How Genes Are Transmitted from Generation to Generation

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