Biology Now, 2e

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CHAPTER 8

END-OF-CHAPTER ANSWERS

1. chromosomes, genes, loci, alleles


2. d


3. gene therapy: 3, in vitro fertilization: 5, preimplantation

genetic diagnosis: 4, chorionic villus sampling: 1, amniocen-

tesis: 2

4. The sex chromosomes are the two in the final pair. Since there

is one large sex chromosome and one small one, the individual

is a male.

5. 
   

6. d


7. e


8. c


9. b, d, e


10. a (where allele T represents stumpy tails and allele t represents

long t a i l s)

11. (a) from their mother, (b) no, (c) yes, (d) two types, (e) no, no,

yes, female only

12. (a) XgXg, (b) XGXg, (c) XgY, (d) XgY, (e) XGXg


13. (a) gg, (b) Gg, (c) Gg, (d) gg

ANSWERS TO FIGURE QUESTIONS

Figure 8.2

Q1: Which two children in this pedigree have cystic fibrosis? How

do you know?

A1: Individuals III-2 and III-3 have cystic fibrosis. They are

depicted by filled symbols.

Q2: Does either parent of these two children have cystic fibrosis? If

so, which one(s)? How do you know?

A2: Neither parent II-1 nor II-2 has cystic fibrosis. They are both

depicted by open symbols.

Q3: Do any of the grandparents of these two children have cystic

fibrosis? If so, which one(s)? How do you know?

A3: Yes. I-2, the paternal grandmother, has cystic fibrosis. She is

depicted by a filled symbol.

HH Hh

H h

Hh

H

h hh

Figure 8.3

Q1: How many male and how many female descendants

(individuals that did not join the family by marriage) does

generation IV of Aldrich’s pedigree contain?

A1: 8 males and 4 females.

Q2: What proportions of the male and female descendants in

generation IV were affected by the disorder?

A2: 50% (4/8, or 0.50) of males, and 0% (0/4, or 0.00) of females.

Q3: Why did Aldrich hypothesize that the disease was X-linked?

(You will need to read ahead to answer this question.)

A3: The disease was observed only in males, but their mothers’

male relatives were also affected.

Figure 8.4

Q1: Is this the karyotype of a male or a female?

A1: Male.

Q2: How would the karyotype of a person with Down syndrome

differ from this karyotype?

A2: There would be three copies of chromosome 21.

Q3: The size of a chromosome correlates roughly with the number

of genes residing on it. Why are an extra copy of chromosome

21 and a missing Y chromosome two of the least damaging

chromosomal abnormalities?

A3: Chromosome 21 and the Y chromosome are two of the smallest

chromosomes, so fewer genes would be affected by there being too

many or too few of them (compared to a larger chromosome).

Figure 8.5

Q1: What are the odds that a given egg cell will contain an X

chromosome? A Y chromosome? What are those odds for a sperm

cell?

A1: An egg cell has a 100% chance of containing an X

chromosome and a 0% chance of containing a Y chromosome. For

the sperm cell, the odds are 50/50 for both cases.

Q2: If a couple has two daughters, does that mean their next two

children are more likely to be sons? Explain your reasoning. (Hint:

Refer back to “What Are the Odds?” on page 124.)

A2: No, the probability for each event is independent of prior and

future events.

Q3: Sisters share the same X chromosome inherited from their

father, but they may inherit different X chromosomes from their

mother. What is the probability that brothers share the same

Y chromosome? What is the probability that brothers share the same

X chromosome?

A3: Brothers have a 100% probability of sharing the same Y

chromosome, and a 50% probability of sharing the same X

chromosome.

Figure 8.6

Q1: Why are changes in chromosome number almost always more

severe than changes in chromosome structure?