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Mendelian Genetics

Mendelian Genetics. Fig. 14-1. Blending theory versus Particulate theory of heredity. Definitions. Character = detectable inheritable feature of an organism Trait = a variant of an inheritable character

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Mendelian Genetics

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  1. Mendelian Genetics

  2. Fig. 14-1

  3. Blending theory versus Particulate theory of heredity

  4. Definitions • Character = detectable inheritable feature of an organism • Trait = a variant of an inheritable character • True Breeding = Always producing offspring with the same traits as the parents when the parents are self-fertilized

  5. Definitions • P generation = parental • F1 generation = first filial (offspring of P generation) • F2 generation = second filial (offspring of the first filial generation)

  6. Definitions • Alleles = alternate forms of a gene • Monohybrid cross = a mating between parents that differ in a single character • Dihybrid cross = a mating between parental varieties that differ in 2 characters

  7. Definitions • Homozygous = 2 identical alleles for a given trait (PP or pp) • Heterozygous = 2 different alleles for a trait (Pp) • Phenotype = An organism’s expressed traits (purple or white flowers) • Genotype = An organism’s genetic makeup (PP or Pp or pp)

  8. How Mendel made his crosses TECHNIQUE 1 2 Fig. 14-2 Parental generation (P) Stamens Carpel 3 4 RESULTS First filial gener- ation offspring (F1) 5

  9. Mendel’s Monohybrid Crosses

  10. EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers Fig. 14-3-2 F1 Generation (hybrids) All plants had purple flowers

  11. EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers Fig. 14-3-3 F1 Generation (hybrids) All plants had purple flowers F2 Generation 224 white-flowered plants 705 purple-flowered plants

  12. F2 generation showed 3:1 ratio (no blending) • From this he reasoned: • Alternate forms of genes are responsible for variations in inherited characters • For each character, an organism inherits two alleles, one from each parent • If the 2 alleles differ, one is fully expressed (dominant allele) and the other is masked, with no noticeable effect on the organism’s appearance (recessive allele) • The 2 alleles for each character segregate during gamete production

  13. Allele for purple flowers Fig. 14-4 Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

  14. Law of segregation • Allele pairs segregate during gamete formation and the paired condition is restored by the random fusion of gametes at fertilization

  15. P Generation Purple flowers Appearance: White flowers Genetic makeup: PP pp p Gametes: P Fig. 14-5-1

  16. P Generation Purple flowers Appearance: White flowers Genetic makeup: PP pp p Gametes: P Fig. 14-5-2 F1 Generation Appearance: Purple flowers Genetic makeup: Pp p Gametes: 1/2 1/2 P

  17. Punnett Square • Possible gametes from one parent are listed on one side of the square • Possible gametes from the other parent are listed on the other side of the square

  18. P Generation Mendel’s Law of Segregation Purple flowers Appearance: White flowers Genetic makeup: PP pp p Gametes: P Fig. 14-5-3 F1 Generation Appearance: Purple flowers Genetic makeup: Pp p Gametes: 1/2 1/2 P Sperm F2 Generation p P P PP Pp Eggs p pp Pp 3 1

  19. Phenotype Genotype PP Purple 1 (homozygous) Fig. 14-6 Pp 3 Purple (heterozygous) 2 Pp Purple (heterozygous) pp White 1 1 (homozygous) Ratio 3:1 Ratio 1:2:1

  20. TECHNIQUE Testcross: breeding of an organism of unknown genotype with a homozygous recessive.  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Fig. 14-7 Predictions If PP If Pp or Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p pp Pp pp Pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white

  21. Mendel’s Dihybrid Crosses

  22. EXPERIMENT YYRR yyrr P Generation Gametes yr YR  F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Fig. 14-8 Predictions Sperm or Predicted offspring of F2 generation 1/4 1/4 1/4 yr 1/4 YR yR Yr Sperm YR yr 1/2 1/2 1/4 YR YYRr YYRR YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 1/16 9/16 3/16 Phenotypic ratio 9:3:3:1 RESULTS Phenotypic ratio approximately 9:3:3:1 315 108 101 32

  23. Mendel’s Dihybrid Crosses • F2 generation showed 9:3:3:1 ratio (not 3:1) • From this he reasoned: • Law of independent assortment = each allele pair segregates independently of other gene pairs during gamete formation.

