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dogs mate, corn plant, inherited characteristics, autosomes, nucleosomes

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Heredity, process of transmitting biological traits from parent to offspring through genes, the basic units of heredity. Heredity also refers to the inherited characteristics of an individual, including traits such as height, eye color, and blood type.

Heredity accounts for why offspring look like their parents: when two dogs mate, for example, they have puppies, not kittens. If the parents are both Chihuahuas, the puppies will also be Chihuahuas, not great Danes or Labrador retrievers. The puppies may be a little taller or shorter, a little lighter or a lot heavier than their parents are. Their faces may look a little different, or they may have different talents and temperaments. In all the important characteristics, however—the number of limbs, arrangement of organs, general size, fur type—they will share the traits of their parents. The principles of heredity hold true not only for a puppy but also for a virus, a roundworm, a pansy, or a human.

Genetics is the study of how heredity works and, in particular, of genes. A gene is a section of a long deoxyribonucleic acid (DNA) molecule, and it carries information for the construction of a protein or part of a protein. Through the diversity of proteins they code for, genes influence or determine such traits as eye color, the ability of a bacterium to eat a certain sugar, or the number of peas in a pod. A virus has as few as a dozen genes. A simple roundworm has 5000 to 8000 genes, while a corn plant has 60,000. The construction of a human requires an estimated 50,000 genes.

If the DNA in a single human cell could be unraveled, it would form a single thread about five feet long and about 50 trillionths of an inch thick. To prevent this fine string of DNA from becoming knotted like a big tangle of yarn, parts of the strand are wrapped around proteins like a thread is wound around spools. These units of wrapped DNA are called nucleosomes, and they coil and fold into structures called chromosomes. Humans have 23 pairs of chromosomes. In each pair, one chromosome comes from the mother and the other from the father. Twenty-two of the pairs are the same in both men and women, and these are called autosomes. The twenty-third pair consists of the sex chromosomes, so called because they are the primary factor in determining the gender of a child. The sex chromosomes are known as the X and Y chromosomes.

Females have two X chromosomes, and males have one X and one Y chromosome. The Y chromosome is about one-third the size of the X chromosome. A sperm, the reproductive cell produced by the male, can carry either one X or one Y chromosome. An egg, the reproductive cell produced by the female, can carry only the X chromosome. When a sperm with an X chromosome unites with an egg, the result is a child with two X chromosomes—a female. When a sperm with a Y chromosome unites with an egg, however, the result is a child with one X and one Y chromosome—a male. Thus, the father determines the gender of the child.


Maugh II, Thomas H., B.S., Ph.D.

Medical Writer, Los Angeles Times.

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