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Pedigree Analysis in Human Genetics Chp.4 Human Pedigrees

Pedigree Analysis in Human Genetics Chp.4 Human Pedigrees. The use of pedigrees is an important method for analyzing the inheritance of traits in human populations. Fig. 4-CO, p. 70. 4.1 Pedigree Analysis and Construction is a Basic Method in Human Genetics.

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Pedigree Analysis in Human Genetics Chp.4 Human Pedigrees

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  1. Pedigree Analysis in Human Genetics Chp.4 Human Pedigrees • The use of pedigrees is an important method for analyzing the inheritance of traits in human populations Fig. 4-CO, p. 70

  2. 4.1 Pedigree Analysis and Construction is a Basic Method in Human Genetics • Analysis of pedigrees using knowledge of Mendelian principles allows us to; • Determine whether the trait has a dominant or recessive pattern of inheritance • Determine whether the gene in question is located on an X or Y chromosome or on an autosome • This kind of information can be used to predict risk

  3. Patterns of Inheritance • Patterns in the pedigree are used to determine how a trait is inherited • Autosomal dominant • Autosomal recessive • X-linked dominant • X-linked recessive • Y-linked • Mitochondrial inheritance

  4. Keep In Mind • Pedigree construction and analysis are basic methods in human genetics

  5. Pedigree Analysis • Pedigree analysis proceeds in several steps: • Rule out patterns of inheritance that are inconsistent with the pedigree • If only one pattern of inheritance is consistent with the pedigree, it is accepted as the pattern for that trait • If more than one pattern in consistent with the pedigree, which one is expected to be more likely? • If due to small sample size, it is impossible to choose among pattern of inheritance, the inclusion of more family members may be necessary

  6. 4.2 Autosomal Recessive Traits • Characteristics of autosomal recessive traits • For rare traits, most affected individuals have unaffected parents • All children of affected parents are affected • The risk of an affected child with heterozygous parents is 25% • The trait is expressed in both males and females

  7. Pedigree: A Rare Autosomal Recessive Trait Fig. 4-2, p. 73

  8. Example of an Autosomal Recessive Trait : Cystic Fibrosis • Cystic fibrosis: A fatal recessive genetic disorder associated with abnormal secretions of the exocrine glands Fig. 4-3a, p. 75

  9. The Frequency of the Gene for Cystic Fibrosis in the Human Population • 1 in 25 Americans of European descent • 1 in 46 Americans of Hispanic descent • 1 in 65 African Americans • 1 in 250 Asian Americans Fig. 4-4, p. 75

  10. Cystic Fibrosis Gene Product (CTRF) • The CFTR gene was identified in 1989 • CFTR protein controls the movement of chloride ions across the plasma membrane Fig. 4-5, p. 76

  11. Exploring Genetics::Was Noah an Albino? • Noah’s “flesh was white as snow” • From the Book of Enoch the Prophet • Phenotype: Lack of pigmentation • Inheritance of albinism • An autosomal recessive trait • Normal, heterozygous parents (may be closely related) • Homozygous recessive offspring (albino)

  12. 4.3 Autosomal Dominant Traits • Characteristics of autosomal dominant traits • Heterozygotes have an abnormal phenotype • Every affected individual has at least one affected parent (except in traits with high mutation rates) • If an affected individual is heterozygous and has an unaffected mate, each child has a 50% chance of being affected • Two affected individuals can have an unaffected child • Usually an affected family member in each generation

  13. Pedigree: An Autosomal Dominant Trait Fig. 4-6, p. 77

  14. Example of an Autosomal Dominant Trait: Marfan Syndrome • Marfan syndrome • An autosomal dominant genetic disorder that affects the skeletal system, cardiovascular system, and eyes • Individuals are tall, thin, long arms and legs. Thin fingers • Heart defects Fig. 4-7, p. 77

  15. Cardiovascular Effects of Marfan Syndrome • Marfan syndrome weakens connective tissue around the base of the aorta

  16. 4.4 Sex Linked Inheritance • Genes on sex chromosomes have a distinct pattern of inheritance • Males (XY) pass their X chromosome to all of their daughters but none of their sons • Females (XX) pass an X chromosome to all of their children • Most genes on the X chromosome are not on the Y chromosome • Males carrying an X-linked recessive allele express the recessive phenotype

  17. Distribution of Sex Chromosomes from Generation to Generation Fig. 4-10, p. 79

  18. Sex Linked Traits • X-linked • Pattern of inheritance that results from genes located on the X chromosome • Y-linked • Pattern of inheritance that results from genes located only on the Y chromosome

