1. The Chromosomal Basis of Inheritance
Chapter 15
The Chromosomal Basis of Inheritance

2. EXPERIMENT RESULTS CONCLUSION Fig. 15-4 + w w + w w + +
Generation F1
All offspring had red eyes
Generation
RESULTS F2
Generation
CONCLUSION P + X w X w
Generation X  Y + w w
Sperm
Eggs F1 + +
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? + w w
Generation w w + w
Sperm
Eggs + + + w w F2 w
Generation + w w w w + w

3. CONCLUSION + P X X Generation  X Y + Sperm Eggs + + F1 + Generation +
Fig. 15-4c
CONCLUSION + P 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

4. Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance
In humans and some other animals, there is a chromosomal basis of sex determination
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

5. Fig. 15-5 X Y
Figure 15.5 Human sex chromosomes

6. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

7. Figure 15.6 Some chromosomal systems of sex determination
44 + XY
44 + XX
Parents
22 + X
22 + Y
22 + X or +
Sperm
Egg
44 + XX
44 + XY or
Zygotes (offspring)
(a) The X-Y system
22 + XX
22 + X
(b) The X-0 system
76 + ZW
76 + ZZ
Figure 15.6 Some chromosomal systems of sex determination
(c) The Z-W system 32
(Diploid) 16
(Haploid)
(d) The haplo-diploid system

8. 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
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9. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10. (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)

11. Each chromosome has hundreds or thousands of genes
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
Genes located on the same chromosome that tend to be inherited together are called linked genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

12. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

13. 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

14. 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

15. 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

16. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17. 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

18. 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

19. A monosomic zygote has only one copy of a particular chromosome
A trisomic zygote has three copies of a particular chromosome
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20. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

21. 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