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Chapter 6 Genetics

Chapter 6 Genetics. Outline Chromosomes Normal Cell Division Lyon Hypothesis Molecular Composition of Chromosomes Genes and Chromosomes. Genetics. (pg. 198) The inheritance and expression of inherited traits

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Chapter 6 Genetics

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  1. Chapter 6 Genetics

  2. Outline Chromosomes Normal Cell Division Lyon Hypothesis Molecular Composition of Chromosomes Genes and Chromosomes

  3. Genetics (pg. 198) The inheritance and expression of inherited traits Some alterations may be found as part of a syndrome, or may occur independently. Cleft lip and palate would be an example. Syndrome A distinctive association of signs and symptoms occurring together Some syndromes are inherited and others are not.

  4. Genetics (cont.) (pg. 199) Genotype The genetic composition Phenotype The observable appearance Refers to the physical, biochemical, and physiologic traits of an individual

  5. Chromosomes (pg. 199) Genes The hereditary units transmitted from one generation to another Genes are found on chromosomes, which are located in the nucleus of a cell. Chromosomes contain DNA, which directs the production of amino acids, polypeptides, and proteins by the cell. DNA has the ability to duplicate itself.

  6. Chromosomes (cont.)

  7. Normal Cell Division Mitosis Stages of mitosis Meiosis

  8. Mitosis (pg. 199) Somatic cells All cells of the body, with exception of ova and spermatozoa Mitosis The process of cellular division in a somatic cell during a part of the cell’s life span called the mitotic cycle.

  9. Mitotic Cycle (pgs. 199-200) After each cell division is complete, the cell enters the gap 1 (G1) phase. This is followed by the S phase, where replication of DNA takes place. The gap 2 (G2) phase follows the S phase and ends when mitotic division takes place.

  10. Stages of Mitosis (pgs. 199-200) Four stages Prophase The chromosomes are lining up toward metaphase. Metaphase Chromosomes at the equatorial plane of the cell Long and short arms are joined at the centromere. Each identical half is called a chromatid. Anaphase The chromatids are in the process of splitting. Telophase Cytokinesis occurs.

  11. Stages of Mitosis (cont.)

  12. Meiosis (pgs. 200-201) Meiosis (reduction division) A two-step process of cell division The primitive germ cells reduce their chromosome number by half and become mature germ cells. Primitive germ cells have two chromosomes for each pair and are called diploid. Mature germ cells have half the number of germ cells and are called haploid. This process maintains the normal number of human chromosomes at 46.

  13. First Meiosis (pgs. 200-201) The members of each pair of chromosomes line up and exchange segments at contacts known as chiasmata. The chromosomes separate, but no splitting of the centromere occurs. Each member of the pair migrates to one of the new cells, each of which contains 23 chromosomes but twice the final amount of DNA.

  14. First Meiosis (cont.)

  15. First Meiosis (cont.) (pg. 201) Nondisjunction Occasionally, both chromosomes that were crossing over do not separate, and both migrate to the same cell. Down syndrome (trisomy 21) An example of this type of abnormality in which three of chromosome 21 are found. In a female, oogenesis (ovum development) occurs around the third month of prenatal life. The older the woman, the greater is the chance of a trisomic ovum.

  16. Second Meiosis (pg. 201) Essentially a mitotic division Each chromosome splits longitudinally. No replication of DNA occurs before the second meiosis.

  17. Second Meiosis (cont.)

  18. Lyon Hypothesis (pgs. 201-202) During the early period of embryonic development, the genetic activity of one of the X chromosomes in each cell of a female embryo is inactivated. The inactivated chromosome forms a contracted structure known as a Barr body. It appears as a dark dot at the periphery of the nucleus.

  19. Lyon Hypothesis (cont.)

  20. Molecular Composition of Chromosomes Deoxyribonucleic acid (DNA) Ribonucleic acid (RNA) Types of ribonucleic acid

  21. Deoxyribonucleic Acid (DNA) (pgs. 202-203) Chromosomes contain DNA. DNA contains the basic code or template that carries all genetic information. Nucleotide The basic unit of DNA Four bases are found in DNA. Adenine, guanine, thymine, cytosine A/T and G/C

  22. Deoxyribonucleic Acid (DNA) (cont.)

  23. Deoxyribonucleic Acid (DNA) (cont.) The bases form chains that are coiled to form the double helix. A sequence of three bases is called a codon. It codes for an amino acid. Several amino acids form a polypeptide, and one or more polypeptides form a protein. Mitochondrial DNA is maternally inherited. It is passed from mother to offspring regardless of sex.

  24. Ribonucleic Acid (RNA) (pg. 203) The genetic code contained in DNA is transcribed into RNA. It is a single strand . Its sugar is a ribose rather than the deoxyribose of DNA. The base uracil replaces thymine in DNA.

  25. Types of Ribonucleic Acid (pg. 203) There are four types of RNA. Messenger (mRNA) Carries the message for the DNA to ribosomes in the cytoplasm Transfer (tRNA) Transfers amino acids from the cytoplasm to the mRNA Ribosomal (rRNA) Combines with several polypeptides to form ribosomes Heterogeneous (hnRNA) The precursor to mRNA, found within the nucleus

  26. Types of Ribonucleic Acid (pg. 203) To produce a protein mRNA carries genetic code for the formation of that protein to the ribosomes. tRNA brings amino acids to the ribosomes from the cellular cytoplasm. The amino acid sequence forms proteins according to the genetic code. Proteins exit the ribosomes as they are formed.

