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Understanding Genetic Inheritance: From Mendel to Modern Science

Learn how genes are transmitted across generations, exploring Mendel's experiments, human traits, pedigree analysis, and various inheritance patterns. Discover autosomal recessive and dominant disorders like albinism, cystic fibrosis, and sickle cell anemia.

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Understanding Genetic Inheritance: From Mendel to Modern Science

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  1. How Genes Are Transmitted from Generation to Generation Chapter 4

  2. Central Points • Genes are transmitted from generation to generation • Traits are inherited according to predictable rules • Dominant, recessive, and X-linked traits follow these rules

  3. Case A: A Family’s Dilemma • Alan’s mother died from Huntington disease (HD) • HD caused by a mutant gene and one copy of gene will cause the disease • Neurologic symptoms develop between ages 30–50, progress slowly, fatal in 10–20 years • Genetic test available

  4. 4.1 How Are Genes Transmitted? • Gregor Mendel: father of genetics • Experiments with pea plants in 1800s • Traits,distinguishing characteristics • Specific patterns in the way traits were passed from parent to offspring

  5. Mendel’s Experiments • Some traits disappeared in the first generation of offspring (all tall) • Reappeared in 3:1 ratio (tall:short) • Dominant traitpresent in the first-generation offspring (tall) • Recessive trait absent in first generation but reappeared in the next generation (short)

  6. Traits Are Passed by Genes • “Factors” or genes transmitted from parent to offspring • Each parent carries a pair of genes for a trait but contributes only one gene to each offspring • Separation of gene pair occurs during meiosis

  7. Genes • Alleles: variations of a gene • Homozygous: identical alleles of a gene • TT or tt • Heterozygous: nonidenticalalleles • Tt

  8. Different Plant Heights

  9. Phenotype and Genotype • Phenotype: what an organism looks like • tall or short • Genotype: genetic makeup • TT, Tt, and tt • Identical phenotypes may have different genotypes • TT or Tt have tall phenotype

  10. Mendel’s Law of Segregation • Two copies of each gene separate during meiosis • One copy of each gene in the sperm or egg • Each parent gives one copy of each gene

  11. Mendel’s Law of Independent Assortment • Members of a gene pair segregate into gametes independently of other gene pairs • Gametes can have different combinations of parental genes

  12. Sorting of Alleles

  13. Animation: Segregation of alleles: pea plants

  14. Human Traits: Albinism • Pigmentation dominant and lack of pigment recessive • AA, Aa: Pigmented • aa: Albino • Both parents Aa, each child has 25% chance of being albino (3:1 ratio)

  15. Segregation of the Albino Allele

  16. Fig. 4-3a, p. 61

  17. Fig. 4-3b, p. 61

  18. Pedigree 1 • Shows all family members and identifies those affected with the genetic disorder

  19. Pedigree 2

  20. Pedigree Symbols

  21. p. 62

  22. p. 62

  23. Proband • Person who is the focus of the pedigree • Indicated by an arrow and the letter P

  24. Animation: Pedigree analysis - predicting future generations

  25. Animation: Observing Patterns in Inherited Traits (Crossing Pea Plants)

  26. Animation: Observing Patterns in Genetic Traits (genetic terms)

  27. Animation: Chromosomes and Human Inheritance (pedigree diagrams)

  28. 4.2 Examining Human Pedigrees • Determine trait has dominant or recessive inheritance pattern • Predict genetic risk for: • Pregnancy outcome • Adult-onset disorder • In future offspring

  29. Three Possible Patterns of Inheritance • Autosomal recessive • Autosomal dominant • X-linked recessive • Autosomal on chromosomes 1–22 • X-linked traits on the X chromosome

  30. Autosomal Recessive • Unaffected parents can have affected children • All children of affected parents are affected • Both parents Aa, risk of affected child is 25% • ~Equal affected male and female • Both parents must transmit the gene for a child to be affected

  31. Consanguinity • Individuals related to each other and indicated by double line between parents

  32. Autosomal Recessive Pedigree

  33. Autosomal Recessive Genetic Disorders

  34. Albinism • A = normal coloring; a = albinism • Group of genetic conditions, lack of pigmentation (melanin) in the skin, hair, and/or eyes • Normally, melanin in pigment granules inside melanocytes • In albinism, melanocytes present but cannot make melanin • Oculocutaneous albinism type I (OCA1)

  35. Cystic Fibrosis (CF) • C = normal; c = cystic fibrosis • CF affects glands that produce mucus and digestive enzyme • CF causes production of thick mucus in lungs blocks airways • Develop obstructive lung diseases and infections • Identified CF gene and protein (CFTR)

  36. Animation: Segregation of alleles: cystic fibrosis

  37. Sickle Cell Anemia (SCA) • S = normal red blood cells; s = sickle) • High frequency in areas of West Africa, Mediterranean Sea, India • Abnormal hemoglobin molecules aggregate to form rods • Red blood cells, crescent- or sickle-shaped, fragile and break open

  38. Normal and Sickled Cells

  39. Autosomal Dominant (1) • Requires one copy of the allele (Aa) rarely present in a homozygous condition (AA) • aa: Unaffected individuals • Affected individual has at least one affected parent • Aa X aa: Each child has 50% chance of being affected

  40. Autosomal Dominant (2) • ~Equal numbers of affected males and females • Two affected individuals may have unaffected children • Generally, AA more severely affected, often die before birth or in childhood

  41. Autosomal Dominant Pedigree

  42. Autosomal Dominant Genetic Disorders

  43. Animation: Chromosomes and Human Inheritance (autosomal-dominant inheritance)

  44. Animation: Chromosomes and Human Inheritance (autosomal-recessive inheritance)

  45. Neurofibromatosis (NF) • N = Neurofibromatosis 1; n = normal • Many different phenotypes • Café-au-lait spots, or noncancerous tumors in the nervous system can be large and press on nerves • Deformities of the face or other body parts (rarely) • NF gene has a very high mutation rate

  46. Neurofibromatosis

  47. Huntington Disease (HD) • H = Huntington disease; h = normal • Causes damage in brain from accumulation of huntingtin protein • Symptoms begin slowly (30–50 years old) • Affected individuals may have already had children (50% chance with one Hh parent) • Progressive neurological signs, no treatment, die within 10–25 years after symptoms

  48. Brain Cells of a Person with HD

  49. Adult-Onset Disorders • Expressed later in life • Present problems in pedigree analysis, genetic testing may be required • Examples: • Huntington disease (HD) • Adult polycystic kidney disease (ADPKD) • Both examples are autosomal dominant

  50. Case A Questions • Who should be tested? • Who should know the results of the test? • How should the test results be used? • See the textbook for further questions on this case

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