2. New Combinations of Alleles: Variation for Natural Selection
Recombinant chromosomes bring alleles together in new combinations in gametes
Random fertilization increases even further the number of variant combinations that can be produced
This abundance of genetic variation is the raw material upon which natural selection works

3. Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry
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

4. 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

5. Recombination frequencies
Figure 15.11
Results
Recombination frequencies 9%
9.5%
Chromosome
17%
Figure Research method: constructing a linkage map b cn vg

6. Genes that are far apart on the same chromosome can have a recombination frequency near 50%
Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes

7. Sturtevant used recombination frequencies to make linkage maps of fruit fly genes
He and his colleagues found that the genes clustered into four groups of linked genes (linkage groups)
The linkage maps, combined with the fact that there are four chromosomes in Drosophila, provided additional evidence that genes are located on chromosomes

8. Long aristae (appendages on head) Red eyes Gray body Red eyes
Figure 15.12
Mutant phenotypes
Short aristae
Maroon eyes
Black body
Cinnabar eyes
Vestigial wings
Down- curved wings
Brown eyes
16.5
48.5
57.5
67.0
75.5
104.5
Figure A partial genetic (linkage) map of a Drosophila chromosome
Long aristae (appendages on head)
Red eyes
Gray body
Red eyes
Normal wings
Normal wings
Red eyes
Wild-type phenotypes

9. Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders
Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
Plants tolerate such genetic changes better than animals do

10. 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

11. Meiosis I Nondisjunction Figure 15.13-1
Figure Meiotic nondisjunction (step 1)

12. Meiosis I Nondisjunction Meiosis II Non- disjunction Figure 15.13-2
Figure Meiotic nondisjunction (step 2)

13. (a) Nondisjunction of homo- logous chromosomes in meiosis I
Figure
Meiosis I
Nondisjunction
Meiosis II
Non- disjunction
Gametes
Figure Meiotic nondisjunction (step 3)
n + 1
n + 1
n − 1
n − 1
n + 1
n − 1 n n
Number of chromosomes
(a) Nondisjunction of homo- logous chromosomes in meiosis I
(b) Nondisjunction of sister chromatids in meiosis II

14. Video: Nondisjunction in Mitosis

15. Aneuploidy results from the fertilization of gametes in which nondisjunction occurred
Offspring with this condition have an abnormal number of a particular chromosome

16. A monosomic zygote has only one copy of a particular chromosome
A trisomic zygote has three copies of a particular chromosome

17. 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

18. 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 orientation of a segment within a chromosome
Translocation moves a segment from one chromosome to another

19. (a) Deletion (c) Inversion (b) Duplication (d) Translocation
Figure 15.14
(a) Deletion
(c) Inversion
A B C D E F G H
A B C D E F G H
A deletion removes a chromosomal segment.
An inversion reverses a segment within a chromosome.
A B C E F G H
A D C B E F G H
(b) Duplication
(d) Translocation
A B C D E F G H
A B C D E F G H
M N O P Q R
A duplication repeats a segment.
A translocation moves a segment from one chromosome to a nonhomologous chromosome.
Figure Alterations of chromosome structure
A B C B C D E F G H
M N O C D E F G H
A B P Q R

20. A deletion removes a chromosomal segment.
Figure 15.14a
(a) Deletion
A B C D E F G H
A deletion removes a chromosomal segment.
A B C E F G H
(b) Duplication
Figure 15.14a Alterations of chromosome structure (part 1: deletion and duplication)
A B C D E F G H
A duplication repeats a segment.
A B C B C D E F G H

21. An inversion reverses a segment within a chromosome.
Figure 15.14b
(c) Inversion
A B C D E F G H
An inversion reverses a segment within a chromosome.
A D C B E F G H
(d) Translocation
A B C D E F G H
M N O P Q R
Figure 15.14b Alterations of chromosome structure (part 2: inversion and translocation)
A translocation moves a segment from one chromosome to a nonhomologous chromosome.
M N O C D E F G H
A B P Q R

22. 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

23. 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

24. Figure 15.15
Figure Down syndrome

25. Figure 15.15a
Figure 15.15a Down syndrome (part 1: karotype)

26. Figure 15.15b
Figure 15.15b Down syndrome (part 2: photo)

27. Aneuploidy of Sex Chromosomes
Nondisjunction of sex chromosomes produces a variety of aneuploid conditions
XXX females are healthy, with no unusual physical features
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

28. 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 severely intellectually disabled 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

29. Translocated chromosome 22 (Philadelphia chromosome)
Figure 15.16
Normal chromosome 9
Normal chromosome 22
Reciprocal translocation
Figure Translocation associated with chronic myelogenous leukemia (CML)
Translocated chromosome 9
Translocated chromosome 22 (Philadelphia chromosome)

30. Concept 15.5: Some inheritance patterns are exceptions to standard Mendelian inheritance
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
In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance

31. 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 depending on which parent passes them on

32. Environmental Effects of Environment.

33. Normal Igf2 allele is expressed.
Figure 15.17
Normal Igf2 allele is expressed.
Paternal chromosome
Maternal chromosome
Normal-sized mouse (wild type)
Normal Igf2 allele is not expressed.
(a) Homozygote
Mutant Igf2 allele inherited from mother
Mutant Igf2 allele inherited from father
Normal-sized mouse (wild type)
Dwarf mouse (mutant)
Normal Igf2 allele is expressed.
Figure Genomic imprinting of the mouse Igf2 gene
Mutant Igf2 allele is expressed.
Mutant Igf2 allele is not expressed.
Normal Igf2 allele is not expressed.
(b) Heterozygotes

34. It appears that imprinting is the result of the methylation (addition of —CH3) of cysteine nucleotides
Genomic imprinting is thought to affect only a small fraction of mammalian genes
Most imprinted genes are critical for embryonic development

35. Inheritance of Organelle Genes
Extranuclear genes (or cytoplasmic genes) are 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

36. Figure 15.18
Figure A painted nettle coleus plant

37. 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

38. Offspring from test- cross of AaBb (F1) × aabb
Figure 15.UN03a
Offspring from test- cross of AaBb (F1) × aabb
Purple stem/short petals (A–B–)
Green stem/short petals (aaB–)
Purple stem/long petals (A–bb)
Green stem/long petals (aabb)
Expected ratio if the genes are unlinked
Expected number of offspring (of 900)
Figure 15.UN03a Skills exercise: using the chi-square test (part 1)
Observed number of offspring (of 900)

39. Testcross Offspring Expected (e) Observed (o) Deviation (o − e)
Figure 15.UN03b
Testcross Offspring
Expected (e)
Observed (o)
Deviation (o − e)
(o − e)2
(o − e)2/e
(A−B−)
220
(aaB−)
210
(A−bb)
231
(aabb)
239
Figure 15.UN03b Skills exercise: using the chi-square test (part 2)
2 = Sum

40. Red = maternal; blue = paternal.
Figure 15.UN04
Sperm
Egg C c B b D d
P generation gametes A E a 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 chromo- somes and is heterozygous for all six genes shown (AaBbCcDdEeFf ).
Red = maternal; blue = paternal. D e C B d
Figure 15.UN04 Summary of key concepts: linkage 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 genetically linked. E F
Each chromosome has hundreds or thousands of genes. Four (A, B, C, F ) are shown on this one. f a c b