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 trait present 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: nonidentical alleles 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. Figure 4.3: Segregation of the Albino Allele in Humans.
Fig. 4-3a, p. 61

17. Figure 4.3: Segregation of the Albino Allele in Humans.
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

51. 4.3 X-Linked Recessive Traits
Genes on X chromosome: X-linked
Genes on Y chromosome: Y-linked
For X-linked traits:
Females XX, XX*, or X*X*
Males XY or X*Y
Males cannot be homozygous or heterozygous, they are hemizygous for genes on X
Distinctive pattern of inheritance

52. X-Linked Recessive Inheritance
Mother gives one X chromosome to offspring
Father gives X to daughters and Y to sons
Sons carry X from mother
For recessive traits, X*X* and X*Y affected
More males affected

53. Pedigrees: X-Linked Inheritance

54. X-Linked Recessive Genetic Disorders

55. Inheritance of X-Linked Disorder

56. Animation: Chromosomes and Human Inheritance (X-linked inheritance)

57. Duchenne Muscular Dystrophy (DMD) (1)
XM = normal; Xm = muscular dystrophy
Most common form, affects ~1/3,500 males
Infants appear healthy, symptoms age ~1–6 years
Rapid, progressive muscle weakness
Usually must use a wheelchair by age 12
Death, age ~20 from respiratory infection or cardiac failure

58. Duchenne Muscular Dystrophy (DMD) (2)
DMD gene on the end of X chromosome
Encodes protein dystrophin that supports plasma membrane during contraction
If dystrophin absent or defective, cells are torn apart
Two forms: DMD, and less-serious Becker muscular dystrophy (BMD)

59. Cells of a Person with MD

60. Hemophilia XH = normal; Xh = hemophilia
Lack of clotting: factor VIII in blood
Affected individuals hemorrhage, often require hospitalization to treat bleeding
Hemophilia A most common form of X-linked hemophilia
Females affected if XhXh, both parents must carry the trait

61. Factor VIII
1980s, half of all people with hemophilia became infected with HIV
Recombinant DNA technology now used to make clotting factors free from contamination

62. Case B: The Franklins Find Out More
Alan and siblings concerned about inheriting HD gene for themselves and future children
Who should be tested and why?
How will it affect health insurance coverage?
See the textbook for further questions on this case