  19. In Males, Genes on the X chromosome Hemizygous • Hemizygous • A gene present on the X chromosome that is expressed in males in both the recessive and dominant condition

  20. X-Linked Dominant Traits • Quite rare inheritance pattern • Affected males produce all affected daughters and no affected sons • A heterozygous affected female will transmit the trait to half of her children • Sons and daughters are equally affected • On average, twice as many daughters as sons are affected

  21. Pedigree of an X-linked Dominant Trait Fig. 4-11, p. 79

  22. X-Linked Recessive Traits • X-linked recessive traits affect males more than females because males are hemizygous for genes on the X chromosome

  23. X-Linked Recessive Inheritance • Affected males receive the mutant X-linked allele from their mother and transmit it to all of their daughters, but not to their sons • Daughters of affected males are usually heterozygous • Sons of heterozygous females have a 50% chance of being affected • Hemizygous males (only one X) and females homozygous for the allele are affected

  24. Pedigree: X-Linked Recessive Inheritance Fig. 4-12, p. 80

  25. Example of an X-linked Recessive Trait: Color Blindness • Color blindness • Defective color vision caused by reduction or absence of visual pigments • Three forms: red, green, and blue blindness • About 8% of the male population in the US affected Fig. 4-13, p. 80

  26. Testing For Color Blindness • People with normal color vision see the number 29 in the chart; those who are color-blind cannot see the number Fig. 4-14, p. 81

  27. Color Blindness: Defect in the Retina • Defects in photoreceptor cells of the retina (cone cells) cause color blindness Fig. 4-15, p. 81

  28. Example of an X-linked Recessive Trait: Muscular Dystrophy • Muscular dystrophy • A group of genetic diseases associated with progressive degeneration of muscle tissue • Duchenne and Becker muscular dystrophy are inherited as X-linked recessive traits • Duchenne muscular dystrophy (DMD) affects 1 in 3,500 males in the US

  29. Molecular Characteristics ofDuchenne Muscular Dystrophy • Dystrophin proteins are flexible and that normally stabilize the muscle cells during contraction are defective • Plasma membranes are torn apart during muscle contraction, causing death of muscle tissue Fig. 4-16, p. 82

  30. 4.5 Paternal Inheritance: Y Chromosome • Only males have Y chromosomes • Genes on the Y chromosome are passed directly from father to son • All Y-linked genes are expressed • Males are hemizygous for genes on the Y chromosome • To date only 36 Y-linked traits have been identified

  31. Pedigree: Y-Linked Traits Fig. 4-18, p. 84

  32. 4.6 Non-Mendelian Maternal Inheritance: Mitochondrial Genes • Mitochondria • Cytoplasmic organelles that convert energy from food into ATP (ATP powers cellular functions) • Carry DNA for 37 mitochondrial genes • Genetic disorders in mitochondrial DNA are associated with defects in energy conversion

  33. Mitochondrial Inheritance • Mitochondria (and genetic disorders caused by mutations in mitochondrial genes) are maternally inherited • Mitochondria are transmitted from mothers to all their offspring through the cytoplasm of the egg

  34. Pedigree: Mitochondrial Inheritance Fig. 4-19, p. 84

  35. Exploring Genetics:Hemophilia and History • Queen Victoria passed the X-linked recessive gene for hemophilia to several of her children p. 85

  36. 4.7 An Online Catalog of Human Genetic Traits • OMIM • Genetic traits are described, cataloged, and numbered in a database called Online Mendelian Inheritance in Man • OMIM is updated daily and contains information about all known human genetic traits • Each trait is assigned an OMIM number • There are more that 10,000 entries

  37. 4.8 Many Factors can Affect the Pattern of Inheritance • Variations in gene expression affect pedigree analysis and assignment of genotypes to members of the pedigree • Several factors can affect gene expression • Interactions with other genes in the genotype • Interactions between genes and the environment

  38. 4.8 Many Factors can Affect the Pattern of Inheritance • Phenotypes are often age related • Example: Huntington disease • Penetrance and expressivity cause variations in phenotype • Penetrance: the probability the the phenotype will appear • Expressivity: The range of phenotypes from a given genotype

  39. An Example of Incomplete Penetrance and Variable Expression • Camptodactyly • A dominant trait (immobile, bent little fingers) with variable expression Fig. 4-22, p. 88

  40. Keep In Mind • Patterns of gene expression can be influenced by many different environmental factors

  41. Common recessive alleles can produce pedigrees that resemble dominant inheritance • Common alleles can enter a pedigree from outside the family and thus appear dominant Fig. 4-23, p. 88

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