Chapter 12
Chromosomal Patterns of Inheritance
Genes are located on the chromosome, which explains the similarity of chromosome and gene behavior during sexual
reproduction. All chromosomes except one pair are called autosomes—the nonsex chromosomes. The other pair are
the sex chromosomes. This pair determines the sex of the new individual. The father can contribute an X chromosome
or a Y chromosome to his offspring, while the mother can only contribute an X chromosome. Therefore, the sex of the
offspring is determined by the genetic contribution of the father.
The inheritance patterns of sex chromosomes explain the inheritance of certain traits, as supported by Morgan's
experiments. X-linked inheritance refers to genes located on the X chromosomes. Because males normally receive only
one X chromosome, they are subject to disorders caused by the inheritance of a recessive allele on the X chromosome.
Well-known X-linked disorders are color blindness, Duchenne muscular dystrophy, and hemophilia. Another X-linked
disorder is fragile X syndrome.
All the genes on one chromosome form a linkage group, which is broken only when crossing-over occurs. Genes that
are linked tend to go together into the same gamete. If crossing-over occurs, a two-trait cross involving dihybrids results
in all possible phenotypes among the offspring, but the expected ratio is greatly changed. The frequency of recombinant
gametes that occurs due to crossing-over has been used to map chromosomes.
Chromosomal mutations include changes in chromosome number, such as polyploidy and aneuploidy. The usual cause
of monosomy and trisomy is nondisjunction during meiosis, which can result in the inheritance of an abnormal number
of autosomes, as in Down syndrome. Nondisjunction of the sex chromosomes can cause abnormal sex chromosome
numbers in offspring, as in Turner syndrome, Klinefelter syndrome, poly-X, and Jacobs’s syndrome. Amniocentesis and
chorionic villi sampling can provide fetal cells for the karyotyping of chromosomes.
Chromosomal mutations also include changes in chromosome structure, such as deletions, duplications, translocations,
and inversions. Examples of deletion syndromes are Williams and cri du chat syndromes, and an example of a
translocation syndrome is Alagille syndrome.
12.1 – Chromosomal Inheritance (204-207)
• Certain chromosomes are called the sex chromosomes because they determine the sex of the individual.
• Certain traits, unrelated to sex, are controlled by genes located on the sex chromosomes.
• Sex-linked traits are usually carried on the X chromosome. Males, having only one X, are more likely to exhibit Xlinked disorders.
• The pattern of inheritance can indicate whether the disorder is X-linked..
1. Match the descriptions with these terms:
(a) X and Y chromosomes
(b) XX
(c) homologous chromosomes
a.______sex chromosomes.
b.______all chromosomes, except the sex chromosomes.
c. ______pairs of chromosomes.
d. ______females.
e. ______male.
(d) autosomes
(e) XY
2. Indicate whether the following statements about Morgan's findings with Drosophila are true (T) or false (F):
a. ______An allele on the X chromosome was not found on the Y chromosome.
b. ______Drosophila flies have the same sex chromosome pattern as humans.
c. ______Carriers are XRXr.
d. ______White eyes are dominant to red eyes.
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Bar eye in Drosophila is an X-linked characteristic in which bar eye (B) is dominant over normal eye (b). The genotype of
a normal-eyed female is a. ____________, and the genotype of a bar-eyed male is b. ____________
4. a. Draw a Punnett square to show the genotypes among the offspring from a cross of the flies in question 3
Determine the chance (percent) of each of the following phenotypes:
b. bar-eyed males
______
c. bar-eyed females
______
d. normal-eyed males
______
e. normal-eyed females
______
5. If XB = normal vision and Xb = color blindness, state the sex and the phenotype of each of these genotypes:
X B XB
a. __________________
B b
XX
b. __________________
xbxb
c. __________________
XB Y
d. __________________
b
XY
e. __________________
6. a. Why do more males than females have X-linked genetic disorders? ______________________________________
b. Why do sons inherit the disorder from their mothers? ______________________________________________
7. a. Indicate the genotype of each
b. How do you know that this is a pedigree for an X-linked recessive trait? ____________________________________
_______________________________________________________________________________________
8. Match the descriptions to these disorders:
hemophilia
muscular dystrophy
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color blindness
a. ________________________muscle weakness
b. ________________________can't see reds and greens
c. ________________________ bleeder's disease
9. Create a Punnett square to show the expected outcome if a color-blind woman reproduces with a man who has
normal vision.
