Essentials of Genetics

William S. Klug, Michael R. Cummings, Charlotte A. Spencer

Chapter 6

Chromosome Mutations: Variation in Number and Arrangement - all with Video Answers

Educators

Chapter Questions

Problem 1

In this chapter, we focused on chromosomal mutations resulting from a change in number or arrangement of chromosomes. In our discussions, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions?
(a) How do we know that the extra chromosome causing Down syndrome is usually maternal in origin?
(b) How do we know that human aneuploidy for each of the 22 autosomes occurs at conception, even though most often human aneuploids do not survive embryonic or fetal development and thus are never observed at birth?
(c) How do we know that specific mutant phenotypes are due to changes in chromosome number or structure?
(d) How do we know that the mutant Bar-eye phenotype in Drosophila is due to a duplicated gene region rather than to a change in the nucleotide sequence of a gene?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 2

Review the Chapter Concepts list on page $99 .$ These all center on chromosome aberrations that create
variations from the "normal" diploid genome. Write a short essay that discusses five altered phenotypes that result from specific chromosomal aberrations.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 3

Define these pairs of terms, and distinguish between them. aneuploidy/euploidy monosomy/trisomy Patau syndrome/Edwards syndrome autopolyploidy/allopolyploidy autotetraploid/amphidiploid paracentric inversion/pericentric inversion

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 4

For a species with a diploid number of $18,$ indicate how many chromosomes will be present in the somatic nuclei of individuals that are haploid, triploid, tetraploid, trisomic, and monosomic.

Jackson Miner

Numerade Educator

Problem 5

What explanation has been proposed to explain why Down syndrome is more often the result of nondisjunction during oogenesis rather than during spermatogenesis?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 6

Contrast the fertility of an allotetraploid with an autotriploid and an autotetraploid.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 7

Why do human monosomics most often fail to survive prenatal development?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 8

Describe the origin of cultivated American cotton.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 9

Predict how the synaptic configurations of homologous pairs of chromosomes might appear when one member is normal and the other member has sustained a deletion or duplication.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 10

Inversions are said to "suppress crossing over." Is this ter. minology technically correct? If not, restate the description accurately.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 11

Predict the genetic composition of gametes derived from tetrads of inversion heterozygotes where crossing over occurs within a pericentric inversion.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 12

Human adult hemoglobin is a tetramer containing two alpha
(a) and two beta ( $\beta$ ) polypeptide chains. The $\alpha$ gene cluster on chromosome 16 and the $\beta$ gene cluster on chromosome 11 share
amino acid similarities such that 61 of the amino acids of the
$\alpha$ -globin polypeptide ( 141 amino acids long) are shared in identical sequence with the $\beta$ -globin polypeptide $(146$ amino acids long. How might one explain the existence of two polypeptides with partially shared function and structure on two different chromosomes? Include in your answer a link to Ohno's hypothesis regarding the origin of new genes during evolution.

Carlene Jimenez

Carlene Jimenez

Numerade Educator

Problem 13

The primrose, Primula kewensis, has 36 chromosomes that are similar in appearance to the chromosomes in two related species, $P .$ floribunda $(2 n=18)$ and $P$. verticillata $(2 n=18) .$ How could
P. kewensis arise from these species? How would you describe $P$ kewensis in genetic terms?

Christina Sorrentino

Christina Sorrentino

Numerade Educator

Problem 14

Certain varieties of chrysanthemums contain $18,36,54,72,$ and 90 chromosomes; all are multiples of a basic set of nine chromosomes. How would you describe these varieties genetically? What feature do the karyotypes of each variety share? A variety with 27 chromosomes has been discovered, but it is sterile. Why?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 15

Drosophila may be monosomic for chromosome $4,$ yet remain fer tile. Contrast the $\mathrm{F}_{1}$ and $\mathrm{F}_{2}$ results of the following crosses involving the recessive chromosome 4 trait, bent bristles:
(a) monosomic IV, bent bristles $\times$ normal bristles;
(b) monosomic $\mathrm{IV}$, normal bristles $\times$ bent bristles.

Hailey Tomashek

Hailey Tomashek

Numerade Educator

Problem 16

Mendelian ratios are modified in crosses involving autotetraploids. Assume that one plant expresses the dominant trait green seeds and is homozygous (WWWW). This plant is crossed to one with white seeds that is also homozygous (wwww). If only one dominant allele is sufficient to produce green seeds, predict the $\mathrm{F}_{1}$ and $\mathrm{F}_{2}$ results of such a cross. Assume that synapsis between chromosome pairs is random during meiosis.

Chelsi Marolf

Numerade Educator

Problem 17

Having correctly established the $\mathrm{F}_{2}$ ratio in Problem $16,$ predict the $\mathrm{F}_{2}$ ratio of a "dihybrid" cross involving two independently assorting characteristics (e.g., $P_{1}=W W W W A A A A \times$ wwwwaaaa).

