Before embarking on this journey let’s focus on the starting point. What structures are involved in reproduction? What physiological process is involved in conceiving a child? What happens at the genetic level when a sperm and an egg unite to form a human being?

The female and male reproductive systems are shown in Figure 2.1a and Figure 2.1b. Notice that the female system has several basic parts:

The pathway that results in fertilization—the union of sperm and egg—begins at ovulation. This is the moment, typically around day 14 of a woman’s cycle, when a mature ovum erupts from the ovary wall. Hormones—chemical substances that target certain tissues and body processes and cause them to change—orchestrate ovulation as well as the other events that program pregnancy.

At ovulation, a fallopian tube suctions the ovum in, and the tube begins vigorous contractions that propel the ovum on its three-day journey toward the uterus.

37

Now the male’s contribution to forming a new life arrives. In contrast to females, whose ova are all mainly formed at birth, the testes—male structures comparable to the ovaries—are continually manufacturing sperm. An adult male typically produces several hundred million sperm a day. During sexual intercourse these cells are expelled into the vagina, where a small proportion enter the uterus and wend their way up the fallopian tubes.

To promote pregnancy, it’s best to have intercourse around ovulation. The ovum is receptive for about 24 hours while in the tube’s outer part. Sperm take a few hours to journey from the cervix to the tube. However, sperm can live almost a week in the uterus, which means that intercourse several days prior to ovulation may also result in fertilization (Marieb, 2004).

Although the ovum emits chemical signals as to its location, the tiny tadpole-shaped travelers cannot easily make the perilous journey upward into the tubes. So, of the estimated several hundred million sperm expelled at ejaculation, only 200 to 300 reach their destination, find their target, and burrow in.

What happens now is a team assault. The sperm drill into the ovum, penetrating toward the center. Suddenly, one reaches the innermost part. Then the chemical composition of the ovum wall changes, shutting out the other sperm. The nuclei of the male and female cells move slowly together. When they meld into one cell, the landmark event called fertilization has occurred. What is happening genetically when the sperm and egg combine?

38

The answer lies in looking at chromosomes, ropy structures composed of ladder-like strands of the genetic material DNA. Arrayed along each chromosome are segments of DNA called genes, which serve as templates for creating the proteins responsible for carrying out the physical processes of life (see Figure 2.2). Every cell in our body contains 46 chromosomes—except the sperm and ova, which have half this number, or 23. When the nuclei of these two cells, called gametes, combine at fertilization, their chromosomes align in pairs to again comprise 46. So nature has a marvelous mechanism to ensure that each human life has an identical number of chromosomes and every human being gets half of its genetic heritage from the parent of each sex.

You can see the 46 paired male chromosomes in Figure 2.3. Notice that each chromosome pair (one from our mother and one from our father) is a match, except for the sex chromosomes. The X is longer and heavier than the Y. Because each ovum carries an X chromosome, our father’s contribution determines our sex. If a lighter, faster-swimming, Y-carrying sperm fertilizes the ovum, we get a boy (XY). If the victor is a more resilient, slower-moving X, we get a girl (XX).

In the race to fertilization, the Y’s are statistically more successful; scientists estimate that 20 percent more male than female babies are conceived. But the prenatal period is particularly hard on males. If a family member learns that she is pregnant, the odds still favor her having a boy; but because more males die in the uterus, only 5 percent more boys than girls make it to birth (Werth & Tsiaras, 2002). And throughout life, males continue to be the less hardy sex, dying off at higher rates at every age. Recall from Chapter 1 that, throughout the developed world, women outlive men by at least two years.

39

Question 2.1

ovaries, fallopian tubes, uterus; fertilization occurs in the fallopian tubes

Question 2.2

ovaries for female; testes for male

Question 2.3

Tell Tiffany that the best time to have intercourse is around the time of ovulation, as fertilization typically occurs when the ovum is in the upper part of the fallopian tube.

Question 2.4

she is more apt to survive and less apt to be conceived.