a. What are the chances of a color-blind daughter? ____________
b. What are the chances of a color-blind son? ____________
10. A son is color blind, but his mother and father are not color blind. Give the genotype of all persons involved.
a. son ____________
b. mother
____________
c. father
____________
11. Hemophilia is an X-linked recessive disorder. A woman who is a hemophiliac reproduces with a man who is not, use:
h = recessive allele
H = dominant allele:
a. The genotype of the woman is
b. The genotype of the man is
c. What is the genotype of sons from this cross?
d. What is the genotype of daughters from this cross?
e. What is the chance (percent) of the daughters' sons having hemophilia?
f. What is the chance of the sons' sons (assume noncarrier wife) having hemophilia?
____________
____________
____________
____________
____________
____________
12. Fragile X syndrome is due to the inheritance of a(n) a. ________________________. The fragile location seems to be
due to a multiple b. __________________ of the bases CGG.
12.2 – Gene Linkage (pg. 209-211)
• Alleles on the same chromosome are said to be linked, and crossing-over makes it possible to determine the
order of genes on the chromosome
13. All the genes on one chromosome form a linkage group and tend to be inherited together. Mendel's law of
independent assortment ☐does ☐does not hold for linked genes.
In questions 14-15, consider that, in humans, arched eyebrow (E) is dominant over curved eyebrow (e) and hitchhiker
thumb (T) is dominant over normal thumb (t). Imagine that these two genes are linked and that two dihybrids having
these gametes reproduce.
14. From the diagram, indicate the phenotype for the
following offspring:
a. __________________
b. __________________
c. __________________
d. __________________
15. What is the phenotypic ratio among the offspring? a. __________________________________________. What
would the ratio have been if the genes were on nonhomologous chromosomes? b. __________________.
16. The percentage of crossing-over indicates the distance between two gene loci. For example, A and B recombine with
a and b in 10% of the offspring, A and C recombine with a and c in 20% of the offspring, and B and C recombine with a
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and c in 10% of the offspring.
How many map units are between the following?
a. A and C
__________________
b. A and B
__________________
c. B and C
__________________
d. Draw a line and map the positions of A, B, and C.
12.3 – Changes in Chromosome Number (pg. 212-217)
• Chromosomal mutations can be caused by a change in chromosome number.
17. The diploid chromosome number of a species is 24. Give the chromosome number if the species has the following
conditions:
a. pentaploid condition
______
b. tetraploid condition
______
c. condition with one trisomy
______
d. condition with one monosomy
______
18. Indicate whether the following statements about Down syndrome are true (T) or false (F):
a. ______Characteristics include a flat face and an eyelid fold.
b. ______In some cases, an extra copy of chromosome 21 is attached to chromosome 16.
c. ______Children of younger women are more likely to be affected than children of older women.
d. ______It most often occurs due to a nondisjunction.
e. ______The Gart gene may be involved in the mental retardation that accompanies Down syndrome.
f. ______ Persons with the defect usually have three copies of chromosome 18.
19. Match the following conditions to each of the descriptions. Terms can be used more than once.
1. Turner syndrome
3. poly-X individual
2. Klinefelter syndrome
4. Jacobs syndrome
a. ______________________________female with no apparent physical abnormalities
b. ______________________________male with some breast development, large hands
c. ______________________________XYY male
d. ______________________________XXY male
e. ______________________________XO female
12.4 – Changes in Chromosome structure (pg. 218-221)
• Chromosomal mutations can be caused by a change in chromosome structure.
20. Identify the types of chromosomal mutations shown in the following illustration.
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21. Label the following photograph, using these terms:
autosomes
homologous pair
karyotype of a male
sex chromosomes
22. Place A for amniocentesis or C for chorionic villi sampling on the appropriate lines.
a. ______14 to 17 weeks of pregnancy
b. ______5 weeks of pregnancy
c. ______Cells are obtained by suction. .
d. ______Cells are obtained by needle.
e. ______Cells are from a cavity about the embryo/fetus.
f. ______Cells are from the extra-embryonic membrane.
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