1. This Biology II course is for UIC students!!!
Announcement
This Biology II course is for UIC students!!!
(1) How to get lecture slides
및 강의
File name: UIC_Chapter00_lecture_Biology II_
(2) Exam : 3-5 pm on Oct. 4th, 2010 (1st exam)
 Problem types: Choice 70% + Short or long answer 30%
Posting of score & expected GPA: on the board at room SB132, New Science Building (과학원)

2. Announcement (continued)
(3) Grading:
1st exam (25%) + 2nd exam (25%) + 3rd exam (25%) +
lab work (20%) + attendance & assignments (5%)
(4) Homework
Summarize each chapter (2 pages) and then submit your handwritten summary before at next class

3. Brief Contents in This Course
Part I: Genetics
Chapter 15 : The chromosomal basis of inheritance (Prof. Jang)
Chapter 16 : The molecular basis of inheritance (Prof. Jang)
Chapter 17 : From gene to protein (Prof. Jang)
Chapter 18 : Regulation of gene expression (Prof. Jang)
What you studied already
Chapter 13 : Meiosis and sexual life cycles
Chapter 14 : Mendel and the gene idea

4. Brief Contents in This Course
Part II: Oct Nov. 16 by H.S. Pai
Chapter 19 : Viruses
Chapter 20: Biotechnology
Chapter 21: Genomes and their evolution
Chapter 39: Plant responses to internal and external signals
Part III: Nov Dec. 21 by K.M. Choe
Chapter 40 : Basic principles of animal form and function
Chapter 41 : Animal nutrition
Chapter 43 : The Immune system
Chapter 45 : Hormones and the endocrine system

5. The Chromosomal Basis of Inheritance
Chapter 15
On by Yeun-Kyu Jang
The Chromosomal Basis of Inheritance
What you learned already
Chapter 13 : Meiosis and sexual life cycles
Chapter 14 : Mendel and the gene idea

6. ☞Genes ( passengers) and chromosomes ( wagon)
Overview: Locating Genes on Chromosomes
☞Genes ( passengers) and chromosomes ( wagon)
Mendel’s “hereditary factors” were genes, though this wasn’t known at the time
Today we can show that genes are located on chromosomes
Q: How to visualize that?
The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

7. Fig. 15-1
FISH (fluorescent in situ hybridization): Chromosomes tagged to reveal a specific gee (yellow color)
Figure 15.1 Where are Mendel’s hereditary factors located in the cell?

8.  Chromosomes: a physical basis for the principles of heredity (=Mendel’s laws of segregation and independent assortment)
This chapter’s aim: Is to study the chromosomal basis for the transmission of genes from parents to offspring

9. Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes
Mitosis and meiosis were first described in the late 1800s
The chromosome theory of inheritance states:
Mendelian genes have specific loci (positions) on chromosomes
Chromosomes undergo segregation and independent assortment
The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment

10. What we learned: Mendel’s laws
The law of segregation
The two alleles (Y & y) for a single heritable character (ex: seed color) separate (segregate) during gamete formation and end up in different gametes
Using the information from a dihybrid cross, Mendel developed the law of independent assortment
Each pair of alleles for two different traits segregates independently during gamete formation (Segregation of one gene’ alleles does not interfere with that of other gene’s alleles)
Ex: The alleles for seed color and seed shape sort into gametes independently of each other.

11. LAW OF INDEPENDENT ASSORTMENT Fertilization among the F1 plants
P Generation
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds (yyrr)
The chromosomal basis of Mendel’s laws Y y r Y R R y r
Starting with two true-breeding pea plants,
we follow two genes through the F1 and F2
generations. The two genes specify seed
color (allele Y for yellow and allele y for
green) and seed shape (allele R for round
and allele r for wrinkled). These two genes are
on different chromosomes. (Peas have seven
chromosome pairs, but only two pairs are
illustrated here.)
Meiosis
Gametes
Fertilization y r R Y
All F1 plants produce
yellow-round seeds (YyRr) R R
F1 Generation y y r r Y Y
Meiosis
LAW OF INDEPENDENT ASSORTMENT
LAW OF SEGREGATION R r
Two equally
probable
arrangements
of chromosomes
at metaphase I r R
Alleles at both loci segregate
in anaphase I, yielding four
types of daughter cells
depending on the chromosome
arrangement at metaphase I.
Compare the arrangement of
the R and r alleles in the cells on the left and right 1 Y y Y y 1
The R and r alleles segregate
at anaphase I, yielding
two types of daughter
cells for this locus. R r r R
Anaphase I Y y Y y
Metaphase II R r r R 2
Each gamete gets
a long and a short
chromosome in
one of four allele
combinations. 2
Each gamete gets one long chromosome with either the R or r allele. Y y y y Y Y Y Y
Gametes Y Y y y R R r r r r R R 1 4 1 4 yr 1 4 Yr 1 4 YR Yr yr yR
F2 Generation 3
Fertilization results
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
Fertilization among the F1 plants
Figure 15.2 3
Fertilization recombines the R and r alleles at random. 9
: 3
: 3
: 1

