1. Chapter 12 The Chromosomal Basis of Inheritance

2. Timeline 1866- Mendel's Paper 1875- Mitosis worked out
1890's- Meiosis worked out
1902- Sutton, Boveri et. al. connect chromosomes to Meiosis.

3. Sutton Developed the “Chromosome Theory of Inheritance”.
Mendelian factors or alleles are located on chromosomes.
Chromosomes segregate and show independent assortment.

6. Morgan Chose to use fruit flies as a test organism in genetics.
Allowed the first tracing of traits to specific chromosomes.

7. Fruit Fly Drosophila melanogaster
Early test organism for genetic studies.

8. Reasons Small Cheap to house and feed Short generation time
Many offspring
Few chromosomes

9. Genetic Symbols Mendel - use of uppercase or lowercase letters.
T = tall
t = short
Morgan: symbol from the mutant phenotype.
+ = wild phenotype

10. Examples Recessive mutation: w = white eyes w+ = red eyes
Dominant Mutation
Cy = Curly wings
Cy+ = Normal wings

11. Morgan Observed:
A male fly with a mutation for white eyes.

12. Morgan crossed
The white eye male with a normal red eye female.

13. The F1 offspring: All had red eyes.
This suggests that white eyes is a genetic _________?
Recessive.

14. F1 X F1 = F2 Morgan expected the F2 to have a 3:1 ratio of red:white
He got this ratio, however, all of the white eyed flies were MALE.
Therefore, the eye color trait appeared to be linked to sex.

15. Morgan discovered: Sex linked traits.
Genetic traits whose expression are dependent on the sex of the individual.

16. Fruit Fly Chromosomes

18. Sex linked traits
Sex linked traits in humans will be covered in a few minutes.

19. Morgan Discovered There are many genes, but only a few chromosomes.
Therefore, each chromosome must carry a number of genes together as a “package”.

20. Linked Genes Traits that are located on the same chromosome. Result:
Failure of Mendel's Law of Independent Assortment.
Ratios mimic monohybrid crosses.

21. Body Color and Wing type

22. Example
b+b vg+vg X bb vgvg (b+ linked to vg+) (b linked to vg) If unlinked: 1:1:1:1 ratio. If linked: ratio will be altered.

26. Crossing-Over Breaks up linkages and creates new ones.
Recombinant offspring formed that doesn't match the parental types.

27. If Genes are Linked: Independent Assortment of traits fails.
Linkage may be “strong” or “weak”.

28. Linkage Strength
Degree of strength related to how close the traits are on the chromosome.
Weak - farther apart
Strong - closer together

29. Genetic Maps Constructed from crossing-over frequencies.
1 map unit = 1% recombination frequency.

30. Comment - only good for genes that are within 50 map units of each other. Why?

31. Genetic Maps
Have been constructed for many traits in fruit flies, humans and other organisms.

33. Sex Linkage in Biology Several systems are known: Mammals – XY and XX
Diploid insects – X and XX
Birds – ZZ and ZW
Social insects – haploid and diploid

35. Chromosomal Basis of Sex in Humans
X chromosome - medium sized chromosome with a large number of traits.
Y chromosome - much smaller chromosome with only a few traits.

38. Human Chromosome Sex Males - XY Females - XX
Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.

41. SRY Sex-determining Region Y chromosome gene. If present - male
If absent - female
SRY codes for a cell receptor.

42. Sex Linkage Inheritance of traits on the sex chromosomes.
X- Linkage (common)
Y- Linkage (very rare if exists at all)

43. Males Hemizygous - 1 copy of X chromosome.
Show ALL X traits (dominant or recessive).
More likely to show X recessive gene problems than females.

44. X-linked Disorders Color blindness Duchenne's Muscular Dystrophy
Hemophilia (types a and b)
Immune system defects

45. Samples of X-linked patterns:

46. X-linked Patterns
Trait is usually passed from a carrier mother to 1/2 of sons.
Affected father has no affected children, but passes the trait on to all daughters who will be carriers for the trait.

47. Comment Watch how questions with sex linkage are phrased:
Chance of children?
Chance of males?

48. Can Females be color-blind?
Yes, if their mother was a carrier and their father is affected.

49. Y-linkage Hairy ear pinnae.
Comment - new techniques have found a number of Y-linked markers that can be shown to run in the males of a family.
Ex: Jewish priests

50. Sex Limited Traits Traits that are only expressed in one sex.
Ex – prostate

51. Sex Influenced Traits
Traits whose expression differs because of the hormones of the sex.
These are NOT on the sex chromosomes.
Ex. – beards, mammary gland development, baldness

52. Baldness Testosterone – the trait act as a dominant.
No testosterone – the trait act as a recessive.
Males – have gene = bald
Females – must be homozygous to have thin hair.

