1. AP Biology Chapter 15 The Chromosome Theory of Inheritance
Genes and Chromosomes
AP Biology
Chapter 15
The Chromosome Theory of Inheritance

2. Genes are Located on Chromosomes
Mendel’s “factors” are now known to be genes—segments of chromosomes.
Where the chromosomes go during Meiosis determines which traits end up in each of the gametes.

3. The Chromosome Theory of Inheritance
First described by Walter S. Sutton in 1902:
Genes have specific loci (positions) along chromosomes
It is the chromosomes that undergo segregation and independent assortment

4. The Chromosomal Basis of Mendel’s Laws:
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)

5. 0.5 mm All F1 plants produce yellow-round seeds (YyRr) F1 Generation
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

6. Fertilization recombines the R and r alleles at random
Fertilization recombines the R and r alleles at random. On a different chromosome, fertilization recombines theY and y alleles also. YyRr X YyRr produces 9:3:3:1 ratio.
F2 Generation
An F1  F1 cross-fertilization 3 3 9
: 3
: 3
: 1
Figure 15.2 The chromosomal basis of Mendel’s laws
So, Mendel’s ratios can be explained by examining the behavior of the chromosomes during meiosis.

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 Mendel’s heritable factors.

8. Morgan’s Choice of Experimental Organism
Thomas Hunt Morgan chose Drosophila melanogaster, a common insect that feeds on the fungi growing on fruit. Why?
They are prolific breeders (a single mating can produce hundreds of offspring)
They can be bred every
two weeks
It has only 4 pairs of
chromosomes
There are many types
of easily identified mutants
which differ from the normal
(wild) type.

9. Wild Type Fruit Flies vs. Mutants
When Thomas Hunt Morgan studied fruit flies, he was looking for naturally occurring variants. After years of study, he finally found one male fruit fly with white eyes instead of the usual red.
The allele for the mutant trait is written as a lower case letter (ex: white eyes is w). The wild-type fly (normal phenotype) is shown with the same letter with a superscript+: w+

10. Correlating behavior of an allele with behavior of the chromosome
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?
In one experiment, Morgan mated a white-eyed male with a red-eyed female.
All of the F1 offspring had red eyes. What does this tell us about the red eye trait?

11. Fig. 15-4b
F2 Results:
RESULTS F2
Generation
When Morgan bred the F1 flies to each other, he observed the classical 3:1 phenotypic ratio in the F2 offspring.
However, surprisingly, the white-eyed trait showed up only in the males! Somehow, the fly’s eye color is related to its sex.
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?

12. CONCLUSION + P X X Generation  X Y + Sperm Eggs + + F1 + Generation +w w X X
Generation  X Y + 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

13. 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
X and Y chromosomes →
The SRY gene on the Y chromosome codes for the development of the testes.

14. (a) The X-Y system 44 + XY 44 + XX Parents 22 + X 22 + X 22 + Y or +
In mammals, the sex of an offspring depends on whether the sperm cell contains an X chromosome or a Y.
Sperm
Egg
Figure 15.6 Some chromosomal systems of sex determination
44 + XX
44 + XY or
Zygotes (offspring)
(a) The X-Y system

15. Fig. 15-6b
22 + XX
22 + X
(b) The X-0 system
Figure 15.6 Some chromosomal systems of sex determination
In grasshoppers, cockroaches, and some other insects, there is only one type of sex chromosome, the X. Females are XX, males have only one sex chromosome (XO). Sex of the offspring is determined by whether the sperm cell contains an X chromosome or no sex chromosome.

16. (c) The Z-W system 76 + ZW 76 + ZZ
Fig. 15-6c
76 + ZW
76 + ZZ
Figure 15.6 Some chromosomal systems of sex determination
(c) The Z-W system
In birds, some fishes, and some insects, the sex chromosomes present in the egg (not the sperm) determine the sex of offspring. The sex chromosomes are designated Z and W. Females are ZW and males are ZZ.

17. (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
There are no sex chromosomes in most species of bees and ants. Females develop from fertilized eggs and are thus diploid. Males develop from unfertilized eggs and are haploid; they have no fathers.

18. 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

19. Inheritance Patterns of Sex Chromosomes.
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

20. The Transmission of Sex-linked recessive traits.
Fig. 15-7
The Transmission of Sex-linked recessive traits.
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)
If a carrier mates with a color-blind male, there is a 50% chance their child will be color-blind.
A color-blind father will transmit the mutant allele to all daughters but not to sons. Daughters are carriers.
If a carrier mates with a male who has normal color vision, there is a 50% chance the son will be color-blind.

21. 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.
The Barr body lies along the inside of the nuclear envelope. Most of the genes in the Barr body are not expressed.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

22. Barr Bodies: the inactive X in each cell of a female
If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character

23. The tortoiseshell gene is on the X chromosome in cats.
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
The tortoiseshell gene is on the X chromosome in cats.
The tortoiseshell color requires the presence of two alleles: one orange and one black. These are located on the X chromosome.
Black fur
Orange fur
Figure 15.8 X inactivation and the tortoiseshell cat
If a female (XX) is heterozygous, the orange and black patches are present in populations of cells with that activated gene.