Qudsiya Anis

Numerade Educator

Problem 18

In a cross between two varieties of corn, $g l_{1} g l_{1} W s_{3} W s_{3}($ egg parent) $\times G I_{1} G l_{1} w s_{3} w s_{3}$ (pollen parent), a triploid offspring was produced with the genetic constitution $G l_{1} G l_{1} g l_{1} W s_{3} w s_{3} w s_{3}$ From which parent, egg or pollen, did the $2 n$ gamete originate? Is another explanation possible? Explain.

Shiksha Dutta

Numerade Educator

Problem 19

A couple planning their family are aware that through the past three generations on the husband's side a substantial number of stillbirths have occurred and several malformed babies were
born who died early in childhood. The wife has studied genetics and urges her husband to visit a genetic counseling clinic, where a complete karyotype-banding analysis is performed. Although the tests show that he has a normal complement of 46 chromosomes, banding analysis reveals that one member of the chromosome 1 pair (in group $A$ ) contains an inversion covering 70 percent of its length. The homolog of chromo. some 1 and all other chromosomes show the normal banding
sequence.
(a) How would you explain the high incidence of past stillbirths?
(b) What can you predict about the probability of abnormality/ normality of their future children?
(c) Would you advise the woman that she will have to bring each pregnancy to term to determine whether the fetus is normal? If not, what else can you suggest?

Sulav Pokhrel

Numerade Educator

Problem 20

A woman who sought genetic counseling is found to be heterozy. gous for a chromosomal rearrangement between the second and third chromosomes. Her chromosomes, compared to those in a normal karyotype, are diagrammed on the next page:
(a) What kind of chromosomal aberration is shown?
(b) Using a drawing, demonstrate how these chromosomes would pair during meiosis. Be sure to label the different segments of the chromosomes.
(c) This woman is phenotypically normal. Does this surprise you? Why or why not? Under what circumstances might you expect a phenotypic effect of such a rearrangement?

Dennis Howard

Numerade Educator

Problem 21

The woman in Problem 20 has had two miscarriages. She has come to you, an established genetic counselor, with these questions:
(a) Is there a genetic explanation of her frequent miscarriages?
(b) Should she abandon her attempts to have a child of her own?
(c) If not, what is the chance that she could have a normal child? Provide an informed response to her concerns.

Harsh Gadhiya

Numerade Educator

Problem 22

In a recent cytogenetic study on 1021 cases of Down syndrome, 46 were the result of translocations, the most frequent of which was symbolized as $t(14 ; 21) .$ What does this designation represent, and how many chromosomes would you expect to be present in $t(14 ; 21)$ Down syndrome individuals?

Ioannis Markopoulos

Ioannis Markopoulos

Numerade Educator

Problem 23

A boy with Klinefelter syndrome $(47, \mathrm{XXY})$ is born to a mother who is phenotypically normal and a father who has the X-linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin.
(a) Which parent contributed the abnormal gamete?
(b) Using the appropriate genetic terminology, describe the meiotic mistake that occurred. Be sure to indicate in which division the mistake occurred.
(c) Using the appropriate genetic terminology, explain the son's skin phenotype.

Bryan Valdivia

Numerade Educator

Problem 24

In a human genetic study, a family with five phenotypically normal children was investigated. Two children were "homozy. gous" for a Robertsonian translocation between chromosomes 19 and 20 (they contained two identical copies of the fused chromosome). They have only 44 chromosomes but a complete genetic complement. Three of the children were "heterozygous" for the translocation and contained 45 chromosomes, with one translocated chromosome plus a normal copy of both chromosomes 19 and $20 .$ Two other pregnancies resulted in stillbirths. It was later discovered that the parents were first cousins. Based on this information, determine the chromosome compositions of the parents. What led to the stillbirths? Why was the discovery that the parents were first cousins a key piece of information in understanding the genetics of this family?

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 25

A 3 -year-old child exhibited some early indication of Turner syndrome, which results from a $45, \mathrm{X}$ chromosome composition. Karyotypic analysis demonstrated two cell types: $46, \mathrm{XX}$ (normal) and $45, \mathrm{X}$. Propose a mechanism for the origin of this mosaicism.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 26

A normal female is discovered with 45 chromosomes, one of which exhibits a Robertsonian translocation containing most of chromosomes 18 and $21 .$ Discuss the possible outcomes in her offspring when her husband contains a normal karyotype.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 27

In a cross in Drosophila, a female heterozygous for the autosomally linked genes $a, b, c, d,$ and $e(a b c d e /+++++)$ was testcrossed with a male homozygous for all recessive alleles (abcde/abcde). Even though the distance between each of the loci was at least 3 map units, only four phenotypes were recovered, yielding the following data:
Why are many expected crossover phenotypes missing? Can any of these loci be mapped from the data given here? If so, determine map distances.

Danielle Ashley

Danielle Ashley

Numerade Educator