clotting proteins • 1 mutation on X chromosome causes Hemophilia A • Males only get 1 X chromosome

42. Huntington’s Disease • dominant allele on autosome • 1st symptoms - mild forgetfulness and irritability in 30’s and 40’s • Eventually lose muscle control, spasms, severe mental illness, and death

43. Treating Genetic Disorders • Most can’t be cured • Genetic Counseling – tells of possible genetic problems with offspring, may be treated if early enough

44. Phenylketonuria (PKU) • Lack enzyme that converts amino acid phenylalanine into tyrosine so it builds up in the body and causes severe mental retardation • Can be placed on phenylalenic diet

45. Gene Therapy • Replace defective genes with normal ones • Isolate copy of gene • Put working copy into a virus • Virus infects and puts gene in • Infected cells are cured • Still trying to get this to work