Chapters 7/8 - Clinical Cytogenetics

Question 1

What are pseudoautosomal regions on the sex chromsomes?

• Homozygous regions of autosomal chromosomes
• Regions of the X and Y chromosomes that don’t recombine during meiosis
• Hemizygous regions of the X and Y chromosomes
• Homologous regions of the X and Y chromosomes

Answer 1

Homologous regions of the X and Y chromosomes.

(The pseudoautosomal regions refer to regions of homology between the X and Y chromosomes in humans that undergo recombination during meiosis.)

Question 2

In humans, the genetic basis for determining the sex “male” is accomplished by the presence of _____.

• A balance between the number of X chromosomes and the number of haploid sets of autosomes
• Multiple alleles scattered throughout the autosomes
• One X chromosome
• High levels of estrogen
• A portion of the Y chromosome

Answer 2

A portion of the Y chromosome.

Question 3

Klinefelter syndrome in humans, which leads to underdeveloped testes and sterility, is caused by which chromosomal condition?

• 47, XYY
• Triploidy
• 45, X
• 47, 21+
• 47, XXY

Answer 3

47, XXY

Question 4

A recessive gene for red-green color blindness is located on the X chromosome in humans. Assume that a woman with normal vision (her father is color blind) marries a color blind male. What is the likelihood that this couple’s first son will be color blind?

• 100%
• 75%
• 50%
• 0%
• 25%

Answer 4

50%

Question 5

Normally in humans, all the sons of a male showing a sex-linked phenotype will inherit the trait.

• True
• False

Answer 5

False

Question 6

Normally in humans, all the sons of a female homozygous for a sex-linked recessive gene will inherit the trait.

• True
• False

Answer 6

True

Question 7

The condition that exists when an organism gains or loses one or more chromosomes but not a complete haploid set is known as _____.

• Triploidy
• Polyploidy
• Aneuploidy
• Euploidy
• Trisomy

Answer 7

Aneuploidy

Question 8

What is the specific term that describes the loss of a single chromosome (2n-1)?

• Polyploidy
• Trisomy
• Aneuploidy
• Monosomy

Answer 8

Monosomy.

Monosomy involves the loss of a single chromosome (e.g., an X chromosome in Turner syndrome).

Question 9

Trisomy 21, or Down Syndrome, occurs when there is a normal diploid chromosomal complement but one (extra) chromosome 21. Although fertility is reduced in both sexes, females have higher fertility rates than males. Van Dyke et al. (1995; Down Syndrome Research and Practice 3(2):6569) summarize data involving children born of Down Syndrome individuals. Assume that children are born to a female with Down Sydrome and a normal 46-chromosme male. What proportion of the offspring would be expected to have Down Syndrome?
None of the offspring would be expected to have Down Syndrome
-One-half of the offspring would be expected to have Down Syndrome
-One-third of the offspring would be expected to have Down Syndrome
-All the children would be expected to have Down Syndrome
-Two-thirds of the offspring would be expected to have Down Syndrome

Answer 9

One-half of the offspring would be expected to have Down syndrome.

Question 10

An individual with Patau syndrome would be called a triploid.

• True
• False

Answer 10

False

Question 11

Chorionic villus sampling (CVS) is sometimes preferred to amniocentesis because results can be provided earlier in the pregnancy.

• True
• False

Answer 11

True

Question 12

Assume that an organism has a diploid chromosome number of 14. There would be 28 chromosomes in a tetraploid.

• True
• False

Answer 12

True

Question 13

The condition known as cri-du-chat syndrome in humans has a genetic constitution designated as _____.

• 46, 5p-
• Trisomy
• Heteroplasmy
• Triploidy
• 45, X

Answer 13

46, 5p-

Question 14

Gene duplications provide an explanation for the origin of gene families.

• True
• False

Answer 14

True

Question 15

A pericentric inversion includes the centromere.

• True
• False

Answer 15

True

Question 16

A paracentric inversion is one whose breakpoints do not flank the centromere.

• True
• False

Answer 16

True

Question 17

In general, inversion and translocation heterozygotes are as fertile as organisms whose chromosomes are in the standard arrangement.