12. Summary for the chromosomal basis of Mendel’s laws
Law of segregation:
Two alleles for each gene segregate during gamete formation and end up in different gametes
 Each gamete gets only one of the two alleles
 In terms of chromosomes, this segregation corresponds to the distribution of homologous chromosomes to different gamete (R or r allele; Y or y allele)

13. Summary for the chromosomal basis of Mendel’s laws
Law of independent assortment:
Alleles of genes on nonhomologous chromosomes assort independently during gamete formation
 Each gamete gets one of four allele combinations (YR:Yr:yR:yr = 1:1:1:1) depending on the alternative arrangements of homologous chromosome pairs in metaphase I

14. Morgan’s Experimental Evidence for chromosomal theory: Scientific Inquiry
Thomas Hunt Morgan (Embryologist at Columbia Univ. early in the 20th century)
Provided the first solid evidence associating a specific gene with a specific chromosome
Namely, provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

15. Morgan’s Choice of Experimental Organism
Several characteristics make fruit flies a convenient organism for genetic studies:
They breed at a high rate
A generation can be bred every two weeks
They have only four pairs of chromosomes

16. Morgan first observed and noted
Wild type, or normal, phenotypes that were common in the fly populations
Traits alternative to the wild type
Are called mutant phenotypes
Morgan’s first mutant: wild-type flies have red eyes (left) and one mutant male showed white eyes (right)
Figure 15.3
This variation made it possible for Morgan to trace a gene for eye color to a specific chromosome

17. Morgan first invented a notation for symbolizing alleles in Drosophila (still widely used)
The allele for white eyes in flies is symbolized by w (indicated as a mutant allele)
A superscript + identifies the allele for the wild-type trait: w+ for the allele for red eyes
Figure 15.3
 Genotype of wild-type:
w+w+
 Genotype of mutant: ww

18. How to Correlate Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair
In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)
The F1 generation all had red eyes
The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes
The white-eye mutant allele must be located on the X chromosome

19. Inquiry: In a cross between a WT female and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?
Morgan determined
That the white-eye mutant allele must be located on the X chromosome
Figure 15.4
The F2 generation showed a typical Mendelian
3:1 ratio of red eyes to white eyes. However, no females displayed the
white-eye trait; they all had red eyes. Half the males had white eyes,
and half had red eyes.
Morgan then bred an F1 red-eyed female to an F1 red-eyed male to
produce the F2 generation.
RESULTS P
Generation F1 X F2
Morgan mated a wild-type (red-eyed) female
with a mutant white-eyed male. The F1 offspring all had red eyes.
EXPERIMENT

20. The recessive trait of white eyes was appeared only in males
CONCLUSION
Since all F1 offspring had red eyes, the mutant white-eye trait (w) must be recessive to the wild-type red-eye trait (w+). Since the recessive trait—white eyes—was expressed only in males in the F2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here. W+ W P
Generation X X X X Y W+
Ova
(eggs) W
Sperm F1
Generation W+ W+ W+
all red-eyes W W+
Ova
(eggs)
Sperm F2
Generation W+ W+ W+
The recessive trait of white eyes was appeared only in males W+ W W W W+

21. Conclusion (1): first experimental evidence for chromosome theory of inheritance
Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait
Was the first solid evidence indicating that a specific gene is associated with a specific chromosome

22. What is the physical basis of these laws?
Concept Check 15.1 (see p289)
Which one of Mendel’s laws relates to the inheritance of alleles for a single character?
Which law relates to the inheritance of alleles for two characters in a dihybrid cross?
What is the physical basis of these laws?

