1. 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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2. 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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3. 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)

4. There are two normal exceptions to Mendelian genetics
Concept 15.5: Some inheritance patterns are exceptions to the standard chromosome theory
There are two normal exceptions to Mendelian genetics
One exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleus
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5. Such variation in phenotype is called genomic imprinting
For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits
Such variation in phenotype is called genomic imprinting
Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production
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6. Normal Igf2 allele is expressed Paternal chromosome Maternal
Fig
Normal Igf2 allele
is expressed
Paternal
chromosome
Maternal
chromosome
Normal Igf2 allele
is not expressed
Wild-type mouse
(normal size)
(a) Homozygote
Mutant Igf2 allele
inherited from mother
Mutant Igf2 allele
inherited from father
Normal size mouse
(wild type)
Dwarf mouse
(mutant)
Figure Genomic imprinting of the mouse Igf2 gene
Normal Igf2 allele
is expressed
Mutant Igf2 allele
is expressed
Mutant Igf2 allele
is not expressed
Normal Igf2 allele
is not expressed
(b) Heterozygotes

7. Normal Igf2 allele is expressed Paternal chromosome Maternal
Fig a
Normal Igf2 allele
is expressed
Paternal
chromosome
Maternal
chromosome
Normal Igf2 allele
is not expressed
Wild-type mouse
(normal size)
Figure Genomic imprinting of the mouse Igf2 gene
(a) Homozygote

8. Mutant Igf2 allele Mutant Igf2 allele inherited from mother
Fig b
Mutant Igf2 allele
inherited from mother
Mutant Igf2 allele
inherited from father
Normal size mouse
(wild type)
Dwarf mouse
(mutant)
Normal Igf2 allele
is expressed
Mutant Igf2 allele
is expressed
Figure Genomic imprinting of the mouse Igf2 gene
Mutant Igf2 allele
is not expressed
Normal Igf2 allele
is not expressed
(b) Heterozygotes

9. Most imprinted genes are critical for embryonic development
It appears that imprinting is the result of the methylation (addition of –CH3) of DNA
Genomic imprinting is thought to affect only a small fraction of mammalian genes
Most imprinted genes are critical for embryonic development
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10. Fig. 15-UN3

11. Inheritance of Organelle Genes
Extranuclear genes (or cytoplasmic genes) are genes found in organelles in the cytoplasm
Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules
Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg
The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

12. Fig
Figure Variegated leaves from Croton dioicus

13. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

14. 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

15. Fig. 15-UN5

16. Fig. 15-UN6

17. Fig. 15-UN7

18. Fig. 15-UN8

19. Fig. 15-UN9

20. You should now be able to:
Explain the chromosomal theory of inheritance and its discovery
Explain why sex-linked diseases are more common in human males than females
Distinguish between sex-linked genes and linked genes
Explain how meiosis accounts for recombinant phenotypes
Explain how linkage maps are constructed
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21. Explain how nondisjunction can lead to aneuploidy
Define trisomy, triploidy, and polyploidy
Distinguish among deletions, duplications, inversions, and translocations
Explain genomic imprinting
Explain why extranuclear genes are not inherited in a Mendelian fashion
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings