1. Single-Gene Inheritance
CHAPTER 2
Single-Gene Inheritance

2. CHAPTER OUTLINE 2.2 Single-gene inheritance patterns
2.3 The chromosomal basis of single-gene inheritance patterns
2.1 Genes and chromosomes
2.5 Sex-linked single-gene inheritance patterns
2.6 Human pedigree analysis

3. Single-gene inheritance patterns

4. The monastery of the father of genetics, Gregor Mendel
Chapter 2 Opener

5. The seven phenotypic pairs studied by Mendel
Figure 2-9

6. Cross-pollination and selfing are two types of crosses
Figure 2-10

7. Mendel’s crosses resulted in specific phenotypic ratios
Figure 2-11

8. Table 2-1

9. A single-gene model explains Mendel’s ratios
Figure 2-12

10. Mendel’s explanation:
1. Existence of genes – hereditary determinants of a particulate nature.
2. Genes are in pairs – alternative phenotypes of a character or trait are determined by
different forms of a single type of gene – alleles.
3. Principle of segregation – members of the gene pair separate equally into gametes.
4. Gametic content – each gamete carries only one member of each gene pair.
5. Random fertilization – union of one gamete from each parent to form the offspring

11. Questions about heredity answered by Mendel:
1. What is inherited?
alleles of genes
2. How are they inherited?
according to principles of segregation and independent
assortment
3. What is the role of chance?
for each individual, inheritance is determined by chance,
but within a population this chance operates in a context
of strictly defined probabilities

12. The chromosomal basis of single-gene inheritance patterns

13. Stages of the asexual cell cycle
Figure 2-13

14. Cell division in common life cycles
Figure 2-14

15. Key stages of meiosis and mitosis
Figure 2-15

16. Stages of Mitosis
Box 2-1

17. Stages of Meiosis
Box 2-2

18. Demonstration of equal segregation within one meiocyte in the yeast S
Demonstration of equal segregation within one meiocyte in the yeast S. cerevisiae
Figure 2-17

19. DNA molecules replicate to form identical chromatids
Figure 2-18

20. Nuclear division at the DNA level
Figure 2-19

21. Genes and chromosomes

22. The nuclear genome
Figure 2-2

23. A diploid genome visualized
Figure 2-3

24. Chromosomal DNA is wrapped around histones
Figure 2-4a

25. Chromosomal DNA is wrapped around histones
Figure 2-4b

26. Chromosomal condensation by supercoiling
Figure 2-5

28. Progressive levels of chromosome packing
1. DNA winds onto nucleosome spools
2. The nucleosome chain coils into a solenoid
3. Solenoid forms loops, and the loops attach to a central scaffold
4. Scaffold plus loops arrange themselves into a giant supercoil

29. Visible chromosome landmarks

30. Chromosome number
Highest known diploid chromosome number
Indian fern Ophioglossum reticulatum (2n = 1260)

31. Chromosome size
and type

34. Heterochromatin and euchromatin
Feulgen stain
Heterochromatin – densely staining region (more condensed)
Euchromatin – poorly staining region (contains most of the active genes)

35. Centromeres
Location of satellite DNA in mouse chromosomes

36. Telomeres

37. Banding patterns
G-banding chromosomes of a human female (staining with Giemsa reagent)

38. Enlargement of chromosome 13

39. Labeling for G bands of chromosome 13

40. Landmarks that distinguish the chromosomes of corn
Features such as size, arm ratio, heterochromatin, number and position of
thickenings, number and location of nucleolar organizers, and banding pattern
identify the individual chromosomes within the set that characterizes a species

41. Some landmarks of tomato chromosome 2
Figure 2-6

42. Representative chromosomal landscapes
Figure 2-7

43. A specific human chromosomal landscape
Figure 2-8

44. Sex-linked single-gene inheritance patterns

46. A dioecious plant species – Osmaronia dioica

47. A dioecious plant species – Aruncus diocius

48. Model Organism: Drosophila

49. Model Organism: Drosophila

50. Human sex chromosomes
Figure 2-25

51. Red-eyed and white-eyed Drosophila
Figure 2-26

52. An example of X-linked inheritance
Figure 2-27

53. Combining probabilities
Product rule
When two independent events occur with the probabilities p and q respectively, then the probability of their joint occurrence is pq.  If the word "and" is used or implied in the phrasing of a problem solution, a multiplication of independent probabilities is usually required.
Example: In test crossing a heterozygous black guinea pig (Bb x bb), let the probability of a black (Bb) offspring be p = 1/2 and of a white offspring be q = 1/2.  The combined probability of the first two offspring being white (i.e. the first offspring is white and the second offspring is white) = q x q = q2 = (1/2)2 = 1/4.
Problem: What is the probability of getting 6(Red) 6(Green) 6(Blue) when all three dice are rolled at the same time?
Each dice has six sides and the probability of obtaining any one side is 1/6.  So the combined probability in the present example is 1/6 x 1/6 x 1/6 = 1/216.

