Fundamentals of Anatomy and Physiology

474 Chapter 19

health Alert

Genetic Inheritance

Sperm and egg cells are produced by the

process of meiosis. Review Chapter 4 where

this is discussed. Humans have 23 pairs of

chromosomes, one member of each pair in-

herited from the father through the sperm cell,

and one member of each pair from the mother

through the egg cell. Meiosis re-duces the

number of chromosomes in half, so the sperm

cell has 23 chromosomes and the egg cell has

23 chromosomes. When fer-tilization of the

egg by the sperm occurs, the number of

chromosomes is restored to 46 (23 pairs).

Genes are located on these chromosomes.

After Watson and Crick published the

structure of the DNA molecule, the human

genome project was started to identify all the

genes on the 46 human chromosomes. It took

almost 40 years to complete this study. We

now know that there are about 30,000 genes

on those 46 chromosomes. We inherit our

genes from our parents at the moment

of conception, when the sperm from our father

fertilizes our mother’s egg. These genes

determine all of our characteristics, such as

eye and hair color, personality, and

immunities.

Of the 23 pairs of chromosomes, 1 pair are

the sex chromosomes that, among other things,

determine our sex. Females have two X

chromosomes (XX) and males have one X and

one Y (XY). Thus, a female’s eggs all carry an X

chromosome, while the male’s sperm will have

50% with an X and 50% with a Y. Therefore, it is

the father who determines the sex of the child. If

a Y-bearing sperm fertilizes an X egg, the child

will be a male. If an X-bearing sperm fertilizes an

X egg, the child will be a female. There are many

other genes, such as the reces-sive genes for

hemophilia and color blindness, that are found

on the X chromosome. Male children express the

gene and have the trait, while females are

carriers of these conditions and do not express

the trait.

Common Disease, Disorder, or Condition

Disorders of The Reproductive System

Sexually transmitted diseases are spread by intimate sexual contact with sexual part-

ners who harbor the infection. These diseases are caused by bacteria, viruses, and

protozoa.

Trichomonas

Trichomonas is a flagellate protozoan that is more commonly found in women, where it

erodes the tissues of the vagina (Figure 19-14). It can be transmitted to men, where it

infects the urethra. The acidity of the vaginal tract can control this organism. If, however,

it grows and reproduces, it results in inflammation of vaginal tissue with an odorous

yellow-green discharge.

Gonorrhea

Gonorrhea is caused by the bacterium Neisseria gonorrhoeae. The bacterium invades

the epithelial lining of the vagina and the male urethra, resulting in a discharge of pus.

Men experience burning sensations during urination and notice a periodic discharge of

pus from the urethral orifice. Women may not notice any symptoms in the early stages

of infection, but if untreated it can lead to pelvic inflammatory disease. Treat-ment is

very successful with antibiotics. Symptoms occur within a week after exposure.