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Chapter 15

Chapter 15. The Chromosomal Basis of Inheritance. Complexity of Genetics. The relationship between genotype and phenotype is rarely simple More than one gene More than two alleles However, the basic principles apply. Single Gene Complexities. Inheritance by a single gene may deviate:

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Chapter 15

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  1. Chapter 15 The Chromosomal Basis of Inheritance

  2. Complexity of Genetics • The relationship between genotype and phenotype is rarely simple • More than one gene • More than two alleles • However, the basic principles apply

  3. Single Gene Complexities • Inheritance by a single gene may deviate: • Incomplete dominance • More than two alleles • Multiple phenotypes

  4. The Spectrum of Dominance • Complete dominance -- phenotypes of the heterozygote and homozygote are identical • Codominance -- two dominant alleles affect the phenotype in separate, distinguishable ways • Incomplete dominance -- the phenotype of F1 hybrids is somewhere in between • What do you notice about the ratios?

  5. P Generation Purple flowers PP White flowers pp Appearance: Genetic makeup: p P Gametes F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 1 p P 2 2 F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1 Complete Dominance LE 14-5_2 Genotype ratio? Phenotype ratio?

  6. Incomplete Dominance Genotype ratio? Phenotype ratio?

  7. Pattern in clover leaflets Co-Dominance Homozygous for lack of pigment at the tips of the leaflets Homozygous for lack of pigment at the center of the leaflets Heterozygous Genotype ratio? Phenotype ratio?

  8. Dominance and Frequency • Not necessarily more common • Polydactyly – 1 out of 400 in the United States • Polydactyly is dominant

  9. Multiple Alleles • Many genes have more than two allelic forms

  10. Multiple Alleles • Many genes have more than two allelic forms • ABO blood group in humans -- three alleles • The IA allele adds the A carbohydrate, the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

  11. Pleiotropy • Multiple phenotypic effects • Pleiotropic alleles are responsible for multiple “symptoms” or effects • cystic fibrosis • sickle-cell disease • albinism

  12. Polygenic Traits • Some traits may be determined by two or more genes • Sometimes additive effect • Broad distribution of potential phenotypes

  13. AaBbCc AaBbCc LE 14-12 aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC 20/64 15/64 Fraction of progeny 6/64 1/64

  14. Gene Interaction • Single trait – at least 2 different genes • “normal” Mendelian ratio – 9: 3: 3: 1 • Normal Mendelian Inheritance • 2 genes with 2 traits

  15. 9 3 Gene Interaction 3 1

  16. Epistasis • One locus alters the phenotypic expression of another gene

  17. Epistasis • One locus alters the phenotypic expression of another gene • Coat color in mammals depends on two genes • Pigment color (B for black and b for brown) • Pigment deposition (C for pigment color and c for no pigment color ) • C is eipstatic to B – it alters the expression of B

  18. BbCc BbCc Sperm bC Bc 1 1 1 1 BC bc 4 4 4 4 1 BC BBCC BbCC BBCc BbCc 4 1 bC BbCC bbCC BbCc bbCc 4 BBcc Bbcc BBCc BbCc 1 Bc 4 bbcc Bbcc 1 bbCc bc BbCc 4 9 3 4 16 16 16 LE 14-11

  19. Nature vs Nurture: The Environmental Impact on Phenotype • Environment • hydrangea flowers -- soil acidity

  20. The enzyme that produces dark fur – mutation renders it inactive above 35°C

  21. Cellular Organization of the Genetic Material • Genome • DNA -- packaged into chromosomes • Humans – 23 pairs chromosomes • Somatic cells -- two sets of chromosomes • Gametes – one set of chromosomes • Sex chromosomes • Human males – XY • Human females -- XX

  22. Many human traits follow Mendelian patterns of inheritance • Humans are not good subjects • generation time is too long; • relatively few offspring; • breeding experiments unacceptable • However, basic Mendelian genetics endures as the foundation of human genetics

  23. Pedigree Analysis • Inheritance patterns of particular traits can be traced and described using pedigrees

  24. First generation (grandparents) Ww ww ww Ww Second generation (parents plus aunts and uncles) Ww ww ww Ww Ww ww Third generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak Dominant trait (widow’s peak) LE 14-14a

  25. First generation (grandparents) Ff Ff ff Ff Second generation (parents plus aunts and uncles) FF or Ff ff ff Ff Ff ff Third generation (two sisters) ff FF or Ff Free earlobe Attached earlobe Recessive trait (attached earlobe) LE 14-14b

