Chapter 2

Question 1

How soon after fertilization occurs within the uterine tube does the blastocyst begin implantation?
(A) Within minutes
(B) By 12 hours
(C) By day 1
(D) By day 2
(E) By day 7

Answer 1

E. The blastocyst begins implantation by day 7 after fertilization.

Question 2

Which of the following events is involved in cleavage of the zygote during week 1 of development?
(A) A series of meiotic divisions forming blastomeres
(B) Production of highly differentiated blastomeres
(C) An increased cytoplasmic content of blastomeres
(D) An increase in size of blastomeres
(E) A decrease in size of blastomeres

Answer 2

E. Cleavage is a series of mitotic divisions whereby the large amount of zygote cytoplasm is successively partitioned among the newly formed blastomeres. Although the number of blas- tomeres increases during cleavage, the size of individual blastomeres decreases until they resem- ble adult cells in size.

Question 3

Individual blastomeres were isolated from a blastula at the 4-cell stage. Each blastomere was cultured in vitro to the blastocyst stage and individually implanted into four pseudopreg- nant foster mothers. Which of the following would you expect to observe 9 months later?
(A) Birth of one baby
(B) Birth of four genetically different babies
(C) Birth of four genetically identical babies
(D) Birth of four grotesquely deformed babies
(E) No births

Answer 3

C. This scenario would result in four genetically identical children. Blastomeres at the 4-cell to 8-cell stage are totipotent, that is, capable of forming an entire embryo. Since blastomeres arise by mitosis of the same cell (zygote), they are genetically identical. This phenomenon is impor- tant in explaining monozygotic (identical) twins. About 30% of monozygotic twins arise by early separation of blastomeres. The remaining 70% originate at the end of week 1 of development by a splitting of the inner cell mass.

Question 4

Embryonic carcinoma (EC) cells were iso- lated from a yellow-coated mouse with a tera- tocarcinoma. The EC cells were then micro- injected into the inner cell mass of a blastocyst isolated from a black-coated mouse. The blasto- cyst was subsequently implanted into the uterus of a white-coated foster mouse. Which of the following would be observed after full- term pregnancy?
(A) A yellow-coated offspring
(B) A black-coated offspring
(C) A white-coated offspring
(D) A yellow- and black-coated offspring
(E) A yellow- and white-coated offspring

Answer 4

D. This scenario would result in a yellow- and black-coated offspring. Because EC cells and inner cell mass cells have very similar biochemical characteristics, they readily mix with each other and development proceeds unencumbered. Because the mixture contains cells with yellow- coat genotype and black-coat genotype, offspring with coats of two colors (yellow and black) will be produced. The offspring are known as mosaic mice.