Organisms do not inherit genes one at a time, but all of them are inherited together. What then will offspring be like with respect to two or more simultaneous traits inherited from particular parents? Mendel discovered a fundamental rule here.

Phrased in modern terms, this law of independent assortment states: The inheritance of a gene pair located on a given chromosome pair is unaffected by the simultaneous inheritance of other gene pairs located on other chromosome pairs.

In other words, two or more traits produced by genes located on two or more different chromosome pairs “assort independently”‘, each trait is expressed independently, as if no other traits were present.

The meaning of the law emerges from an examination of the simultaneous inheritance of for example, two traits of fruit flies, body colour and wing shape. As already noted, body colour can be either wild-type gray or recessive ebony. Wing shape can be either normal or vestigial. In the latter condition the wings are reduced in size to such an extent that the animal cannot fly.

ADVERTISEMENTS:

Such stunted wings can be shown to develop whenever a recessive gene vg is homozygous, vgvg. Normal wings represent the dominant wild type, produced by either VgVg or Vgvg gene combination. The body-colour and wing shape genes are located on different chromosome pairs of Drosophila, and the wing genes, like the colour genes, obey the law of segregation.

What will now be the result of a mating between two EeVgvg flies, individuals that are heterozygous for both traits simultaneously? After meiosis, each gamete will contain only one colour gene and only one wing gene. But which of each pair-the dominant or the recessive gene? This is a matter of chance. Thus a gamete might contain the genes E and Vg, or E and vg, or e and Vg, or e and vg. If many gametes are produced, all four combinations will occur with roughly equal frequency.

Fertilization, too, is governed by chance. Consequently any one of the 4 sperm types might fertilize any of the 4 egg types. Hence there are 16 different combinations that can occur in fertilization. If large numbers of fertilizations take place simultaneously, all 16 combinations will occur with roughly equal frequency. These 16 combinations can be determined from a grid in which the gametes of one parent are put along a horizontal edge and the gametes of the other parent along a vertical edge.

Among the 16 offspring types so formed, some individuals contain both dominant genes at least once, some contain none of the dominant genes. A count reveals gray- normal gray-vestigial, ebony-normal, and ebony-vestigial to be present in a ratio of 9: 3: 3: 1.

ADVERTISEMENTS:

This result proves the law of independent assortment. For if body colour is considered alone, there are 9 plus 3, or 12 animals out of every 16 that are gray, and 3 plus 1, or 4 that are ebony. But 12: 4 is a 3: 1 ratio. Similarly, if wing shape is considered alone, again 12 out of every 16 animals have normal wings and 4 have vestigial wings: here, too, the ratio is 3: 1.

Evidently, although the colour and wing traits are inherited simultaneously and yield a 9: 3: 3: 1 offspring ratio overall, each trait considered separately nevertheless gives a 3: 1 ratio of offspring. Each trait is therefore is inherited as if the other trait were not present; or in Mendel’s phrase, the traits assort independently.

Mendel’s second law applies specifically to gene pairs located on different chromosome pairs. The law will therefore hold for as many different gene pairs as there are chromosome pairs in each cell of an organism. Suppose we considered the inheritance of three different chromosome pair, in a mating of two triple heterozygotes AaBbCc x AaBbCc.

A double heterozygote AaBb produces four different gamete types. Applying the same principles, it can be verified readily that a triple heterozygote produces eight different gamete types: ABC, ABc, Abe, aBC, aBc, abC, and abc.

ADVERTISEMENTS:

To determine all possible genotypes of the offspring, we can use grid 8 squares by 8 squares and place the 8 gamete types of each parent along the edges, as above. The result will be 64 offspring types, of which 27 will express all the three traits in dominant form. The complete phenotype ratio can be easily verified as27:9:9:9:3:3:3: 1.

Carrying this progression further, a 23-fold hybrid can be shown to produce 223 or over 8 million genetically different gamete types. Hence in considering just 23 gene pairs on different chromosome pairs, a grid of 8 million by 8 million would be required to represent the over 64 trillion possible genotypes.