53. Barr Body Inactive X chromosome observed in the nucleus.
Way of determining genetic sex without doing a karyotype.

54. Lyon Hypothesis Which X inactivated is random.
Inactivation happens early in embryo development by adding CH3 groups to the DNA.
Result - body cells are a mosaic of X types.

55. Examples
Calico Cats.
Human examples are known such as a sweat gland disorder.

56. Calico Cats XB = black fur XO = orange fur
Calico is heterozygous, XB XO.

58. Question? Why don’t you find many calico males?
They must be XB XOY and are sterile.

59. Chromosomal Alterations
Changes in number.
Changes in structure.

60. Number Alterations
Aneuploidy - too many or too few chromosomes, but not a whole “set” change.
Polyploidy - changes in whole “sets” of chromosomes.

61. Nondisjunction When chromosomes fail to separate during meiosis
Result – cells have too many or too few chromosomes which is known as aneuploidy

63. Meiosis I vs Meiosis II Meiosis I – all 4 cells are abnormal
Meiosis II – only 2 cells are abnormal

64. Aneuploidy
Caused by nondisjunction, the failure of a pair of chromosomes to separate during meiosis.

65. Types
Monosomy: 2N - 1
Trisomy: 2N + 1

66. Turner Syndrome 2N - 1 or 45 chromosomes Genotype: X_ or X0.
Phenotype: female, but very poor secondary sexual development.

67. Characteristics Short stature. Extra skin on neck. Broad chest.
Usually sterile
Normal mental development except for some spatial problems.

68. Question Why are Turner Individuals usually sterile?
Odd chromosome number.
Two X chromosomes need for ovary development.

69. Other Sex Chromosome changes
Kleinfelter Syndrome
Meta female
Supermale

70. Kleinfelter Syndrome 2N + 1 Genotype: XXY
Phenotype: male, but sexual development may be poor. Often taller than average, mental development fine, usually sterile.

72. Meta female 2N + 1 or 2N + 2 Genotype: XXX or XXXX
Phenotype: female, but sexual development poor. Mental impairment common.

73. Super male 2N + 1 or 2N + 2 Genotype: XYY or XYYY
Phenotype: male, usually normal, fertile.

75. Trisomy events Trisomy 21: Down's Syndrome Trisomy 13: Patau Syndrome
Both have various physical and mental changes.

77. Question? Why is trisomy more common than monosomy?
Fetus can survive an extra copy of a chromosome, but being hemizygous is usually fatal.

78. Question? Why is trisomy 21 more common in older mothers?
Maternal age increases risk of nondisjunction.

79. Polyploid
Triploid= 3N
Tetraploid= 4N
Usually fatal in animals.

80. Question?
In plants, even # polyploids are often fertile, why odd # polyploids are sterile. Why?
Odd number of chromosomes can’t be split during meiosis to make spores.

81. Structure Alterations
Deletions
Duplications
Inversions
Translocations

83. Translocations

84. Result Loss of genetic information.
Position effects: a gene's expression is influenced by its location to other genes.

85. Cri Du Chat Syndrome Part of p arm of #5 missing.
Good survival, but low birth weight and slow gain.
Severe mental impairment.
Small sized heads common.

86. Cri Du Chat Syndrome

87. Philadelphia Chromosome
An abnormal chromosome produced by an exchange of portions of chromosomes 9 and 22.
Causes chronic myeloid leukemia.

88. Parental Imprinting of Genes
Gene expression and inheritance depends on which parent passed on the gene.
Usually caused by different methylations of the DNA.

91. Example: Prader-Willi Syndrome and Angelman Syndrome
Both lack a small gene region from chromosome 15.
Male imprint: Prader-Willi Female imprint: Angelman

92. Cause:
Imprints are "erased" in gamete producing cells and re-coded by the body according to its sex.
Gametes are methylated to code as “male “ or “female”.

93. Result
Phenotypes don't follow Mendelian Inheritance patterns because the sex of the parent does matter.

94. Extranuclear Inheritance
Inheritance of genes not located on the nuclear DNA.
DNA in organelles.
Mitochondria
Chloroplasts

95. Result Mendelian inheritance patterns fail.
Maternal Inheritance of traits where the trait is passed directly through the egg to the offspring.

96. Chloroplasts Gives non-green areas in leaves, called variegation.
Several different types known.
Very common in ornamental plants.

97. Variegation in African Violets

98. Variegated Examples

99. Mitochondria Myoclonic Epilepsy Ragged Red-fiber Disease
Leber’s Optic Neuropathy
All are associated with ATP generation problems and affect organs with high ATP demands.

100. Comment Cells can have a mixture of normal and abnormal organelles.
Result - degree of expression of the maternal inherited trait can vary widely.

101. Summary Know about linkage and crossing-over.
Sex chromosomes and their pattern of inheritance.

102. Summary
Be able to work genetics problems for this chapter.