1. Inheritance: Beyond Mendel

2. Overview: Locating Genes Along Chromosomes
Mendel’s “characters” were genes, though this wasn’t known at the time
Today we can show that genes are pieces of DNA that are located on chromosomes
The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene
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3. Fig. 15-1
Figure 15.1 Where are Mendel’s hereditary factors located in the cell?

4. Mendelian inheritance has its physical basis in the behavior of chromosomes
Mendel’s principles of segregation and independent assortment are based on what happens to chromosomes (and the genes that are located on them) during meiosis
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5. yellow-round seeds (YyRr)
Fig. 15-2a
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds ( yyrr)
P Generation y Y r  R R r Y y
Meiosis
Fertilization y r R Y
Figure 15.2 The chromosomal basis of Mendel’s laws
Gametes
All F1 plants produce
yellow-round seeds (YyRr)

6. 0.5 mm All F1 plants produce yellow-round seeds (YyRr) F1 Generation
Fig. 15-2b
All F1 plants produce
yellow-round seeds (YyRr)
0.5 mm
F1 Generation R R y y r r Y Y
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
Meiosis
LAW OF INDEPENDENT
ASSORTMENT Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation. R r r R
Metaphase I Y y Y y 1 1 R r r R
Anaphase I Y y Y y R r
Metaphase II
Figure 15.2 The chromosomal basis of Mendel’s laws r R 2 2 Y y Y y y Y Y Y Y y y y
Gametes R R r r r r R R 1 4 YR 1 4 yr 1 4 Yr 1 4 yR 3 3

7. Morgan’s Experimental Evidence: Scientific Inquiry
The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist
Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of genes
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8. 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
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9. Traits that are not the wild type are called mutant phenotypes
Morgan noted wild type, or normal, phenotypes that were common in the fly populations
Traits that are not the wild type are called mutant phenotypes
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10. Fig. 15-3
Figure 15.3 Morgan’s first mutant

11. Correlating 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
Morgan determined that the white-eyed mutant allele must be located on the X chromosome
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12. EXPERIMENT RESULTS P  Generation F1 All offspring Generation
Fig. 15-4a
EXPERIMENT P 
Generation F1
All offspring
had red eyes
Generation
Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?
RESULTS F2
Generation

13. w+ = wild type (red eyes) w = mutant (white eyes)
Fig. 15-4c
CONCLUSION + P w w X X
Generation  X Y +
w+ = wild type (red eyes)
w = mutant (white eyes) w w
Sperm
Eggs + + F1 w w +
Generation w w + w
Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?
Sperm
Eggs + + w w + F2 w
Generation w w w + w

14. The Chromosomal Basis of Sex
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
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15. Fig. 15-5 X Y
Figure 15.5 Human sex chromosomes

16. Females are XX, and males are XY
Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome
Other animals have different methods of sex determination
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17. (a) The X-Y system 44 + XY 44 + XX Parents 22 + X 22 + X 22 + Y or +
Fig. 15-6a
44 + XY
44 + XX
Parents
22 + X
22 + X
22 + Y or +
Sperm
Egg
Figure 15.6 Some chromosomal systems of sex determination
44 + XX
44 + XY or
Zygotes (offspring)
(a) The X-Y system

18. (b) The X-0 system 22 + XX 22 + X Fig. 15-6b
Figure 15.6 Some chromosomal systems of sex determination

19. (c) The Z-W system 76 + ZW 76 + ZZ Fig. 15-6c
Figure 15.6 Some chromosomal systems of sex determination

20. (d) The haplo-diploid system
Fig. 15-6d 32
(Diploid) 16
(Haploid)
Figure 15.6 Some chromosomal systems of sex determination
(d) The haplo-diploid system