This number is far larger than the totality of human beings ever produced, and a good many millions or billions of these genotypes therefore have probably never yet arisen during the entire history of man.

Accordingly, chances are great that every newborn human being differs from every other one, past or present, in at least some genes controlling just 23 traits. And the genetic differences for all traits must be enormous indeed. Here is one major reason for the universal generalization that no two organisms produced by separate fertilizations are precisely identical.

ADVERTISEMENTS:

A chromosome contains not just one gene but anywhere from a few hundred to a few thousand. What is the inheritance patter of two or more gene pairs located on the same chromosome pair? This question leads beyond Mendel’s two laws.

Linkage

Genes located on the same chromosome are said to be linked; as the chromosome is inherited, so are all its genes inherited. Such genes clearly do not assort independently but are transmitted together in a block. The traits controlled by linked genes are similarly expressed in a block.

In fruit flies, for example, the same chromosome pair that carries the wing-shape genes also carries one of many known pairs of eye-colour genes: a dominant allele Pr produces red, wild-type eyes, and recessive distinctly purple eyes.

ADVERTISEMENTS:

If now a normal-winged, red-eyed heterozygous fly VgvgPrpr produces gametes, only two types should be expected, VgPr and vgpr, 50 per cent of each. In actuality, however, four gamete types are produced, in proportions. If these four types occurred in approximately equal numbers each about 25 per cent of the total, then the result could be regarded simply as a case without linkage, governed by Mendel’s second law.

But the actual results include significantly more than 25 per cent of each of the expected gamete types and significantly less than 25 per cent of each of the unexpected types. To explain odd results of this sort, T.H. Morgan, a renowned American biologist of the early twentieth century, proposed a new hypothesis. He postulated that, during meiosis, paired chromosomes in some cases might twist around each other and might break where they were twisted.

The broken pieces might then fuse again in the “wrong”‘ order. Such occurrences could account for the large percentage of expected and the small percentage of unexpected gamete types. The hypothesis was tested by microscopic examination of cells undergoing meiosis, and it could indeed be verified chat chromosomal crossing over actually takes place.

Crossing over is now believed to involve a breaking of DNA chains and subsequent synthesis of new connections between the broken ends of these chains. The implications of crossing have proved to be far-reaching. It has been reasoned that the frequency of crossovers should be an index of the distant between two genes would occur between these close points.

ADVERTISEMENTS:

But if two genes are relatively far apart, twists between them should be rather frequent. In general, the frequency of crossovers should be directly proportional to the distance between two genes. Inasmuch as the crossover percentage of two genes can be determined by breeding experiments, it should therefore be possible to construct gene maps showing the actual location of particular genes on a chromosome.

Indeed, since Morgan’s time the exact positions of a few hundred genes have been mapped in the fruit fly. Smaller numbers of genes have similarly been located in corn plants, in mice, and in various other organisms. A second implication of crossing over is that genes on a chromosome must be lined up single file. Only if this is the case can linkage and crossing over occur as it actually does occur.

This generalization has become known as the law of the linear order of genes. It represents the third major rule that governs Mendelian inheritance. Third, crossing over makes meiosis a source of genetic variations.

For example, when a diploid cell in a testis undergoes meiosis and produces four haploid sperms, these four do not contain merely the same whole chromosomes as the original cell. Rather, because of crossing over, the chromosomes in each sperm will be quilt works composed of various joined pieces of the original chromosomes; and the four sperms are almost certain to be genetically different not only from one another but also from the original diploid cell.

Moreover, any two genetically identical diploid cells are almost certain to give rise to two genetically different quarters of sperms. Genetic variations thus are produced by both phases of sex: through fertilization, which doubles chromosome numbers, and through meiosis, which reduces numbers and also shuffles the genes of the chromosomes.

The three rules of heredity here outlined describe and predict the consequences of sexual recombination-the various results possible when different sets of genes become joined through fertilization and are pooled in tin zygote. In other words, sexual recombination of genes leads to

Mendelian inheritance. However, a great many hereditary events have been found that do not obey the three basic rules.