  26. Other Inherited Traits • Tongue rolling (R) • Freckles (F) • Bent Phalanges (B) • Long Palomar Muscles (P) • Hitchhiker's Thumb (t) • Dimples (D) • PTC taster (T)

  27. Inherited Disorders • Many genetic disorders -- recessive • Individuals must be homozygous for the allele • Carriers -- heterozygous individuals • Many single-gene disorders • Polygenic • Multifactorial disorders • Genetic and Environmental component

  28. Multifactorial Disorders • Many disorders [not diseases] -- both genetic and environment components • heart disease and cancer • Little is understood about the genetic contribution

  29. Hemophilia

  30. Cystic Fibrosis • Most common lethal genetic disorder-- one out of every 2,500 people of European descent • Defective or absent chloride transport channels in plasma membranes • Symptoms -- mucus buildup in lungs and abnormal absorption of nutrients in the small intestine

  31. Sickle-Cell Disease • One in 400 African-Americans • Single amino acid change in the hemoglobin protein • Physical weakness, pain, organ damage, and even paralysis

  32. Dominantly Inherited Disorders • Achondroplasia – lethal form of dwarfism when homozygous for the dominant allele • Huntington’s disease -- degenerative disease of the nervous system • No symptoms until 35-40

  33. MENDEL'S WORK • True breeding strains for several traits • Hybridized plants that differed in a single trait • First hybrid generation is called the F1 generation • F1 generation was selfed to produce the F2 generation • segregation for traits occurred in the second generation

  34. MENDEL'S WORK • Postulates (Modernized) • Traits are controlled by heritable factors (genes) • Factors (genes) are passed from parent to child in reproductive cells • Each individual contains pairs of factors (alleles) in every cell except reproductive cells

  35. MENDEL'S WORK • Postulates (Modernized) • Paired factors (alleles) segregate during the formation of reproductive cells (meiosis) [Law of segregation] • There is an equal probability that a given reproductive cell will get either factor (allele) • Each factor (allele) has an equal chance of combining with the other factor (allele) during fertilization

  36. MENDEL'S WORK • Postulates (Modernized) • Sometimes one factor (allele) dominates the other in phenotype • When considering two or more traits, each trait assorts independently of the others [Law of independent assortment]

  37. Genes and Chromosomes • Relationship not obvious • Genes are located on chromosomes • Several researchers proposed in the early 1900s that genes are located on chromosomes • The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment

  38. The chromosome theory of inheritance states that: Genes have specific loci (positions) on chromosomes Chromosomes that undergo segregation and independent assortment Chromosome Theory

  39. Genes vs Chromosomes • Thomas Hunt Morgan -- fruit flies • Characteristics that make fruit flies a convenient organism for genetic studies: • They breed at a high rate • A generation can be bred every two weeks • They have only four pairs of LARGE chromosomes

  40. Morgan’s Experimental Evidence: Scientific Inquiry • Thomas Hunt Morgan -- fruit flies • Looked for “wild type” phenotype • Alternate forms -- mutant phenotypes

  41. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair • Mate male flies with white eyes (mutant) with female flies with red eyes (wild type) • The F1 generation all had red eyes • What are you going to get in the F2?

  42. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair • Mate male flies with white eyes (mutant) with female flies with red eyes (wild type) • The F1 generation all had red eyes • The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes • White-eye mutant allele -- Y chromosome • Finding supports the chromosome theory

  43. P Generation F1 Generation F2 Generation P Generation Ova (eggs) Sperm F1 Generation Ova (eggs) Sperm F2 Generation LE 15-4

  44. Parents Ova Sperm Zygotes (offspring) The X-Y system The X-0 system The Z-W system The haplo-diploid system The Chromosomal Basis of Sex • Sex -- inherited phenotype determined by the presence or absence of chromosomes

  45. Sperm Sperm Sperm Ova Ova Ova Inheritance of Sex-Linked Genes • genes for many characters unrelated to sex • sex-linked gene • Specific patterns of inheritance • Affected mom X Normal dad → all daughters carriers; all sons affected

  46. X inactivation in Female Mammals • One X chromosome in each cell is randomly inactivated during embryonic development • Heterozygous? -- a mosaic for that character

  47. Two cell populations in adult cat: Active X Early embryo: Orange fur X chromosomes Cell division and X chromosome inactivation Inactive X Inactive X Black fur Allele for orange fur Active X Allele for black fur X inactivation in Female Mammals • One X chromosome in each cell is randomly inactivated during embryonic development • Heterozygous? -- a mosaic for that character

  48. Table 9.1

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