24. Linked Genes
Genes located near each other on the same chromosome tend to be inherited together.
These are called linked genes.
These are not to be confused with sex-linked traits (traits that come from the sex chromosomes)

25. 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
Wild type—normal wings/color Ebony body Vestigial (short) wings
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

26. Linked Genes
In the fruit fly Drosophila melanogaster, flies reared in the laboratory occasionally exhibit mutations in their genes.
Two such mutations, affecting body color and wing structure are linked. Morgan did experiments studying these two traits.
Grey body and normal wings are dominant traits.
Black body and vestigial wings are recessive

27. b+ vg+ b vg  Parents in testcross b vg b vg b+ vg+ b vg Most or
Fig. 15-UN1
Homozygous for black body, vestigial wings
Heterozygous for grey body, normal wings X
b+ vg+
b vg 
Parents
in testcross
b vg
b vg
b+ vg+
b vg
Most
offspring or
b vg
b vg
Grey body, normal wings
Black body, vestigical wings

28. EXPERIMENT P Generation (homozygous) b+ b+ vg+ vg+ 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?

29. 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?

30. EXPERIMENT P Generation (homozygous) b+ b+ vg+ vg+ 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

31. EXPERIMENT RESULTS P Generation (homozygous) b+ b+ vg+ vg+ 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

32. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

33. Genetic Recombination
However, nonparental phenotypes were also produced
Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent
Recombinant phenotypes
Original phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

34. Linked genes animation
nt/chp10/ html

35. Recombination of Unlinked Genes
When traits appear that are different from either one of the parents, it is due to independent assortment when genes are not on the same chromosome.
Parental types: resemble the parents
Recombinants: contain new combinations of genes
If genes are located on different chromosomes, there will be a 50% recombination rate.
Recombinants
Parental types

36. Recombinants YR yr Yr yR yr YyRr yyrr Yyrr yyRr
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

37. Recombinants from Linked Genes
Recombinations in traits that are located on the same chromosome (linked genes) are due to crossing over.

38. Recombination due to crossing over:

39. Mapping the distance between two genes:
Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome
Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency

40. Genetic Maps

41. Genetic Linkage Maps
A linkage map is a genetic map of a chromosome based on recombination frequencies
Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency
Map units indicate relative distance and order, not precise
locations of genes

42. A linkage map from Drosophila:
RESULTS
Recombination
frequencies 9%
9.5%
Chromosome
17%
Figure Constructing a linkage map b cn vg
Genes that are far apart on a chromosome can have recombination frequencies close to 50%.
(These behave almost the same as if they were on difference chromosomes)

43. How to figure out recombination frequencies:
Determine which traits are like the original parents (parental traits). Determine which traits are new combinations of the genes (these are the recombinants)
Figure out the total number of recombinant offspring and divide by the total number of offspring X 100 = Recombination %
What is the recombination frequencies of the b and vg genes?

44. Now You Try:
A wild-type fruit fly heterozygous for gray body color and normal wings, b+b vg+vg, is mated with a black fly with vestigial wings bbvgvg. The offspring have the following phenotypic distribution:
Wild-type (gray, normal wings): 778
Black-vestigial: 785
Black-normal wings: 158
Gray-vestigial: 162
What is the recombination frequency between these genes for body color and wing size?

45. Answer Number recombinants = 320 Total offspring = 1883
Recombinant frequency = 320/1883 X 100 =
17%

46. Now You Try:
Determine the sequence of genes along a chromosome based on the following recombination frequencies:
A—B , 8 map units
A—C, 28 map units
A—D, 25 map units
B—C, 20 map units
B—D, 33 map units

47. Answer
D A B C

48. Chromosome Alterations/Mutations Cause Genetic Disorders
Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
Children with Down’s Syndrome/ Trisomy 21

49. 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
Offspring with
this condition
is called
aneuploidy

50. 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

51. 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

52. 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

53. Monosomic, Trisomic Zygotes
A monosomic zygote has only one copy of a particular chromosome
A trisomic zygote has three copies of a particular chromosome
Turner’s Syndrome Karyotype: XO (monosomic zygote)
Down’s Syndrome Karyotype:
Trisomy 21

54. Polyploidy
Polyploidy is a condition in which an organism has more than two complete sets of chromosomes
Polyploidy is common in plants, but not animals. In plants, it may result in hybrids that are more vigorous.
Polyploidy can result when a 2N zygote fails to divide after replicating its chromosomes. This will produce a 4N embryo. (Note the hybrid vigor in the middle plant)

55. 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
Cri du chat results from a deletion of a portion of chromosome 5.

56. Reciprocal translocation
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

57. Translocated chromosome 22 (Philadelphia chromosome)
Translocation
Reciprocal
translocation
Normal chromosome 9
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Normal chromosome 22
Figure Translocation associated with chronic myelogenous leukemia (CML)
The cancerous cells in nearly all CML (chromic myelogenous leukemia) patients contain an abnormally short chromosome 22, the so-called Philadelphia chromosome, and an abnormally long chromosome 9. These altered chromosomes result from translocation.