23. What will you examine here
Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance
What will you examine here
We consider the role of sex chromosome in heritance
First step: reviewing the chromosomal basis of sex determination in humans and some other animals

24. The Chromosomal Basis of Sex
How to determine sex in an organism?
An organism’s sex
Is an inherited phenotypic character determined by the presence or absence of certain chromosomes (sex chromosomes)

25. Fig. 15.5: Human sex chromosomes
In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome
Only the ends of the Y chromosome have regions that are homologous with the X chromosome
The SRY gene on the Y chromosome codes for the development of testes
Fig. 15.5: Human sex chromosomes

26. Some chromosomal systems of sex determination
In humans and other mammals
There are two varieties of sex chromosomes, X and Y
Figure 15.6a
(a) The X-Y system
44 + XY XX
Parents
22 + X Y
Sperm
Zygotes
(offspring)
Ova (eggs)
In mammals, the sex of an offspring depends on whether the sperm cell contains an X chromosome or a Y

27. (d) The haplo-diploid system
Different systems of sex determination
Are found in other organisms
22 + XX X
76 + ZZ ZW 16
(Haploid)
(Diploid)
(b) The X–0 system
(c) The Z–W system
(d) The haplo-diploid system
In grasshoppers, cockroaches, there is only one type of sex chromosome (the X)
In birds, some fishes, and some insects, the sex chromosome present in the egg (not the sperm) determines the sex of offspring (Z and W)
In most species of bees and ants, there are no sex chromosomes. Female develop from fertilized eggs and are thus diploid. Males develop from unfertilized eggs and haploid (no fathers)
Figure 15.6b–d

28. Inheritance of Sex-Linked Genes
Key points to keep in mind
The sex chromosomes have genes for many characters unrelated to sex
A gene located on either sex chromosome is called a sex-linked gene
In humans, sex-linked usually refers to a gene on the larger X chromosome

29. Sex-linked trait by recessive allele on X chromosome
Female: has two X chromosomes
Homozygous recessive alleles : expression of phenotype
Heterozygous: no expression of phenotype (called carrier)
Male: has only one X chromosome
You don’t use the terms homozygous and heterozygous: use “hemizygous”
Any male receiving the recessive allele from his mother will express the trait  far more males than females have sex-linked recessive disorders (The freq. of sex-linked recessive disorders in male is higher than that in female)

30. What kinds of X-lined disorders are found?
Some recessive alleles found on the X chromosome in humans cause certain types of disorders
Color blindness:
Duchenne muscular dystrophy : 1/3500 males born in US; progressive weakening of the muscles; the absence of a key muscle protein called dystrophin
Hemophilia: shows delayed blood clotting due to the absence of one or more of the proteins required for blood clotting; treated with intravenous injections of the missing protein

31. How do sex-lined recessive traits transmit to offspring?
XAXa
(a)
A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation.
Figure 15.7a–c

32. The transmission of sex-linked recessive traits
XaXA
(b)
If a carrier female mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder.
Figure 15.7a–c

33. The transmission of sex-linked recessive traits
XaXa
(c)
If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele.
Figure 15.7a–c

34. X inactivation in Female Mammals
In mammalian females
One of the two X chromosomes in each cell is randomly inactivated during embryonic development
As a result, the cells of females and males have the same effective dose (one copy) of genes on the X chromosome (Got even!! = Dosage compensation)
The inactive X condenses into a compact object called a Barr body (discovered by a cytologist Murray Barr)
Most of genes are not expressed but Barr-body chromosomes are reactivated during oogenesis to form female gamete ovum
Random selection of X inactivation results in females with mosaic trait (Mosaicism)

35. Mystery of the mottled coloration in cat coat
On random inactivation of an X chromosome in a particular cell, all mitotic descendants of that cell have the same inactive X.
If a female is heterozygous for a particular gene located on the X chromosome  She will be a mosaic for that character: about half will express one allele while the others will express the alternate allele
Two cell populations
in adult cat:
Active X
Early embryo:
Orange
fur
X chromosomes
Cell division
and X
chromosome
inactivation
Inactive X
Inactive X
Black
fur
Allele for
orange fur
Allele for
black fur
Active X
X-inactivation and the tortoiseshell cat
Figure 15.8

36. Mystery of the presence or absence of sweat glands in female skin
In humans, mosaicism can be observed in a recessive X-linked mutation that prevents the development of sweat glands
A woman who is heterozygous for this trait has patches of normal skin and patches of skin lacking sweat glands

37. Concept Check 15.2 (see p292)
During early embryonic development of female carriers for color blindness, the normal allele is inactivated by chance in about half the cells. Why, then, aren’t 50% of female carriers color-blind?

