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Study Guide Chapter 15 -Chromosomal Basis of Inheritance-Answers
15.1 Mendelian Inheritance and chromosome theory______________________________
1. Thomas Hunt Morgan identified the first solid evidence associating a
specific___gene_____ on a specific chromosome.
2. Why did Morgan choose Drosophila as his experimental organism? (List 3 reasons)
They reproduce quickly, a new generation of adults forms every two weeks!
Prolific (single matting = hundreds of offspring)
Only 4 chromosome pairs = simple genetics
3A. The normal phenotype for a character (phenotype most common in nature) is
called___wild type_____.
3B. Provide at least two examples this phenotype in Drosophila.
Red eyes
gray body
4A. An alternative phenotype for a character (phenotype not common in nature) is
called____mutant____.
4B. Provide at least two examples of this phenotype in Drosophila.
White eyes
Black body
5. Morgan and his students invented a notation to symbolize alleles in Drosophila that
differed from the notation Mendel used to represent alleles. Describe how Morgan’s
Drosophila characters are named and the symbol used for the allele type.
You can think of Morgan’s wild-type allele as equivalent to the dominant allele in
Mendel’s naming system. And you can think of Morgan’s mutant allele as equivalent to
Mendel’s recessive allele. So whenever you have one wild-type allele and one mutant
allele, that organism will have the wild-type phenotype.
Mendel
Used the first letter of the dominant character name to represent the dominant allele. This
letter was capitalized for the dominant allele, lower case for the recessive allele
Ex: purple flower allele dominant to white flower allele: P= dominant allele, p=recessive
allele
Morgan
-Used the first letter of the mutant character name to represent the wild-type allele. The
letter for the wild-type and mutant alleles are both lower case. The wild-type allele gets a
+ sign and the mutant does not.
Ex: red eye allele wild-type to white eye mutant: w+ = wild-type allele, w= recessive
allele.
6. Morgan and his students invented a notation to symbolize alleles in Drosophila.
Which of the following genotypes would produce a fly that is wild-type for eye color (red
vs. white eyes)?
w+ w+
w+ w
ww
w+w+ and w+w
7. Morgan performed an experiment that yielded a 3:1 ratio of offspring in the F2
generation; however, only the males of this F2 generation had white eyes.
White- eye females DO EXIST, so why were only white-eye males observed in this cross?
Complete the following chart and describe why Morgan observed these results and how it
allowed him to conclusively determine that the gene for eye color was located on the X
chromosome.
Morgan’s experiment (see Figure 15.4) Use notation developed by Morgan and his
students
If the gene for eye color is located on the X chromosome….
P generation
Red eye Female genotype for eye color = w+w+
White eye Male genotype for eye color= w
F1 generation
Genotype of offspring =
Red eye females w+w
Red eye males w+
F2 generation (F1 M crossed to F1 F)
Genotypes of offspring:
Red eye females w+w+
Red eye females w+w
Red eye males w+
8. If the eye-color gene locus in Drosophila was located on an autosome what would you
expect to observe in the F2 offspring produced by the crosses in the Morgan’s experiment
described in the previous question?
He would have observed equal numbers of white -eye males and females.
15.2 Sex-linked genes inheritance patterns______________________________________
9. Compare the X an Y chromosomes in Mammals.
X = larger, has more genes, two found in females, one in males
Y = smaller, has fewer genes, one in males, none in females
10A. Following Meiosis in mammals, each ovum (egg) contains one __X_____(X or Y)
sex chromosome, whereas following meiosis in males each sperm cell has either an X
OR Y chromosome.
10B. Which gender M/F determines the sex of offspring in mammals? Male
11. Your friend Bill has a “sex-linked” disease. Which chromosome, X or Y, is more
likely to contain the mutant allele that is responsible for Bill’s disease Why?
X-because it is bigger than the Y, it has many more genes. For this reason, if a person has
a “sex-linked disease” it is most likely on the X chromosome. It is so rare to have a sexlinked disease as a result of a mutant gene on the Y chromosome that sex-linked diseases
are also called X-linked diseases.
12. In mammals, sex-linked genes are passed from Father’s to
__daughters__(daughters/son) only. Mother’s can pass sex-linked genes to sons OR
daughters.
13A. Name at least two sex-linked disorders found in humans.
Colorblindness, hemophilia
13B. Why are sex-linked disorders more common in males?
More common in males because males only have one X chromosome, whereas females
have two X chromosomes. If a male inherits an X chromosome from his mom with a
diseased allele, he will have the disease because he has only one X chromosome and does
not have another X chromosome to inherit a good (non-diseased allele). Females, because
they have two X chromosomes, have two chances to inherit a non-diseased allele.
14. Define the term “Hemizygous”. When is this term used? How does it differ from the
term “carrier”?
The term “hemizygous” is used to describe a male who has a mutant allele for a gene on
his X chromosome. Being a male, he does not have another X chromosome. Therefore, a
male CANNOT be a “carrier” for a sex-linked disease…he either has the disease or he
does not have the disease.
