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CHAPTER 4
Conceptual Questions
C1.
Answer: In the X-Y system the male is heterogametic whereas in the Z-W system
the female is heterogametic.
C2. Answer:
A.
Dark males and light females; reciprocal: all dark offspring
B.
All dark offspring; reciprocal: dark females and light males
C.
All dark offspring; reciprocal: dark females and light males
D.
All dark offspring; reciprocal: dark females and light males
C3. Answer:
A.
The fly is a male because the ratio of X chromosomes to sets of autosomes
is 1/2, or 0.5.
B.
The fly is female because the ratio of X chromosomes to sets of autosomes
is 1.0.
C.
The fly is male because the ratio of X chromosomes to sets of autosomes
is 0.5.
D.
The fly is female because the ratio of X chromosomes to sets of autosomes
is 1.0.
C4.
Answer:
A.
Female; there are no Y chromosomes.
B.
Female; there are no Y chromosomes.
C.
Male; the Y chromosome determines maleness.
D.
Male; the Y chromosome determines maleness.
C5.
Answer: Dosage compensation refers to the phenomenon that the level of
expression of genes on the sex chromosomes is similar in males and females, even
though they have different numbers of sex chromosomes. In many species it seems
necessary so that the balance of gene expression between the autosomes and sex
chromosomes is similar between the two sexes.
C6.
Answer: A Barr body is a mammalian X chromosome that is highly condensed. It
is found in somatic cells with two or more X chromosomes. Most genes on the Barr body
are inactive.
C7.
Answer: In mammals, one of the X chromosomes is inactivated in females; in
Drosophila, the level of transcription on the X chromosome in males is doubled; in C.
elegans, the level of transcription of the X chromosome in hermaphrodites is decreased
by 50% of that of males.
C8.
Answer: X inactivation in heterozygous females produces a mosaic pattern of
gene expression. During early embryonic development, some cells have the maternal X
chromosome inactivated and other cells have the paternal X chromosome inactivated;
these embryonic cells will divide and produce billions of cells. In the case of a female
that is heterozygous for a gene that affects pigmentation of the fur, this produces a
variegated pattern of coat color. Because it is a random process in any given animal, two
female cats will vary as to where the orange and black patches occur. A variegated coat
pattern could not occur in female marsupials due to X inactivation because the paternal X
chromosome is always inactivated in the somatic cells of females.
C9.
Answer: X inactivation begins with the counting of Xics. If there are two X
chromosomes, in the process of initiation, one is targeted for inactivation. During
embryogenesis, this inactivation begins at the Xic locus and spreads to both ends of the X
chromosome until it becomes a highly condensed Barr body. The Xist gene, which is
located in the Xic region, remains transcriptionally active on the inactivated X
chromosome. It is thought to play an important role in X inactivation by coating the
inactive X chromosome. After X inactivation is established, it is maintained in the same
X chromosome in somatic cells during subsequent cell divisions. In germ cells, however,
the X chromosomes are not inactivated, so that an egg can transmit either copy of an
active (noncondensed) X chromosome.
C10. Answer: The male is XXY. The person is male due to the presence of the Y
chromosome. Because two X chromosomes are counted, one of the X chromosomes is
inactivated to produce a Barr body.
C11. Answer:
A.
One
B.
Zero
C.
Two
D.
Zero
C12. Answer:
A.
In females, one of the X chromosomes is inactivated. When the X
chromosome that is inactivated carries the normal allele, only the defective color
blindness allele will be expressed. Therefore, on average, about half of a female’s eye
cells are expected to express the normal allele. Depending on the relative amounts of
cells expressing the normal versus the color-blind allele, the end result may be partial
color blindness.
B.
In this female, as a matter of chance, X inactivation occurred in the right
eye to always, or nearly always, inactivate the X chromosome carrying the normal allele.
The opposite occurred in the left eye. In the left eye, the chromosome carrying the color
blindness allele was primarily inactivated.
C13. Answer: The offspring inherited XB from its mother and XO and Y from its
father. It is an XXY animal, which is male (but somewhat feminized).
C14. Answer: The spreading stage is when the X chromosome is inactivated (i.e.,
condensed) as a wave that spreads outward from the X-inactivation center (Xic). The
condensation spreads from Xic to the rest of the X chromosome. The Xist gene is
transcribed from the inactivated X chromosome. It encodes an RNA that coats the X
chromosome, which subsequently attracts proteins that are responsible for the
compaction.
C15. Answer: The outcome is that all the daughters will be affected and all the sons
will be unaffected. The ratio is 1:1.
C16. Answer: First set up the following Punnett square:
A.
1/4
B.
(3/4)(3/4)(3/4)(3/4) = 81/256
C.
3/4
D.
The probability of an affected offspring is 1/4 and the probability of an
unaffected offspring is 3/4. For this problem, you use the binomial expansion equation
where x = 2, n = 5, p = 1/4, and q = 3/4. The answer is 0.26, or 26%, of the time.
C17. Answer: 1 affected daughter : 1 unaffected daughter : 1 affected son : 1
unaffected son