From Encyclopedia of Sex and Sexuality
The passing of traits from parents to offspring. When sexual reproduction occurs, both parents contribute to the cells of the new being a copy of a chemical code that controls its development and inherited traits—its heredity. Each new individual resembles its parents in some ways and differs in others.
The sperm produced in the testicles of the male and the ova, or eggs, stored in the ovaries of the female contain the chemical blueprint for each new life. This information is contained in the chromosomes, strands of genetic material composed of deoxyribonucleic acid (DNA). Hereditary characteristics such as skin color, hair color, body type, and height are determined by the precise combinations of DNA that we acquire from each parent and from subsequent environmental influences.
All human cells contain forty-six chromosomes, except for the sperm and egg cells (the sex cells), which contain twenty-three chromosomes each. When a sperm joins with an egg during fertilization, the chromosomes of the two sex cells combine to form a zygote with forty-six chromosomes—a complete set of instructions for the formation of a unique individual. Each gene, or piece of DNA strand, accounts for a single trait handed down in reproduction to the new individual.
All eggs produced by a female can potentially join with sperm to create either male or female offspring. The factor that determines a baby’s sex is contributed by the sperm cell. All eggs carry a sex chromosome, called the “X” chromosome. When the sperm cell that joins with an egg also contains an “X” chromosome, the new zygote gets two “X’s” (XX) and the new individual is a girl. If the chromosome contributed by the sperm cell is a “Y” chromosome, the result is an “XY” individual, or a male.
In some ways all human beings are alike. That is, all of us carry the genes for certain traits. Such traits, like the ability to speak, are called species traits. Individual traits make people different from one another (unless they happen to have an identical twin). Scientists have long debated which of our traits result from heredity and which result from environmental effects, or the outside forces that act upon each individual (see also heterosexuality).