Essentials of Anatomy and Physiology

1. Where does fertilization usually take place? How
   many chromosomes are present in a human
   zygote? Explain what happens during cleavage, and
   describe the blastocyst stage. (p. 476)


2. Describe the process of implantation, and state
   where this takes place. (p. 476)


3. How long is the period of embryonic growth?
   How long is the period of fetal growth?
   (p. 480)


4. Name two body structures derived from ectoderm,
   mesoderm, and endoderm. (p. 477)
   5. Name the embryonic membrane with each of these
   functions: (pp. 477, 479)
   a. Forms the fetal portion of the placenta
   b. Contains fluid to cushion the embryo
   c. Forms the first blood cells for the embryo
   6. Explain the function of: placenta, umbilical arter-
   ies, and umbilical vein. (p. 480)
   7. Explain the functions of the placental hormones:
   hCG, progesterone, and estrogen and proges-
   terone (together). (pp. 480–481)
   8. Describe the three stages of labor, and name the
   important hormone. (pp. 483–485)


5. Labor: second stage—delivery of the infant; oxy-
   tocin causes more powerful contractions of the
   myometrium. If a vaginal delivery is not possible, a
   cesarean section may be performed.


6. Labor: third stage—delivery of the placenta; the
   uterus continues to contract to expel the placenta,
   then contracts further, decreases in size, and com-
   presses endometrial blood vessels.

The Infant at Birth (see Box 21–6)

1. Umbilical cord is clamped and severed; increased
   CO 2 stimulates breathing, and lungs are inflated.


2. Foramen ovale closes, and ductus arteriosus con-
   stricts; ductus venosus constricts; normal circula-
   tory pathways are established.


3. Jaundice may be present if the infant’s immature
   liver cannot rapidly excrete bilirubin.

Genetics—the study of inheritance; chromo-
somes—46 per human cell, in 23 homolo-
gous pairs

1. A homologous pair consists of a maternal and a
   paternal chromosome of the same type (1 or 2,
   etc.).


2. There are 22 pairs of autosomes and one pair of sex
   chromosomes (XX or XY).


3. DNA—the hereditary material of chromosomes.


4. Gene—the genetic code for one protein; an indi-
   vidual has two genes for each protein or trait, one
   maternal and one paternal.


5. Alleles—the possibilities for how a gene may be
   expressed.

Genotype—the alleles present in the genetic

makeup

1. Homozygous—having two similar alleles (genes).


2. Heterozygous—having two different alleles.

Phenotype—the appearance or expression of

the alleles present

1. Depends on the dominance or recessiveness of alle-
   les or the particular pattern of inheritance involved.

Inheritance—dominant–recessive

1. A dominant gene will appear in the phenotype of a
   heterozygous individual (who has only one domi-
   nant gene). A recessive gene will appear in the phe-
   notype only if the individual is homozygous, that is,
   has two recessive genes.


2. See Figs. 21–7 and 21–8 for Punnett squares.

Inheritance—multiple alleles

1. More than two possible alleles for each gene:
   human ABO blood type.


2. An individual will have only two of the alleles (same
   or different).


3. See Table 21–5 and Fig. 21–9.

Inheritance—sex-linked traits

1. Genes are recessive and found only on the X chro-
   mosome; there are no corresponding genes on the
   Y chromosome.


2. Women with one gene (and one gene for normal
   functioning) are called carriers of the trait.


3. Men cannot be carriers; they either have the trait
   or do not have it.


4. See Fig. 21–10.

494 Human Development and Genetics

REVIEW QUESTIONS