• True
• False

Answer 17

False

Question 18

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

• Dispermic fertilization
• Mitotic nondisjunction of X chromosomes
• Reciprocal translocation between two X chromosomes
• Meiotic nondisjunction of X chromosome
• Inactivation of X chromosomes

Answer 18

Mitotic nondisjunction of X chromosomes.

Question 19

Which of the following accurately describes a possible meiotic nondisjunction event?

• Homologs fail to separate during meiosis I.
• Meiosis fails to proceed to completion.
• Sister chromatids fail to separate during meiosis I.
• Fusion of gametes results in trisomy.

Answer 19

Homologs fail to separate during meiosis I.

(Correct. Ordinarily, homologs separate during meiosis I. Failure of this separation is one of the ways in which nondisjunction can occur.)

Question 20

All of the following events occur during normal meiosis except _______All of the following events occur during normal meiosis except _______.

• homologous chromosomes separate from one another during meiosis I
• two haploid gametes fuse to form a diploid cell
• one diploid cell produces four haploid cells
• sister chromatids separate from one another during meiosis II

Answer 20

two haploid gametes fuse to form a diploid cell.

Fusion of haploid gametes occurs after meiosis.

Question 21

In order to create the possibility of generating a trisomy, nondisjunction must occur during meiosis II.

• True
• False

Answer 21

False.

Nondisjunction during either meiosis I or meiosis II creates gametes that will generate trisomies if fertilized.

Question 22

Which of the following arrangements would yield the greatest reduction in gamete viability?

• An inversion heterozygote for a large inversion
• An inversion homozygote for a small inversion
• An inversion homozygote for a large inversion
• An inversion heterozygote for a small inversion

Answer 22

An inversion heterozygote for a large inversion.

(A large heterozygous inversion will result in greater numbers of genes that are duplicated or deleted when crossing over occurs.)

Question 23

All of the following could result from meiosis in a pericentric inversion heterozygote in which a single crossover occurred within the inversion loop except a _______.

• chromosome with some duplicated regions
• completely normal chromosome
• chromosome with two centromeres
• chromosome with some deleted regions

Answer 23

-chromosome with two centromeres

When the centromere is contained within the inversion, it is neither duplicated nor deleted.

Question 24

Inversion loops do NOT form during meiosis in paracentric inversion heterozygotes.

• True
• False

Answer 24

False
(Loop formation allows pairing within inverted regions of homologous chromosomes regardless of whether the centromere is included in the inversion.)

Question 25

A Robertsonian translocation is considered non-reciprocal because _______.

• for every viable gamete formed, there are two inviable gametes formed
• an uneven number of gametes is produced in each meiosis
• trisomies of chromosome 21 are viable, whereas monosomies of the same chromosome are not
• the smaller of the two reciprocal products of translocated chromosomes is lost

Answer 25

The smaller of the two reciprocal products of translocated chromosomes is lost.

(After several cell divisions, only the larger of the two translocated chromosomes remains. Its reciprocal is lost.)

Question 26

Which of the following statements regarding familial Down syndrome is false?

• It accounts for approximately 5% of all Down syndrome cases.
• If one parent is a translocation heterozygote for chromosomes 14/21, a child born to this parent is as likely to have Down syndrome as he is to be normal.
• Heterozygous carriers of the Robertsonian translocation that cause familial Down syndrome can have children who are also carriers of the translocation.
• Heterozygous carriers of the Robertsonian translocation that causes familial Down syndrome can have children who are phenotypically normal and are not carriers of the translocation.

Answer 26

If one parent is a translocation heterozygote for chromosomes 14/21, a child born to this parent is as likely to have Down syndrome as he is to be normal.

(Three viable zygote types are possible when fertilization occurs between one normal and one heterozygous parent. One-third has a normal chromosome complement, one-third is a carrier, and one-third has Down syndrome.)

Question 27

With respect to the chromosomes involved in the translocation, 14 and 21, what is the total number of different gametes possible for a heterozygous carrier parent to produce?

• 6
• 2
• 4
• 3

Answer 27

6

(During meiosis, the Robertsonian chromosome may sort to one pole of the cell alone, with the normal copy of chromosome 14, or the normal copy of chromosome 21. The other chromosomes migrate to the opposite pole.)