38. Answer:
The cells in the eye must come from multiple cells in the early embryo
The descendants of half of these cells express normal allele
Thus having half the number of mature eye cells expressing the normal allele must be sufficient for normal color vision

39. Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome
Each chromosome has hundreds or thousands of genes
Location of some genes on the same chromosome is expected
The genes tend to be inherited together in genetic crosses: are said to be linked genes
The linked genes do not always follow Mendel’s law of independent assortment

40. How Linkage Affects Inheritance
Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters
Morgan crossed flies that differed in traits of body color and wing size

41. b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most or
Fig. 15-UN1
b+ vg+
b vg 
Parents
in testcross
b vg
b vg
b+ vg+
b vg
Most
offspring or
b vg
b vg

42. Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes)
He noted that these genes do not assort independently, and reasoned that they were on the same chromosome

43. Inquiry: Are the genes for body color and wing size in fruit flies located on the same chromosome or different chromosomes?
EXPERIMENT
Morgan first mated true-breeding
wild-type flies with black, vestigial-winged flies to produce
heterozygous F1 dihybrids, all of which are wild-type in
appearance. He then mated wild-type F1 dihybrid females with black, vestigial-winged males, producing 2,300 F2 offspring, which he “scored” (classified according to
phenotype).
P Generation
(homozygous)
Double mutant
(black body,
vestigial wings)
Wild type
(gray body,
normal wings) x
Double mutant
(black body,
vestigial wings)
b+ b+ vg+ vg+
b b vg vg
If these two genes were on
different chromosomes, the alleles from the F1 dihybrid would sort into gametes independently, and we would expect to see equal numbers of the four types of offspring.
If these two genes were on the same chromosome, we would expect each allele combination, B+ vg+ and b vg, to stay together as gametes formed. In this case, only offspring with parental phenotypes would be produced.
Since most offspring had a parental phenotype, Morgan concluded that the genes for body color and wing size are located on the same chromosome. However, the production of a small number of offspring with nonparental phenotypes indicated that some mechanism occasionally breaks the linkage between genes on the same chromosome (Genetic recombination)
CONCLUSION
F1 dihybrid
(wild type)
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
Double mutant
(black body,
vestigial wings)
TESTCROSS x
b b vg vg
b+ b vg+ vg
RESULTS
b+vg+
b vg
b+ vg
b vg+
Ova (eggs)
965
Wild type
(gray-normal)
944
Black-
vestigial
206
Gray-
vestigial
185
Black-
normal
b vg
Sperm
bb vgvg
bb vg+vg
b+b vg+vg
b+b vgvg
Parental-type
offspring
Recombinant (nonparental-type)
offspring
Figure 15.9

44. EXPERIMENT RESULTS Fig. 15-9-4 P Generation (homozygous) b+ b+ vg+ vg+
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings) 
b+ b+ vg+ vg+
b b vg vg
F1 dihybrid
(wild type)
Double mutant
TESTCROSS 
b+ b vg+ vg
b b vg vg
Testcross
offspring
Eggs
b+ vg+
b vg
b+ vg
b vg+
Wild type
(gray-normal)
Black-
vestigial
Gray-
vestigial
Black-
normal
b vg
Figure 15.9 How does linkage between two genes affect inheritance of characters?
Sperm
b+ b vg+ vg
b b vg vg
b+ b vg vg
b b vg+ vg
PREDICTED RATIOS
If genes are located on different chromosomes: 1 : 1 : 1 : 1
If genes are located on the same chromosome and
parental alleles are always inherited together: 1 : 1 : :
RESULTS
965 :
944 :
206 :
185

45. However, nonparental phenotypes were also produced
Genetic recombination might be involved in breaking the linkage between genes on the same chromosome

46. Morgan concluded that
Genes that are close together on the same chromosome are linked and do not assort independently
Unlinked genes are either on separate chromosomes or are far apart on the same chromosome and so assort independently
Appearance of non-parental phenotypes suggest that the body-color and wing size genes are only partially linked genetically
To understand this unexpected result, we need to further explore genetic recombination

47. Genetic Recombination and Linkage
What you learned in the Chapter 13
Meiosis and random fertilization generate genetic variation among offspring of sexually reproducing organisms
What will you examine next
What is the chromosomal basis of recombination in relation to the genetic findings of Mendel and Morgan?