15. If a female homozygous dominant for the normal form of a sex-linked gene mates
with a male who is hemizygous for the mutant form of the gene…
MAKE A PUNNETT SQUARE FOR THIS CROSS!
A. What fraction of the total offspring from this mating will have the disease?
0/4 = 0 (When asked for total offspring, look at the ENTIRE punnett square...0 out of the
4 offspring will have the disease.)
B. What fraction of the total offspring from this mating will be carriers for the disease?
2/4 = 1/2 (When asked for total offspring, look at the ENTIRE punnett square...2 out of
the 4 offspring will be carriers.)
C. What fraction of the female offspring from this mating will be carriers for the disease?
2/2 =1, (When asked about the female offspring, look at the females ONLY…All females
will be carriers.)
D. What percentage of the female offspring from this mating will have the disease?
0/2 = 0 (look at the females ONLY…0 out of the 2 females will have the disease)
E. What percentage of the male offspring from this mating will be carriers for the
disease? NONE, Males CANNOT be “carriers” for a sex-linked disease!
F. What percentage of the male offspring from this mating will have the disease?
0/2 = 0 (When asked about the male offspring, look at the males ONLY…0 out of the 2
males will have the disease.)
16. If a female heterozygous for a sex-linked disease mates with a male who is
hemizygous for the mutant form of the gene…
MAKE A PUNNETT SQUARE FOR THIS CROSS!
A. What fraction of the total offspring from this mating will have the disease?
2/4 = 1/2 (When asked for total offspring, look at the ENTIRE punnett square...2 out of
the 4 offspring will have the disease.)
B. What fraction of the total offspring from this mating will be carriers for the disease?
1/4 (When asked for total offspring, look at the ENTIRE punnett square...1 out of the 4
offspring will be a carrier.)
C. What fraction of the female offspring from this mating will be carriers for the disease?
1/2 (When asked about the female offspring, look at the females ONLY…one out of the
two females will be a carrier.)
D. What percentage of the female offspring from this mating will have the disease?
1/2 (look at the females ONLY…1 out of the 2 females will have the disease.)
E. What percentage of the male offspring from this mating will be carriers for the
disease? NONE, Males CANNOT be “carriers” for a sex-linked disease!
F. What percentage of the male offspring from this mating will have the disease?
1/2 (When asked about the male offspring, look at the males ONLY…1 out of the 2
males will have the disease.)
17. During embryogenesis, one of the X chromosomes in every female cell becomes
inactivated. What is the name given to this inactivated X chromosome? Barr body
18A. Random inactivation of one of the X chromosomes (X inactivation) occurs in every
body cell of female mammals. In a particular breed of cat (tortoiseshell cat) the gene for
fur color is located on the X chromosome. Two alleles exist, orange fur allele and black
fur allele. A female with a heterozyougous genotype will be tortoiseshell (orange and
black). Male tortoiseshell cats do not exist. Explain this finding.
The heterozygous fur color genotype female cat has one X with the orange fur allele,
other X has black fur color allele. X with orange allele is active in some cells, inactive
(Barr body) in other cells. X with black allele active  black fur, X with orange allele
active  orange fur. This is why the cat has patches of orange and patches of black fur.
A male has only ONE X chromosome, so he can only have the orange fur color allele on
his X OR the black fur color allele on his X. He can only be orange OR black.
B. A Female orange (tortoiseshell species) cat mates with a black male (tortoiseshell
species) cat.
Orange allele= b
Black allele =B
What are the fur color genotypes of the parent cats (REMEMBER…this allele is on the X
chromosome!) ?
Female: Xb Xb
Male: XB Y
C. What are the possible genotypes of their offspring? (construct a punnett square)
XB Xb
Xb Y
D. What are the offspring phenotypes associated with the genotypes listed in the previous
question?
XB Xb tortoiseshell female
Xb Y orange male
15.4 Alteration in chromosome structure or number  genetic disorders
19. Describe how nondisjunction can occurs during meiosis.
Meiosis I = homologous chromosomes do not separate
Meiosis II = sister chromatids do not separate
20. If a gamete with an abnormal number of chromosomes (produced as a result of
nondisjunction) unites with a normal gamete at fertilization, what condition will result?
Aneuploidy (Ex: trisomic, monosomic)
21A. A chromosome present in triplicate in fertilized egg (2n + 1 chromosomes) is said to
be _____trisomic_________________.
21B. What disease is caused by the presence of chromosome 21 in triplicate? Down
syndrome
22. Breakage of a chromosome can lead to 4 types of changes in chromosome structure.
List these 4 possible changes and describe how they occur.
Deletion –when nonsister chromatids cross over during meiosis at wrong place
Duplication- when nonsister chromatids cross over during meiosis at wrong place
Inversion- when when nonsister chromatids cross over during meiosis at the right place,
but one of the broken chromatids attaches to the nonsister chromatid upside down
Translocation- when when nonsister chromatids cross over during meiosis at the right
place, but attaches to a non homologous chromosome