1. Chapter 15 Thought Questions
Ms. Whipple – Brethren Christian Jr/Sr High School

2. 1.What is the Chromosome Theory of Inheritance?
Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels, and a chromosome theory of inheritance began to take form. It states that:
Genes occupy specific loci on chromosomes.
Chromosomes undergo segregation and independent assortment during meiosis.
Section 15.1

3. Figure 15.2
P Generation
Yellow-round seeds (YYRR)
Green-wrinkled seeds (yyrr) Y r y  Y R R r y
Meiosis
Fertilization R Y y r
Gametes
All F1 plants produce yellow-round seeds (YyRr).
F1 Generation R R y y r r Y Y
Meiosis
LAW OF SEGREGATION The two alleles for each gene separate during gamete formation.
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
Figure 15.2 The chromosomal basis of Mendel’s laws. R r r R 2 2 Y y
Metaphase II 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
Section 15.1
F2 Generation
An F1  F1 cross-fertilization 3
Fertilization recombines the R and r alleles at random. 3
Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. 9
: 3
: 3
: 1

4. 2. Why was Drosophila melanogaster a good choice of organism to study chromosomal inheritance? Were there any hindrances to this choice?
Like Mendel, Morgan made an insightful choice in his experimental animal. Morgan worked with Drosophila melanogaster, a fruit fly that eats fungi on fruit.
Fruit flies are prolific breeders and have a generation time of two weeks.
Fruit flies have only 4 chromosomes - three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males).
However, it was hard to find varieties of flies so Morgan spent a year looking for variant individuals among the flies he was breeding.
He discovered a single male fly with white eyes instead of the usual red.
The most commonly observed character phenotype is called the wild type (red eyes)
Section 15.1

5. Wild Type Mutant Section 15.1 Figure 15.3
Figure 15.3 Morgan’s first mutant.
Wild Type
Mutant
Section 15.1

6. 3. What were Thomas Morgan’s Fruit Fly findings and how support the theory of chromosomal inheritance?
When Morgan crossed his white-eyed male with a red-eyed female, all the F1 offspring had red eyes, suggesting that the red allele was dominant to the white allele.
Crosses between the F1 offspring produced the classic 3:1 phenotypic ratio in the F2 offspring.
Surprisingly, the white-eyed trait appeared in only F2 males.
All the F2 females and half the F2 males had red eyes.
Morgan concluded that a fly’s eye color was linked to its sex.
Section 15.1

7. 3. What were Thomas Morgan’s Fruit Fly findings and how support the theory of chromosomal inheritance?
Morgan reasoned that the gene with the white-eyed mutation is on the X chromosome, with no corresponding allele present on the Y chromosome.
A female (XX) can have white eyes only when both X chromosomes carry a recessive mutant allele (w).
Males (XY) have only a single allele. They will have red eyes if they have a red-eyed allele or white eyes if they have a white-eyed allele.
Morgan’s finding of the correlation between a particular trait and an individual’s sex provided support for the chromosome theory of inheritance.
A specific gene (for eye color) is carried on a specific chromosome (the X chromosome).
Section 15.1

8. P Generation F1 Generation F2 Generation
Figure 15.4a
EXPERIMENT
P Generation
F1 Generation
All offspring had red eyes.
RESULTS
Figure 15.4 Inquiry: 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?
F2 Generation
Section 15.1

9. P Generation F1 Generation F2 Generation
Figure 15.4b
CONCLUSION
P Generation w w X X X Y w w
Sperm
Eggs
F1 Generation w w w w w
Sperm
Figure 15.4 Inquiry: 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?
Eggs w w
F2 Generation w w w w w
Section 15.1 w

10. In bees and ants, females are diploid and males are haploid.
4. In the human chromosomal system, males are XY and females are XX. Is this the only system utilized by all organisms? Please describe an alternate system and which organism uses it.
The X-0 system is found in some insects. Females are XX and males are X.
In birds, some fishes, and some insects, females are ZW and males are ZZ.
In bees and ants, females are diploid and males are haploid.
Section 15.2