  27. Types of Ribonucleic Acid (cont.)

  28. Genes and Chromosomes Chromosomal Abnormalities Gross Chromosomal Abnormalities Patterns of Inheritance

  29. Genes and Chromosomes (cont.) (pg. 203) Alleles The genes that are located at the same level (locus) in homologous chromosomes and that dictate the same functions or characteristics When allelic genes are identical, the person is homozygous for that gene. When genes are different, the person is heterozygous for that gene.

  30. Genes and Chromosomes (cont.) (pg. 203) If a gene can express its effect clinically with a single dose as in combination, it is dominant. If a gene requires a double dose to be expressed, the resulting characteristic or function is recessive.

  31. Chromosomal Abnormalities (pgs. 204-205) Can be divided into Molecular abnormalities Occur at the DNA level Not detectable microscopically Most inherited disorders are at the level of one or both allelic genes. Gross abnormalities Can be observed in a karotype A photographic representation of a person’s chromosomal constitution

  32. Chromosomal Abnormalities (cont.)

  33. Gross Chromosomal Abnormalities Alterations in Number and Structure of Chromosomes Clinical Syndromes Resulting from Gross Chromosomal Abnormalities

  34. Alterations in Number and Structure of Chromosomes (pg. 204) Caused by either alterations in chromosome number or alterations in structure Alterations in number Euploid A complete second set of chromosomes Polyploid Three or four complete sets of chromosomes Aneuploid Any extra number of chromosomes that do not represent an exact multiple of the total

  35. Alterations in Number and Structure of Chromosomes (cont.) (pg. 204) Alterations in structure Deletion Loss of part of a chromosome Translocation A portion of a chromosome attached to another chromosome Inversion A portion of a chromosome is upside-down Duplication A chromosome is larger than normal; the extra segment is identical to a segment of a normal chromosome

  36. Clinical Syndromes Resulting from Gross Chromosomal Abnormalities Trisomy 21 Trisomy 13 Turner Syndrome Klinefelter Syndrome Cri du Chat Syndrome and Wolf-Hirschhorn Syndrome

  37. Trisomy 21 (pg. 204) Down syndrome The most frequent trisomy The facies are characterized by slanted eyes. More than 30% have heart abnormalities. Fissured tongue is frequently seen. Gingival and periodontal disease has been reported in 90% of affected individuals. Hypodontia, abnormally shaped teeth, anomalies in eruption with malposition and crowding of teeth are common.

  38. Trisomy 13 (pgs. 204, 206) Characterized by multiple abnormalities in various organs 70% die within the first 7 months of life. Characteristic facial clinical findings include bilateral cleft lip and palate, and microphthalmia or anophthalmia.

  39. Trisomy 13 (cont.)

  40. Turner Syndrome (pgs. 204, 206) Female karyotype Usually only one X chromosome Clinically of short stature with a webbing of the neck and edema of the hands and feet Smears have a lack of Barr bodies.

  41. Turner Syndrome (cont.)

  42. Klinefelter Syndrome (pg. 206) Most are from nondisjunction of the X chromosome. Male phenotype The maxilla is slightly hypoplastic (underdeveloped). May be XXXY or XXXXY The greater the number of X chromosomes, the more pronounced are the clinical manifestations.

  43. Cri du Chat (Cat Cry) Syndrome and Wolf-Hirschhorn Syndrome (pg. 206) Abnormalities caused by deletions Cri du chat syndrome A deletion on the short arm of chromosome 5 Cat-like cry at birth, mentally retarded Wolf-Hirschhorn syndrome A deletion on the short arm of chromosome 4 Cleft palate and IQ < 30

  44. Patterns of Inheritance Autosomal Dominant Inheritance Autosomal Recessive Inheritance X-Linked Inheritance Lyon hypothesis and X-linked recessive traits Genetic Heterogeneity Examples of Molecular Chromosomal Abnormalities

  45. Autosomal Dominant Inheritance (pg. 207) Transmitted vertically from one generation to the next Males and females are equally affected. Lack of penetrance may occur if an individual carries a gene without presenting any clinical manifestation. Penetrance refers to the number of individuals affected. Expressivity pertains to the degree to which an individual is affected.

  46. Autosomal Recessive Inheritance (pg. 207) Individuals exhibiting an autosomal recessive trait must be homozygous for the gene. Risk is a mathematical estimate of the probability of an event occurring. The chance of having deleterious genes in common increases among close relatives. Consanguinity A familial relationship

  47. X-Linked Inheritance (pg. 207) Women have two X chromosomes. They may be either heterozygous or homozygous for a gene that is located on the X chromosome. Men have only one X and one Y chromosome. If a deleterious gene occurs on the X chromosome in a male, the condition or trait will be seen clinically regardless of the dominant or recessive behavior of the same gene in women.

  48. Lyon Hypothesis and X-linked Recessive Traits (pgs. 207-208) One of the X chromosomes in the female is genetically cancelled at an early stage of embryonic development. This cancellation affects X chromosomes from both maternal and paternal lines.

  49. Genetic Heterogeneity (pg. 208) Used when a condition has more than one inheritance pattern as well as differences in the degree of clinical manifestations for each of the inherited varieties Recessive – the person must be homozygous for the trait to be seen Oligogenic inheritance – characteristics or traits that are inherited by the participation of several genes

  50. Examples of Molecular Chromosomal Abnormalities Inherited disorders affecting the gingiva and periodontium Inherited disorders affecting the jawbones and facies Inherited disorders affecting the oral mucosa Inherited disorders affecting the teeth

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