48. When Mendel followed the inheritance of two characters unlinked
Recombination of Unlinked Genes can occur via Independent Assortment of Chromosomes
When Mendel followed the inheritance of two characters unlinked
He observed that some offspring have combinations of traits that do not match either parent in the P generation (called recombinant types or recombinants or nonparental types)
Gametes from green-
wrinkled homozygous
recessive parent (yyrr)
Gametes from yellow-round
heterozygous parent (YyRr)
Parental-
type offspring
Recombinant
offspring
YyRr
yyrr
Yyrr
yyRr YR yr Yr yR

49. Mendel observed that combinations of traits in some offspring differ from either parent
Offspring with a phenotype matching one of the parental phenotypes are called parental types
Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants (Half the number of offspring would be recombinant types)
A 50% frequency of recombination is observed for any two genes on different chromosomes

50. Recombination of Linked Genes: Crossing Over
Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes
Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome
That mechanism was the crossing over of homologous chromosomes

51. Fig
Testcross
parents
Gray body, normal wings
(F1 dihybrid)
Black body, vestigial wings
(double mutant)
b+ vg+
b vg
Chromosomal basis for recombination of linked genes: Linked genes exhibit RF less than 50%
b vg
b vg
Replication
of chromo-
somes
Replication
of chromo-
somes
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
Meiosis I: Crossing
over between b and vg
loci produces new allele
combinations.
b vg
b vg
Meiosis I
Meiosis I and II:
Even if crossing over
occurs, no new allele
combinations are
produced.
b+ vg+
Meiosis I and II
b+ vg
Meiosis II: Segregation
of chromatids produces
recombinant gametes
with the new allele
combinations.
b vg+
b vg
Meiosis II
Recombinant
chromosomes
Gametes
b+ vg+
b vg
b+ vg
b vg+
Eggs
Testcross
offspring
965
Wild type
(gray-normal)
944
Black-
vestigial
206
Gray-
vestigial
185
Black-
normal
b vg
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Sperm
Parental-type offspring
Recombinant offspring
RF: the percentage of recombinants in the total offspring
Recombination
frequency
391 recombinants =
 100 = 17%
2,300 total offspring

52. Relation of RF with Gene-to-Gene distances
The percentage of recombinant offspring, the recombination frequency (RF), is related to the distance between linked genes

53. The discovery of linked genes and recombination via crossing over
Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry
The discovery of linked genes and recombination via crossing over
Led one of Morgan’s students (Alfred H. Sturtevant) to develop a method for constructing a genetic map
A genetic map (Gene Address Book)
Is an ordered list of the genetic loci along a particular chromosome
Can be developed using recombination frequencies

54. Sturtevant’s work for constructing a genetic map
He hypothesized that RFs depend on the distances between genes on a chromosome
-He assumed that crossing over is a random event and thus the chance is approximately equal at all points along a chromosome
He predicted that the farther apart two genes are, the higher the probability of a crossover between them and therefore the higher the RF
Using recombination data, he assigned relative positions to genes on the same chromosomes (Constructing linkage map: genetic map based on RF)

55. How to construct a linkage map
 Is the actual map of a chromosome based on recombination frequencies
 shows the relative locations of genes along a chromosome
 The distances between genes are expressed as map units (centimorgans): one map unit (1 m.u.) equivalent to 1% RF

56. How to construct a linkage map (continued)
Example
Consider three genes: the body-color (b), wing-size (vg), and cinnabar eyes (cn) [brighter red than wt eye color]
The observed recombination frequencies between three Drosophila gene pairs : b–cn 9%, cn–vg 9.5%, and b–vg 17%
Make a linear order : cn is positioned about halfway between the other two genes:
Recombination
frequencies 9%
9.5%
17% b cn vg
Chromosome
1st cross-over
2nd CO
b, black body color
vg, vestigial wing
cn, cinnabar eyes
Figure 15.11

57. How to construct a linkage map (continued)
Example
Recombination
frequencies 9%
9.5%
17% b cn vg
Chromosome
1st cross-over
2nd CO
Inquiry: The b–vg recombination frequency (17%) is slightly less than the sum (18.5%) of the b–cn and cn–vg frequencies. Why?
Because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes.
A second crossover would “cancel out” the effect of the first crossover and thus reduce the observed b–vg RF.