11. Section 15.2 Parents or Sperm Egg or Zygotes (offspring)
Figure 15.6
44  XY
Parents
44  XX
22  X or
22  Y
22  X
Sperm
Egg
44  XX or
44  XY
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)
Section 15.2
(d) The haplo-diploid system

12. 5. King Henry VIII famously executed a few of his wives for giving birth to daughters instead of sons. Now that we have a modern understanding of genetics and inheritance, how is offspring gender determined in humans? Was it the King or Queen’s “fault”?
Gender of offspring is determined by the sperm which may contain an X or a Y for the 23rd chromosome. All woman’s eggs contain an X so depending on which sperm fertilizes it, the baby will be male (XY) or female (XX)
It was the kings “fault” more than the queens but really it was just independent assortment at work.
Section 15.2

13. Figure 15.5 X Y
Figure 15.5 Human sex chromosomes.
Section 15.2

14. Genes located on the Y chromosome are called Y-linked genes.
6. For the 23rd chromosomal pair, how many genes have been sequenced from the Y chromosome? How many on the X chromosome? How does this affect the inheritance of genes?
Researchers have sequenced the Y chromosome and identified 78 genes coding for about 25 proteins.
Half of the genes are expressed only in the testes, and some are required for normal testicular function and the production of normal sperm.
Genes located on the Y chromosome are called Y-linked genes.
The Y chromosome is passed along virtually intact from a father to all his sons.
Because there are so few Y-linked genes, very few disorders are transferred from father to son on the Y chromosome.
A rare example is that in the absence of some of the Y-linked genes, an XY individual is male but does not produce normal sperm.
Section 15.2

15. The human X chromosome contains approximately 1,100 X-linked genes.
6. For the 23rd chromosomal pair, how many genes have been sequenced from the Y chromosome? How many on the X chromosome? How does this affect the inheritance of genes?
The human X chromosome contains approximately 1,100 X-linked genes.
Because males and females inherit a different number of X chromosomes, the pattern of inheritance of X-linked genes differs from that of genes located on autosomes.
While most Y-linked genes help determine sex, the X chromosomes have genes for many characters unrelated to sex.
Section 15.2

16. Section 15.2 XNXN XnY XNXn XNY XNXn XnY Sperm Xn Y Sperm XN Y Sperm Xn
Figure 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 X-linked recessive traits.
(a)
(b)
(c)
Section 15.2

17. 7. What is X-inactivation in female mammals
7. What is X-inactivation in female mammals? Is it the same X chromosome that is deactivated each time? What do tortoiseshell female cats have to do with it?
Males and females have the same effective dose (one copy) of genes on the X chromosome.
During female development, one X chromosome per cell condenses into a compact Barr body.
Most of the genes on the Barr-body X chromosome are not expressed.
The condensed Barr-body chromosome is reactivated in ovarian cells that produce eggs.
Section 15.2

18. 7. What is X-inactivation in female mammals
7. What is X-inactivation in female mammals? Is it the same X chromosome that is deactivated each time? What do tortoiseshell female cats have to do with it?
Mary Lyon, a British geneticist, demonstrated that selection of which X chromosome forms the Barr body occurs randomly and independently in each embryonic cells present at the time of X inactivation.
As a consequence, females consist of a mosaic of two types of cells, some with an active paternal X chromosome and others with an active maternal X chromosome.
After an X chromosome is inactivated in a particular cell, all mitotic descendants of that cell will have the same inactive X.
If a female is heterozygous for a sex-linked trait, approximately half her cells will express one allele, and the other half will express the alternate allele.
Similarly, the orange-and-black pattern on tortoiseshell cats is due to patches of cells expressing an orange allele while other patches have a non- orange allele.
Section 15.2

19. Cell division and X chromosome inactivation
Figure 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.
Section 15.2

20. 1. What are linked genes? How is there relationship to each other unique?
Each chromosome (except the Y chromosome) has hundreds or thousands of genes.
Genes located near each other on the same chromosome tend to be inherited together. These genes are called linked genes.
The results of crosses with linked genes differ from those expected according to the law of independent assortment.

