Most traits of organisms occur in two or more variant forms: some traits are exhibited in more or less sharp alternatives, like the different eye colour in man, while others form graded series between extremes, like body height in man.

By studying the offspring from mating between such variant organisms, the patterns of trait inheritance can often be determined. For example, in the fruit fly Drosophila, one of the most widely used organisms in genetic research; the trait of body pigmentation is expressed in at least two alternative forms. In one the general coloration of the animal is gray and abdomen bears thin transverse bands of black melanin pigment.

A gray body represents the wild type, or predominant form of coloration in nature. By contrast, some flies are pigmented black uniformly all over the skin, a coloration pattern referred to as the ebony trait. If two gray-bodied wild-type flies are mated all offspring produced are also gray- bodied. Indeed, all later generations again develop only wild- type colorations. Similarly, a mating of two ebony flies yields ebony offspring in all later generations.

Gray and ebony body colours here are said to be true- breeding traits. In Mendel’s time it was generally supposed that if alternative forms of a trait are cross-bred, a blending of the trait would result. Thus if gray and black were mixed together, like paints, a dark-gray colour should be produced.

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And if blending really occurred dark-gray should be true breeding as well; for mixed traits, like mixed paints, should be incapable of “unbending”. In reality, however, the result of cross-breeding is strikingly different.

When a wild type and an ebony fly are mated (parental generation, P), all offspring (first filial generation, F.) are gray bodied, exactly like the wild-type parent. And when two such gray-bodied F] flies are then mated in turn some of the offspring are obtained are gray-bodied, others are ebony; colour mixtures do not occur.

Numerically, some 75 percent of the second generation (F2) is gray-bodied, like their parents and one of their grandparents; and the remaining 25 per cent are ebony, unlike their parents but like the other grandparent. Evidently, the colour traits of the offspring do not breed true; from gray-bodied flies in the F, can arise ebony flies in the F2.

Large numbers of tests in this kind have clearly established that, quite generally for any trait, blending inheritance does not occur and traits remain distinct and intact. If they become joined together in one generation, they can again become separated, or segregated, in a following generation. Mendel was the first to reach such a conclusion from studies on plants.

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Moreover, he not only negated the old idea of blending but postulated a new interpretation. He realized that traits back to the gametes for each trait. When that offspring in turn becomes adult and produces gametes, each gamete must similarly contribute one factor to the next generation.

Hence before gametes are mature, two factors must be reduced to one. Mendel therefore postulated the existence of a factor-reducing process. With this he in effect predicted meiosis. Near the end of the nineteenth century meiosis was actually discovered, and it was later recognized that chromosome reduction during meiosis corresponded precisely to Mendel’s postulated factor reduction.

Chromosomes therefore came to be regarded as the carriers of the factor, and the chromosome theory of heredity emerged. This theory has since received complete confirmation, and Mendel’s factors eventually became the genes of today.

Segregation

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On the basis of the chromosome theory of the fruit fly data above can be interpreted as follows. A true-breeding wild-type fly contains a pair of gray-colour-producing genes on some pair of chromosomes in each cell.

These genes can be symbolized by the letters EE. Thus the gene content, or genotype, is EE, and the visible appearance, or phenotype is gray. When such an animal produces gametes, meiosis occurs.

Mature gametes therefore contain only one of the two chromosomes, hence only one of the two genes. It is entirely a matter of chance which of the two adult chromosomes will become incorporated in a particular gamete.

Since both adult chromosomes here carry the same colour gene all gametes will be genetically alike in this respect. That is why EE animals are true-breeding, and why a mating of EE x EE will produce only gray-bodied offspring. In similar fashion, the genotype of a true-breeding ebony fly can be symbolized.

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A mating of two such flies will yield only black-bodied offspring. If now a wild type and an ebony fly are mated, all offspring will be gray-bodied. In such offspring the E and e genes are present together, yet the effect of the e gene evidently is overridden or masked completely. The single gene E by itself exerts the same effect as two E genes.

By contrast, the single gene e by itself is without visible effect; a double dose, eye is required if a visible result is to be produced. Gene that exert a maximum effect in a single does, like E, are said to produce dominant traits. Such genes mask more or less completely the effect of corresponding genes such as e, which are said to produce recessive traits.

Genes that affect the same trait in different ways and that occur at equivalent (homologous) locations in a chromosome pair are called allelic genes, or alleles. Genes such as E and e are alleles, and pairs such as EE, eye and Eye are different allelic pairs.

If both alleles of a pair are the same, as in EE, or eye, the combination is said to be homozygous combination is one such as Eel in which one allele produces a dominant and the other a recessive trait.

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Thus the F resulting from a mating of a wild type and an ebony fly as above is heterozygous, and this F reveals that the wild-type trait is dominant over the ebony trait. That the heterozygous F condition is not a true-breeding blend is now shown if two F flies are mated.

After meiosis, each fly will produce two types of gamete. Of the genes Eye, either the E gene or the e gene could by chance become incorporated in any one gamete. Approximately 50 per cent of the gametes will therefore carry the E gene, and the other 50 per cent, the e gene. In almost all organisms it is wholly a matter of chance which of the two genetically different sperm types fertilizes which of the two genetically different egg types. If much fertilization occurs simultaneously, as is usually the case, then all possibilities will be realized with appropriate frequency.

The result is that three-quarters of the offspring are gray-bodied and resemble their parents in this respect. One- quarter is ebony and these offspring resemble one of their grandparents. Evidently, the result can be explained fully on the basis of no blending, freely segregating genes and the operation of chance. Offspring in ratios of,3/4: ‘A (3:1) are usually characteristic for mating of heterozygous organisms as above. However, not all genes produce sharply dominant and sharply recessive forms of a trait. Many allelic genes give rise to traits that are neither dominant nor recessive.

In such cases each allele in a heterozygous combination such as Aa can exert a definite effect, and the result usually is a visible trait intermediate between those produced by AA and aa combinations. For example, snapdragon plants occur m true-breeding red flowered (AA) and true-breeding white-flowered (aa) forms.

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If two such plants are cross-bred, all F offspring (Aa) are pink-flowered: neither gene in Aa combination is fully dominant, and neither is fully recessive. But even here the intermediate result is again not produced by colour blending, for the pink- flowered condition is not true-breeding.

A mating of two pink-flowered plants segregates red and pink and white F offspring, in a characteristic phenotype ratio of V: “2 : Vi (or 1 : 2 : 1). The inheritance pattern of the genotypes here is precisely the same as where genes have sharply dominant and recessive effects, and only the phenotype ratios are different.

Evidently, when genes are inherited in a particular pattern, the expressions of visible traits can differ according to the particular effects that the genes have on one another and on cell metabolism generally. In modern terminology, Mendel’s first law, the law of segregation, can now be stated as follows: Genes do not blend but behave as independent units.

They pass intact from one generation to the next, where they may or may not produce visible traits depending on their dominance characteristics. And genes segregate at random, thereby producing predictable ratios of visible traits among the offspring.

Implied in this law are chromosome reduction by meiosis and the operation of chance in the transmission of genes.