58. A partial genetic (linkage) map of Drosophila chromosome
Many fruit fly genes
Were mapped initially using recombination frequencies
Figure 15.12
Mutant phenotypes
Short
aristae
Black
body
Cinnabar
eyes
Vestigial
wings
Brown
Long aristae
(appendages
on head)
Gray
Red
Normal
Wild-type phenotypes II Y I X IV
III
48.5
57.5
67.0
104.5
A partial genetic (linkage) map of Drosophila chromosome

59. What kind of changes to the genome affecting phenotype?
Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders
What kind of changes to the genome affecting phenotype?
 Gene mutations, chromosome structure, chromosome number
 Many causes can damage chromosomes or alter their number in a cell
 Large-scale chromosomal alterations:
Often lead to spontaneous abortions or cause a variety of developmental disorders

60. Abnormal Chromosome Number
When nondisjunction occurs
Pairs of homologous chromosomes do not separate properly during either meiosis I or meiosis II
Gametes receive two copies (n+1) or no copies (n-1) of a particular chromosome
Meiosis I
Nondisjunction
Meiosis II
Meiotic nondisjunction: gametes with an abnormal chromosome number can arise by nondisjunction in either meiosis I or meiosis II
Nondisjunction
Gametes
n + 1
n + 1
n  1
n – 1
n + 1
n –1 n n
Number of chromosomes
(a)
Nondisjunction of homologous
chromosomes in meiosis I
(b)
Nondisjunction of sister
chromatids in meiosis II
Figure 15.13a, b

61. The consequences of nondisjunction after fertilization?
Aneuploidy
Results from the fertilization of aberrant gametes (nondisjunction occurred) with a normal one
Is a condition in which offspring have an abnormal number of a particular chromosome

62. Chromosome number of haploid (gamete): n
Chromosome number of normal zygote: 2n
If a zygote is trisomic (2n+1 chromosomes)
It has three copies of a particular chromosome
If a zygote is monosomic (2n-1 chromosomes)
It has only one copy of a particular chromosome

63. Polyploidy
Is a condition in which there are more than two complete sets of chromosomes in an organism
Triploidy (3n): can be produced by fertilization of an abnormal diploid egg produced by nondisjunction
Tetraploidy (4n): the failure of 2n zygote to divide after replication of its chromosomes and then subsequent mitosis may produce a 4n embryo
Figure 15.14
A tetraploid mammal : a burrowing rodent (굴쥐)
This species may have arisen when an ancestor doubled its chromosome number, presumably by errors in mitosis or meiosis within the animal’s reproductive organs

64. Alterations of Chromosome Structure
Breakage of a chromosome can lead to four types of changes in chromosome structure:
 Called “Chromosomal rearrangement”
Deletion
Duplication
Inversion
Translocation

65. Alterations of chromosome structure
Figure 15.15a–d A B C D E F G H
Deletion
Duplication M N O P Q R
Inversion
Reciprocal
translocation
(a) A deletion removes a chromosomal
segment.
(b) A duplication repeats a segment.
(c) An inversion reverses a segment
within a chromosome.
(d) A translocation moves a segment from one chromosome to another,
nonhomologous one. In a reciprocal
  translocation, the most common type,
nonhomologous chromosomes exchange
fragments. Nonreciprocal translocations
also occur, in which a chromosome
transfers a fragment without receiving a
fragment in return.
(Give & Take mode: exchange their fragments each other)
Breakage points

66. Human Disorders Due to Chromosomal Alterations
Alterations of chromosome number and structure
Are associated with a number of serious human disorders

67. Down Syndrome Down syndrome
Ch21
Ch14-21
Down syndrome
Is usually the result of an extra chromosome 21, trisomy 21 (45+XX)
5% of persons with this syndrome contain a combined chromosome via translocation of ch21 to ch14
Shows characteristic facial features, short stature, heart defects, respiratory infection, mental retardation prone to developing leukemia and Alzheimer’s disease
egg
sperm
zygote
Figure 15.16
The child exhibits the facial features characteristic of Down syndrome. The karyotype shows trisomy 21, the most common cause of this disorder