21. Inheritance of Sex-Linked Genes
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 (it’s larger, so it contains more genes)
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22. Sex-linked genes follow specific patterns of inheritance
For a recessive sex-linked trait to be expressed
A female needs two copies of the allele
A male needs only one copy of the allele
Sex-linked recessive disorders are much more common in males than in females
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23. Some disorders caused by recessive alleles on the X chromosome in humans:
Color blindness
Duchenne muscular dystrophy
Hemophilia
Possible Female Genotypes: Possible Male Genotypes:
XAXA = healthy XAY = healthy
XAXa = healthy carrier XaY = has the disorder
XaXa = has the disorder
Can males be carriers for sex-linked disorders?
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24. (a) (b) (c) Sperm Sperm Sperm Eggs Eggs Eggs XNXN  XnY XNXn  XNY
Fig. 15-7
XNXN 
XnY
XNXn 
XNY
XNXn 
XnY
Sperm Xn Y
Sperm XN Y
Sperm Xn Y
Eggs XN
XNXn
XNY
Eggs XN
XNXN
XNY
Eggs XN
XNXn
XNY XN
XNXn
XNY Xn
XnXN
XnY Xn
XnXn
XnY
Figure 15.7 The transmission of sex-linked recessive traits
(a)
(b)
(c)

25. X Inactivation in Female Mammals
In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development
The inactive X condenses into a Barr body, and its genetic instructions are not used in that cell
If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character (some cells will express the allele from one X, some cells will express the allele from the other).
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26. X chromosomes Allele for orange fur Early embryo: Allele for black fur
Fig. 15-8
X chromosomes
Allele for
orange fur
Early embryo:
Allele for
black fur
Cell division and
X chromosome
inactivation
Two cell
populations
in adult cat:
Active X
Inactive X
Active X
Black fur
Orange fur
Figure 15.8 X inactivation and the tortoiseshell cat

27. 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
Genes located on the same chromosome that tend to be inherited together are called linked genes
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28. 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
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29. 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

30. EXPERIMENT Fig. 15-9-1 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
Figure 15.9 How does linkage between two genes affect inheritance of characters?

31. EXPERIMENT Fig. 15-9-2 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
Figure 15.9 How does linkage between two genes affect inheritance of characters?

32. EXPERIMENT Fig. 15-9-3 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

33. 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

34. 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
However, some offspring were still recombinant types (not parental types)
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35. Independent Assortment of Chromosomes
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
The process that makes recombinants is called genetic recombination
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36. The frequency of recombination = the percentage of offspring that are recombinant types
When 2 genes are on different chromosomes, independent assortment occurs, and we get a 50% frequency of recombination
Means 50% of offspring are parental types and 50% are recombinant types
When two genes are linked, there will be more parental type offspring than recombinants
Frequency of recombination will be less than 50%

37. YR yr Yr yR yr YyRr yyrr Yyrr yyRr Gametes from yellow-round
Fig. 15-UN2
Gametes from yellow-round
heterozygous parent (YyRr) YR yr Yr yR
Gametes from green-
wrinkled homozygous
recessive parent ( yyrr) yr
YyRr
yyrr
Yyrr
yyRr
Parental-
type
offspring
Recombinant
offspring

38. Recombination of Linked Genes: Crossing Over
Complete linkage means that ALL offspring are parental types (Freq. of recombination = 0%)
After finding some recombinants but more parental types, Morgan proposed that some linkage can be incomplete
Some process must sometimes break the physical connection between genes on the same chromosome - that process was crossing over
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39. Black body, vestigial wings
Fig a
Testcross
parents
Gray body, normal wings
(F1 dihybrid)
Black body, vestigial wings
(double mutant)
b+ vg+
b vg
b vg
b vg
Replication
of chromo-
somes
Replication
of chromo-
somes
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
Meiosis I
b+ vg+
Meiosis I and II
b+ vg
b vg+
Figure Chromosomal basis for recombination of linked genes
b vg
Meiosis II
Recombinant
chromosomes
b+ vg+
b vg
b+ vg
b vg+
b vg
Eggs
Sperm

40. 965 944 Black- vestigial 206 Gray- vestigial 185 Black- normal
Fig b
Recombinant
chromosomes
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+
Figure Chromosomal basis for recombination of linked genes
b vg
b vg
b vg
b vg
Sperm
Parental-type offspring
Recombinant offspring
Recombination
frequency
391 recombinants =
 100 = 17%
2,300 total offspring