  24. Probability  Segregation and independent assortment of alleles are random events. • Random events are independent of each other.

  25. Probability • Rule of multiplication = the probability that independent events will occur simultaneously is the product of their individual probabilities. • Rule of Addition = The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.

  26. Rr Rr  Segregation of alleles into sperm Segregation of alleles into eggs Sperm Fig. 14-9 1/2 1/2 R r R R R 1/2 r R 1/4 1/4 Eggs r r r R 1/2 r 1/4 1/4

  27. Variations on Mendelian Genetics

  28. Dominance • Incomplete dominance • Complete dominance • Codominance

  29. P Generation Red White CRCR CWCW CR CW Gametes Fig. 14-10-1

  30. P Generation Red White CRCR CWCW CR CW Gametes Fig. 14-10-2 Pink F1 Generation CRCW 1/2 1/2 CR CW Gametes

  31. P Generation Red White CRCR CWCW CR CW Gametes Fig. 14-10-3 Pink F1 Generation CRCW 1/2 1/2 CR CW Gametes Sperm 1/2 1/2 CR CW F2 Generation 1/2 CR CRCW CRCR Eggs 1/2 CW CRCW CWCW

  32. ABO Blood Group • An example of Codominance and Multiple alleles

  33. Carbohydrate Allele Phenotype (blood group) Red blood cell appearance Genotype IA A B IB i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Fig. 14-11 IAIA or IA i A B IBIB or IB i AB IAIB ii O (b) Blood group genotypes and phenotypes

  34. Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Pleiotropy

  35. In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in mice and many other mammals, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair Epistasis

  36. BbCc BbCc Sperm 1/4 1/4 1/4 1/4 BC bC Bc bc Eggs Fig. 14-12 1/4 BC BBCc BBCC BbCC BbCc 1/4 bC bbCC bbCc BbCC BbCc 1/4 Bc BBcc Bbcc BBCc BbCc 1/4 bc BbCc bbCc Bbcc bbcc : 4 9 : 3

  37. Polygenic inheritance • Additive effect of 2 or more genes determines a single phenotypic character • Quantitative characters = characters that vary by degree in a continuous distribution rather than by discrete (either/or) qualitative differences

  38. AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Fig. 14-13 1/8 1/8 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 1/64 6/64 Number of dark-skin alleles: 2 6 0 3 4 5 1

  39. Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity The Environmental Impact on Phenotype

  40. Fig. 14-14 Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype

  41. Pedigree Analysis Key Mating Male Affected male Offspring, in birth order (first-born on left) Female Affected female

  42. 1st generation (grandparents) Ww Ww ww ww 2nd generation (parents, aunts, and uncles) Fig. 14-15b Ww Ww ww ww Ww ww 3rd generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait?

  43. 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) Ff Ff FF or ff ff Ff ff Fig. 14-15c 3rd generation (two sisters) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?

  44. Some Human Disorders

  45. Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair The Behavior of Recessive Alleles

  46. Fig. 14-16 Parents Normal Normal Aa Aa  Sperm a A Eggs Aa AA Normal (carrier) A Normal Aa aa Normal (carrier) a Albino

  47. Some Human Disorders Recessively inherited disorders • Cystic fibrosis • Tay-Sachs disease • Sickle-cell disease

  48. Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Cystic Fibrosis

  49. Sickle-cell disease affects one out of 400 African-Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include physical weakness, pain, organ damage, paralysis, etc, etc. Sickle-Cell Disease

  50. Some human disorders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Achondroplasia is a form of dwarfism caused by a rare dominant allele Dominantly Inherited Disorders

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