54. Sum rule
Mutually exclusive events are those in which the occurrence of any one of them excludes the occurrence of the others.  The word "or" is usually required or implied in the phrasing of problem solutions involving mutually exclusive events, signaling that an addition of probabilities is to be performed.
Example: With two dice, what is the probability of getting either two 4s or two 5s? The probability of getting two 4s is 1/6 x 1/6 = 1/36. The probability of getting two 5s is 1/6 x 1/6 = 1/36. The probability of getting two 4s or two 5s is 1/36 + 1/36 = 1/18.
Problem: What is the probability of getting two 6s and one 5 on any dice when three dice (Red, Green, Blue) are rolled at the same time?
Three ways to get two 6s and one 5: 6R, 6G, 5B = 1/6 x 1/6 x 1/6 or                    + 6R, 5G, 6B = 1/6 x 1/6 x 1/6 or                    + 5R, 6G, 6B = 1/6 x 1/6 x 1/6
                    = 3/216 = 1/72.

55. Human pedigree analysis

56. Pedigree Analysis
pedigree analysis is a scrutiny of records of matings
pedigrees use standard sets of symbols to depict family trees and
lineages
pedigrees provide concise and accurate records of families
pedigrees are helpful in following and diagnosing heritable traits
(for example, diseases and medical conditions) by describing patterns
of inheritance
- pedigrees are useful in mapping (locating and isolating) genes
“responsible” for certain traits

57. Pedigree construction
- use standard set of symbols
- one generation per row (oldest at the top)
- siblings are shown in order of birth (from left to right)
- generations are given Roman numerals (I, II, III, IV, etc)
- individuals within a generation (row) are given Arabic
numerals (1, 2, 3, 4, etc)

58. Pedigree symbols
Figure 2-28

59. Analyzing pedigrees
- trial and error: consider one pattern of inheritance at a time for each
mating in the pedigree and try to find evidence against it; repeat for
each pattern of inheritance, for example, autosomal recessive or
dominant, X-linked recessive or dominant, etc
- patterns of inheritance follow Mendelian rules; Mendelian ratios
are rarely observed
- assumption: for rare traits unaffected people entering into a family
pedigree (for example, by marriage) are considered homozygous
normal
- result: pedigrees can frequently rule out, but not necessarily prove,
a certain pattern of inheritance

60. Autosomal recessive I II III IV
- the trait is found equally in both males and females
- affected individuals usually have unaffected parents
- the pattern of inheritance is often horizontal with several generations
of unaffected individuals, but then several siblings in one generation
are affected

61. Autosomal dominant I II III IV
- the trait is found equally in both males and females
- every affected individual has at least one affected parent
- trait shows vertical pattern of inheritance, that is affected
males and females are observed in each generation

62. Pseudoachondroplasia phenotype
Figure 2-30

63. Inheritance of an autosomal dominant disorder
Figure 2-31

64. Late onset of Huntington disease
Figure 2-32

65. Polydactyly
Figure 2-33a

66. Polydactyly
Figure 2-33b

67. X-linked recessive I II III IV - more males than females are affected
- all the sons of an affected mother will be affected
- half the sons of a carrier mother will be affected
- all daughters of carrier mothers will be normal, but half will be carriers
- affected males do not transmit the trait to their sons
- trait often skips a generation

68. Inheritance of an X-linked recessive disorder
Figure 2-36

69. Inheritance of hemophilia in European royalty
Figure 2-37a

70. X-linked dominant I II III IV
- trait observed in both males and females
- affected males ALWAYS transmit the trait to their daughters, but to
NONE of their sons
- affected females will transmit the trait to both sons and daughters
- trait does not skip generation

71. Inheritance of an X-linked dominant disorder
Figure 2-39

72. Y-linked I II III IV - only males are affected
- the trait is passed from an affected father to all of his sons

73. Hairy ears: a phenotype proposed to be Y linked
Figure 2-40

74. Sex-influenced and Sex-limited Traits
Not due to X-linked genes
Due to autosomal genes expression influenced by sex hormones
both parents contribute equally to offspring
no notable mother-to-son or father-to-daughter patterns
example: pattern baldness – sex-influenced trait
alleles B and B’
B for bald B’ for nonbald
B > B’ in males, B’ > B in females
genotype BB --- bald in both sexes
genotype BB’ --- bald in males, nonbald in females
genotype B’B’ -- nonbald in both sexes
There are also traits that are sex-influenced, which means that their expression is influenced by the individual's sex. This does not imply that the gene is sex-linked. A human example is pattern baldness. The gene's expression is influenced by hormonal levels and only one copy of the baldness allele is sufficient to cause baldness in a man, whereas two copies are needed in a woman. In effect, it behaves as a dominant in males and as a recessive in females. Though half the sons of a female carrier will be affected, a heterozygous male will also pass the trait to half his sons.
Thus, any trait that appears more frequently in males than females is suspect as either sex-linked or sex-influenced. If it is passed from the father or the mother to half the sons, it is likely sex-influenced. If it seems to skip a generation and the pattern is grandfather to grandson, it is likely sex-linked.

75. Mitochondrial inheritanceII
III IV
- both males and females are affected
- the trait is passed from an affected mother to all her progeny
- affected males do not transmit the trait to any of their progeny