41. The alleles of unlinked genes are either on separate chromosomes
Fig. 15-UN4
Sperm
Egg C B A c b a
P generation
gametes D E d e + F f
The alleles of unlinked
genes are either on
separate chromosomes
(such as d and e) or so
far apart on the same
chromosome (c and f )
that they assort
independently.
This F1 cell has 2n = 6
chromosomes and is
heterozygous for all six
genes shown (AaBbCcDdEeFf ).
Red = maternal; blue = paternal. D e C B d
Each chromosome
has hundreds or
thousands of genes.
Four (A, B, C, F) are
shown on this one. A
Genes on the same chromo-
some whose alleles are so
close together that they do
not assort independently
(such as a, b, and c) are said
to be linked. E F f a c b

42. Alterations of chromosome number or structure cause some genetic disorders
Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
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43. Abnormal Chromosome Number
In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis
As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
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44. Meiosis I (a) Nondisjunction of homologous
Fig
Meiosis I
Nondisjunction
Figure Meiotic nondisjunction
For the Cell Biology Video Nondisjunction in Mitosis, go to Animation and Video Files.
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II

45. Meiosis I Meiosis II (a) Nondisjunction of homologous
Fig
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Figure Meiotic nondisjunction
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II

46. Meiosis I Meiosis II Gametes (a) Nondisjunction of homologous
Fig
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
Figure Meiotic nondisjunction
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

47. A monosomic zygote has only one copy of a particular chromosome
Aneuploidy results from the fertilization of gametes in which nondisjunction occurred
Offspring with this condition have an abnormal number of a particular chromosome
A monosomic zygote has only one copy of a particular chromosome
A trisomic zygote has three copies of a particular chromosome
Monosomy and trisomy are both types of aneuploidy
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48. Polyploidy is common in plants, but not animals
Polyploidy is a condition in which an organism has more than two complete sets of chromosomes
Triploidy (3n) is three sets of chromosomes
Tetraploidy (4n) is four sets of chromosomes
Polyploidy is common in plants, but not animals
Polyploids are more normal in appearance than aneuploids
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49. Fig
Figure A tetraploid mammal

50. Alterations of Chromosome Structure
Breakage of a chromosome can lead to four types of changes in chromosome structure:
Deletion removes a chromosomal segment
Duplication repeats a segment
Inversion reverses a segment within a chromosome
Translocation moves a segment from one chromosome to another
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51. Reciprocal translocation
Fig
A B C D E F G H
A B C E F G H
Deletion
(a)
A B C D E F G H
A B C B C D E F G H
Duplication
(b)
A B C D E F G H
A D C B E F G H
(c)
Inversion
Figure Alterations of chromosome structure
A B C D E F G H
M N O C D E F G H
(d)
Reciprocal
translocation
M N O P Q R
A B P Q R

52. Human Disorders Due to Chromosomal Alterations
Alterations of chromosome number and structure are associated with some serious disorders
Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond
These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy
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53. Down Syndrome (Trisomy 21)
Down syndrome is an aneuploid condition that results from three copies of chromosome 21
It affects about one out of every 700 children born in the United States
The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained
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54. Fig a
Figure Down syndrome

55. Fig b
Figure Down syndrome

56. Aneuploidy of Sex Chromosomes
Nondisjunction of sex chromosomes produces a variety of aneuploid conditions
Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals
Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans
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57. Disorders Caused by Structurally Altered Chromosomes
The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5
A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood
Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes
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58. Translocated chromosome 22 (Philadelphia chromosome)
Fig
Reciprocal
translocation
Normal chromosome 9
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Normal chromosome 22
Figure Translocation associated with chronic myelogenous leukemia (CML)

59. Inheritance of Organelle Genes
Extranuclear genes (or cytoplasmic genes) are genes found in organelles in the cytoplasm
Mitochondria and chloroplasts carry small circular DNA molecules (like bacterial DNA)
Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg
A mother passes all her mitochondrial alleles to all her children; a father does not give mitochondria to his children
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60. Fig
Figure Variegated leaves from Croton dioicus
The yellow spots on these leaves come from genes in chloroplast DNA, and are inherited maternally.

61. Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems
For example, mitochondrial myopathy and Leber’s hereditary optic neuropathy
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62. What do we know about the genes on each human chromosome?