1. The answer is d. Genes are the functional units of DNA. The DNA molecule consists of chemical compounds called nucleotides, which are linked together in two long chains. These chains coil to form a double helix, which looks like a twisted ladder. The key is the sequence in which the nucleotide bases are arranged along and between the ladder (i.e., on the “sides” and the “rungs”). A code word refers to a specific arrangement of three bases, and multiple combinations of code words along the DNA strand form genes. Where and how in the longer strand of DNA they start and stop, and whether they overlap, depend on the careful placement of specific code words. Genes—sometimes alone, sometimes in combination—contain the coded information that determines particular traits in an individual person.

2. The answer is a. Skin cells are somatic cells—that is, the type of cells that make up the major organs and body systems. Genes are arranged in linear order on chromosomes. Somatic cells contain 46 chromosomes arranged in 23 pairs of homologous chromosomes. One member of each pair is inherited from each parent. Of the 23 pairs, 22 pairs are autosomes and 1 pair is the sex chromosomes. Autosomes control most body traits, whereas the sex chromosomes determine gender as well as other traits. Females contain two X chromosomes: XX. Males contain one X chromosome and one Y chromosome: XY.

3. The answer is b. Meiosis is the process of cell division by which one diploid somatic cell produces four haploid gamete cells. First, the 46 chromosomes in the diploid somatic cell replicate, so that each of the 46 chromosomes has a doubled structure. Then, rather than separate into two identical cells as in mitosis, the chromosomes literally intertwine. As chromosomes carrying similar genes touch, they exchange genetic material. This exchange is responsible for genetic variability, such as differences in eye color or height. When the original cell divides, it forms two cells, each containing 23 chromosomes that are doubled in structure, but in combinations that are different from those found in the original cell.

4. The answer is c. After the ovum is released by the ovaries, it is carried into the fallopian tubes. Fertilization usually takes place in the outer third of the fallopian tubes, called the ampulla. The mature ovum is fertile for only 24 hours after ovulation. Sperm can survive in the female reproductive tract for 72 hours, but are at peak performance during the first 24 hours. Of the 200 to 400 million sperm released during ejaculation, only approximately 200 will reach the ampulla in about 4 hours. Only one sperm fertilizes an ovum.

5. The answer is a. Embryo is the name given to the fertilized ovum during the first 8 weeks of prenatal development. The first 2 weeks are also called the germinal period, or the period of the ovum. From 3 to 8 weeks is the critical period of organogenesis, during which all of the fetus’s major organs develop.

6. The answer is a. During the 4th week, the heart begins to beat; limb buds are visible; eyes, ears, nerves, and the skeletomuscular and digestive systems begin to form; and the neural tube closes. The embryo is about 0.5 cm long and weighs 0.4 g. Women who are planning to become pregnant should be sure they have adequate amounts of folic acid in their diets to prevent spina bifida.

7. The answer is d. One of the myths concerning infertility is that it is essentially a female problem. Today, research clearly indicates that either the male or the female may be responsible in any given case of infertility.

8. The answer is c. These figures testify to the extent of this problem. Research has opened many new possibilities for infertile couples.

9. The answer is a. Concern about the possibility of disease has caused sperm banks to exercise extreme caution in their screening procedures.

10. The answer is c. Once detailed information about the reproductive process became available, the timing for the union of sperm and egg was seen as a key element in ensuring a successful procedure.