68. The consequences of aneuploidy of Sex Chromosomes?
Nondisjunction of sex chromosomes
Produces a variety of aneuploid conditions
Klinefelter syndrome (XXY male)
Is the result of an extra chromosome in a male, producing XXY individuals
Occurs once in approximately 2,000 live births; small testes and sterile
Turner syndrome (XO female)
Is the result of monosomy X, producing an X0 karyotype (only known viable monosomy in humans)
1/5,000 live births; immature sex organs and sterile

69. Disorders Caused by Structurally Altered Chromosomes
Cri du chat (“cry of the cat”)
Is a disorder caused by a specific deletion in chromosome 5
Shows mental retardation, small head with unusual facial features, cry that sounds like the mewing of a cat, die in infancy or early childhood

70. Certain cancers Are caused by translocations of chromosomes
Figure 15.17
Normal chromosome 9
Reciprocal
translocation
Translocated chromosome 9
Philadelphia
chromosome
Normal chromosome 22
Translocated chromosome 22
Chromosomal translocation associated with chronic myelogenous leukemia (CML): the reciprocal translocation between ch9 and ch22 results in an abnormally short ch22 (called Philadelphia chr) and abnormally long ch9

71. Two normal exceptions to Mendelian genetics include
Concept 15.5: Some inheritance patterns are exceptions to the standard chromosome theory
Two normal exceptions to Mendelian genetics include
Genes located in the nucleus: genomic imprinting at Igf2 gene
Genes located outside the nucleus: extranuclear genes contained in mitochondria, and plastids including chloroplast

72. Genomic Imprinting What exceptions will you consider by now
What you learned in Mendelian genetics
We assumed that a specific allele will have the same effect regardless of whether it was inherited from the mother or the father
What exceptions will you consider by now
Some traits in mammals depend on which parents passed along the alleles
The phenotypic effects of certain genes depend on which allele is inherited from the mother and which is inherited from the father:
such variation in phenotype is called genomic imprinting

73. Genomic imprinting
Involves the silencing of certain genes that are “stamped” with an imprint during gamete production
A zygote expresses only one allele of an imprinted gene (either from the mother or the father)
 The imprints are transmitted to all the body cells during development
A gene imprinted for maternal allele expression is always imprinted for maternal allele expression, generation to generation

74. (a) A wild-type mouse is homozygous for the normal igf2 allele.
Genomic imprinting (Example)
Normal Igf2 allele
(expressed)
Genomic imprinting of the mouse Igf2 gene: (a) The paternal Igf2 allele is only expressed because methylation of a certain DNA sequence on the paternal chromosome leads to expression of the paternal Igf2 allele
(b) Matings between wild-type and those homozygous for the recessive mutant Igf2 allele produce heterozygous offspring that can be either normal or dwarf, depending on which parent passes on the mutant allele
Paternal
chromosome
Wild-type mouse
(normal size)
Maternal
chromosome
Normal Igf2 allele
with imprint
(not expressed)
(a) A wild-type mouse is homozygous for the normal igf2 allele.
Normal Igf2 allele
Paternal
Normal size mouse
Maternal
Heterozygous mice with normal Igf2 allele inherited from the father
Mutant lgf2 allele
Mutant lgf2 allele
Paternal
Dwarf mouse
Maternal
Heterozygous mice with mutant Igf2 allele inherited from the father
Normal Igf2 allele
with imprint
(b) When a normal Igf2 allele is inherited from the father, heterozygous mice grow to normal size. But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype.
Figure 15.18a, b

75. Inheritance of Organelle Genes
Extranuclear genes
Are found in organelles in the cytoplasm
Mitochondria, chloroplast, plastids
They do not display Mendelian inheritance

76. The inheritance of traits controlled by genes present in the chloroplasts or mitochondria
Depends solely on the maternal parent because the zygote’s cytoplasm comes from the egg
Figure 15.19
Discovery of organelle genes by Karl Correns in 1909
 Variegated leaves from Croton diocus result from mutations in pigment genes located in plastids, which is generally inherited from the maternal parent

77. Mitochondrial mutations and human diseases
Some diseases affecting the muscular and nervous systems
Are caused by defects in mitochondrial genes that prevent cells from making enough ATP
Some mitochondrial mutations inherited from the mother may contribute to some cases of diabetes, heart disease and Alzheimer’s disease

78. Concept Check 15.5 (see p302)
Gene dosage, the number of active copies of a gene, is important to proper development.
Identify and describe two processes that help establish the proper dosage of certain genes.