21. 2. Briefly describe the experiment that led T. H
2. Briefly describe the experiment that led T.H. Morgan to discover linked genes.
Morgan followed the inheritance of characters for body color and wing size in Drosophila.
The wild-type body color is gray (b+) and the mutant is black (b).
The wild-type wing size is normal (vg+) and the mutant has vestigial wings (vg).
The mutant alleles are recessive to the wild-type alleles; neither gene is on a sex chromosome.

22. 2. Briefly describe the experiment that led T. H
2. Briefly describe the experiment that led T.H. Morgan to discover linked genes.
Morgan crossed F1 heterozygous females (b+bvg+vg) with homozygous recessive males (bbvgvg).
According to independent assortment, this should produce four phenotypes in a 1:1:1:1 ratio.
Morgan observed a large number of wild-type (gray-normal) and double-mutant (black-vestigial) flies among the offspring: the parental phenotypes.
The other two phenotypes (gray-vestigial and black-normal) were rarer than expected based on independent assortment.
Morgan reasoned that body color and wing shape are usually inherited together because the genes for these characters are on the same chromosome.

23. Wild type (gray-normal)
Figure
EXPERIMENT
P Generation (homozygous)
Double mutant (black body, vestigial wings)
Wild type (gray body, normal 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 Inquiry: 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

24. 3. Describe the process by which Unlinked Genes are recombined.
Unlinked genes are recombined by the independent assortment of chromosomes during gamete formation.
A 50% frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes.

25. About 17% of offspring, however, were recombinants.
4. Describe the process by which Linked Genes are recombined. If two genes are very close to each other on a chromosome, will it be more or less likely to be recombined by this process?
Most of the offspring from the Drosophila testcross for body color and wing size had parental phenotypes.
That suggested that the two genes were on the same chromosome, since the occurrence of parental types with a frequency greater than 50% indicates that the genes are linked.
About 17% of offspring, however, were recombinants.
Morgan proposed that some mechanism must occasionally break the physical connection between genes on the same chromosome.
This process, called crossing over, accounts for the recombination of linked genes.

26. 5. How did Alfred H. Sturtevant propose creating a map of genes along a chromosomes? What is this map called? What are the units used? Are there limits to this?
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
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

27. Recombination frequencies
Figure 15.11
RESULTS
Recombination frequencies 9%
9.5%
Chromosome
17%
Figure Research Method: Constructing a Linkage Map b cn vg

28. 5. How did Alfred H. Sturtevant propose creating a map of genes along a chromosomes? What is this map called? What are the units used? Are there limits to this?
There 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

29. 6. What is Non-Disjunction? When does it occur?
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

30. Nondisjunction of homo- logous chromosomes in meiosis I (a)
Figure
Meiosis I
Nondisjunction
Meiosis II
Non- disjunction
Gametes
Figure Meiotic nondisjunction.
n  1
n  1
n  1
n  1
n  1
n  1 n n
Number of chromosomes
Nondisjunction of homo- logous chromosomes in meiosis I
(a)
Nondisjunction of sister chromatids in meiosis II
(b)

31. 7. Define the following terms associated with non-disjunction:
Aneuploidy: the fertilization of gametes in which nondisjunction occurred. Offspring with this condition have an abnormal number of a particular chromosome
Monosomic: A monosomic zygote has only one copy of a particular chromosome

32. 7. Define the following terms associated with non-disjunction:
Trisomic: A trisomic zygote has three copies of a particular chromosome
Polyploid: 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

33. Deletion: removes a chromosomal segment Duplication: repeats a segment
8. Describe the following processes that can lead to alteration of a chromosome’s 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

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

35. 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 Alterations of chromosome structure.
A translocation moves a segment from one chromosome to a nonhomologous chromosome. G M N O C H F E D A B P Q R

36. 9. Describe the Aneuploid condition Down’s syndrome
9. Describe the Aneuploid condition Down’s syndrome. What causes this disorder? What is the frequency of this disorder? What does a woman’s age at time of pregnancy have anything to do with it?
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

37. Figure 15.15
Figure Down syndrome.

38. 10. Describe one syndrome associated with non-disjunction of the 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
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