The effects of chromosome activity upon the inherited traits of organisms have been formulated into a number of principles, or laws. Some of these principles were observed long before anything was known about chromosomes and genes.

An Austrian monk, Gregor Mendel (1822-1884), made the first truly scientific study of heredity and published his findings in 1866. Mendel's method was to crossbreed various kinds of garden peas that had opposing pairs of obvious traits. The principles he established—segregation and dominance, and independent assortment—became the basis for the science of genetics. The four other principles discussed here were discovered by later geneticists.

In his first experiments, Mendel crossbred varieties that differed in only one pair of opposing traits. He crossed red-blossomed peas with white-blossomed peas; smooth-seeded with wrinkle-seeded peas; tall-stemmed with dwarf-stemmed peas. Altogether, nine alternative traits were investigated by a great number of crossings. (Three of the nine—flower color, seed-coat color, and seedling-axil color—were later considered aspects of the same trait.) The results in each case followed the same general mathematical pattern, which can be simplified by using symbols for the successive generations.

P1 is the parent generation; F1 is the first filial, or hybrid, generation; F2 is the second filial generation, produced by crossing the hybrid F1 plants. The results were as follows: F1 resembled only one of the P1; F2 resembled both members of the P1 in a proportion of approximately three to one.

For example, red-blossomed peas crossed with white-blossomed peas produced only red-blossomed plants. When the hybrid red-blossomed plants were crossed, the result was about three red-blossomed to one white-blossomed.

Mendel concluded that each plant carried a pair of factors (what are today called genes), one inherited from each parent, which controlled such traits as blossom color. (In the illustration, the factors are indicated by the letters R for red and r for white.) When the gametes were formed, the paired factors segregated, or separated, and each gamete received only one factor from each parent. The arrows in the illustration show how the various factors combine in different patterns. In F1, for example, the left R combines with the right r to produce Rr; the left R combines with the right R to produce RR. Similarly, the left r combines with the right R (Rr) and the right r (rr).

Mendel observed that a plant carrying both red and white factors developed red blossoms, not pink ones, as might have been expected. He concluded that one factor was dominant over the other factor, which he called recessive. For example, the red-blossom factor was dominant; the white-blossom factor, recessive. Therefore only those plants (about one-fourth of the F2) that had both factors for white would be white.

Later experiments, however, showed that one factor is not always completely dominant over the other. Some paired factors show no dominance. Therefore, although the factors do not blend, there may be a blending effect of traits. For example, when certain yellow-blossomed plants are crossed with white-blossomed ones, the offspring may have cream-colored blossoms.

In modern terms, the paired genes (factors) are called alleles (a$-lēlz') of each other. If they are identical, the organism is said to be pure, or homozygous, for the pair. If they are different (as are the genes for the red and white blossoms), the organism is said to be hybrid, or heterozygous, for the pair.

In other experiments Mendel crossed plants having two pairs of obvious opposing traits. The F2 generation consisted of four different kinds of individuals in the ratio 9:3:3:1. One-sixteenth of the F2 showed both recessive traits. Nine-sixteenths showed both dominant traits. Three-sixteenths showed one dominant trait but not the other. Another three-sixteenths showed the second dominant but not the first. This ratio is the expected one according to the mathematical laws of combinations and indicates that the two pairs of genes were on two different pairs of homologous chromosomes, and segregated, or assorted, independently.

By chance, all the plants with which Mendel experimented showed, in the F2 generation, traits resulting from independent assortment of genes. Later research by other geneticists indicated that independent assortment does not always take place. In many cases the two pairs of genes are on the same pair of homologous chromosomes and stay together during sexual recombination. Such genes are said to be linked.

Traits controlled by linked genes will appear together in the F2 generation and frequently in successive generations. However, they do not always appear together because genes originally linked are sometimes separated during meiosis by a process called crossing over. This separation occurs if parts of a homologous pair of chromosomes break during synapsis and exchange places.

In animals and certain plants, there is an exception to the rule that chromosomes of somatic cells appear as homologous pairs. The male and female of such species differ to some extent in chromosome structure and function. For example, the human male has only 22 paired homologous chromosomes; he has two unpaired chromosomes, called X and Y. Each somatic cell of the female has 23 pairs, including a pair of X chromosomes.

Half the human male gametes contain X chromosomes, and half contain Y chromosomes. At fertilization, if a male gamete containing an X chromosome unites with a female gamete, a female organism is produced. If a male gamete containing a Y chromosome unites with a female gamete, the new organism will be male. The male parent, therefore, determines the sex of the offspring.

Techniques have been developed that can separate sperm (male gametes) containing X chromosomes from sperm containing Y chromosomes. It is possible to predetermine the sex of a child by artificial insemination with sperm containing either X chromosomes or Y chromosomes.

Traits controlled by genes found on X and Y chromosomes are said to be sex linked. Such traits may be exhibited in the successive generations in different ways than traits controlled by other genes. For example, recessive traits appear more frequently in males than in females, if the traits are carried on X chromosomes. This higher frequency is due to the fact that the male organism receives only one X chromosome, from the female parent. Since there is apt to be no allele on the Y chromosome to counteract the recessive gene found on the X chromosome, the recessive trait will appear. A female will not exhibit the trait unless the recessive gene is on both her X chromosomes.

Such diseases and conditions as hemophilia (a disease of the blood), color blindness, and muscular dystrophy are recessive, sex-linked traits that are more common in men than in women.

Although genes are called units of heredity, this does not mean that one gene controls one trait. The genes in an organism interact. One gene may affect many traits, and one trait may be determined by many genes.