Chapter 21: Human Development and Genetics

Human Development—growth of a fertilized egg into a human individual

Fertilization—the union of the nuclei of egg and sperm; usually takes place in the fallopian tube

1. Sperm undergo final maturation (capacitation) within the female reproductive tract; the acrosome contains enzymes to digest the membrane of the ovum.

2. The 23 chromosomes of the sperm join with the 23 chromosomes of the egg to restore the diploid number of 46 in the zygote.

3. A zygote has 22 pairs of autosomes and one pair of sex chromosomes; XX in females, XY in males (see Fig. 21-1).

Implantation (see Fig. 21-2)—5 to 8 days after fertilization

1. Within the fallopian tube, the zygote begins mitotic divisions called cleavage to form two-cell, four-cell, eight-cell stages, and so on.

2. A morula is a solid sphere of cells that divides further to form a hollow sphere called a blastocyst.

3. A blastocyst consists of an outer layer of cells called the trophoblast and an inner cell mass that contains the potential embryo; the cells are stem cells, not yet specialized. The trophoblast secretes enzymes to form a crater in the endometrium into which the blastocyst sinks. Progesterone prevents inflammation at the site and is necessary for successful implantation.

Embryo—from zygote through 8 weeks of gestation (see Fig. 21-3)

1. In the embryonic disc, three primary germ layers develop: ectoderm, mesoderm, and endoderm (see Table 21-1).

2. By the eighth week of gestation (end of 2 months), all organ systems are formed (see Table 21-2).

Embryonic Membranes (see Fig. 21-3)

1. The yolk sac forms the first blood cells and the cells that become spermatogonia or oogonia.

2. The amnion surrounds the fetus and contains amniotic fluid; this fluid absorbs shock around the fetus.

3. The chorion develops chorionic villi that will contain blood vessels that form the fetal portion of the placenta.

Fetus—weeks 9 through 40 of gestation (see Table 21-2)

1. The organ systems grow and mature.

2. The growing fetus brings about structural and functional changes in the mother (see Table 21-3).

Placenta and Umbilical Cord

1. The placenta is formed by the chorion of the embryo and the endometrium of the uterus; the umbilical cord connects the fetus to the placenta.

2. Fetal blood does not mix with maternal blood; fetal capillaries are within maternal blood sinuses (see Fig. 21-5); this is the site of exchanges between maternal and fetal blood.

3. Two umbilical arteries carry blood from the fetus to the placenta; fetal CO2 and waste products diffuse into maternal blood; oxygen and nutrients enter fetal blood.

4. Umbilical vein returns blood from placenta to fetus.

5. The placenta is delivered after the baby and is called the afterbirth.

Placental Hormones

1. hCG—secreted by the chorion; maintains the corpus luteum so that it secretes estrogen and progesterone during the first few months of gestation. The corpus luteum is too small to secrete sufficient hormones to maintain a full-term pregnancy.

2. Estrogen and progesterone secretion begins within 4 to 6 weeks and continues until birth in amounts great enough to sustain pregnancy.

3. Estrogen and progesterone inhibit FSH and LH secretion during pregnancy and prepare the mammary glands for lactation.

4. Progesterone inhibits contractions of the myometrium until just before birth, when progesterone secretion begins to decrease.

5. Relaxin inhibits contractions of the myometrium and permits stretching of the pubic symphysis.

Parturition and Labor

1. Gestation period ranges from 37 to 42 weeks; the average is 40 weeks.

2. Labor: first stage—dilation of the cervix; uterine contractions force the amniotic sac into the cervix; amniotic sac ruptures and fluid escapes.

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

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

The Infant at Birth (see Box 21-6)

1. Umbilical cord is clamped and severed; increased CO2 stimulates breathing, and lungs are inflated.

2. Foramen ovale closes, and ductus arteriosus constricts; ductus venosus constricts; normal circulatory pathways are established.

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

Genetics—the study of inheritance; chromosomes— 46 per human cell, in 23 homologous pairs

1. A homologous pair consists of a maternal and a paternal chromosome of the same type (designated #1, #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 individual has two genes for each protein or trait, one maternal and one paternal.

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

6. Genotype—the alleles present in the genetic makeup

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

   2. Heterozygous—having two different alleles.

7. Phenotype—the appearance or expression of the alleles present

   1. Depends on the dominance or recessiveness of alleles or the pattern of inheritance involved.

Inheritance—dominant–recessive

1. A dominant gene will appear in the phenotype of a heterozygous individual (who has only one dominant gene). A recessive gene will appear in the phenotype 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). See Table 21-5 and Fig. 21-9.

Inheritance—sex-linked traits

1. Genes are recessive and found only on the X chromosome; 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. See Fig. 21-10.