Book - Evolution and Genetics 9

From Embryology

Morgan (1925) Evolution and Genetics: 1 Different Kinds of Evolution | 2 Four Great Historical Speculations | 3 Evidence for Organic Evolution | 4 Materials of Evolution | 5 Mendel's Two Laws of Heredity | 6 Chromosomes and Mendel’s Two Laws | 7 Linkage Groups and the Chromosomes | 8 Sex-Linked Inheritance | 9 Crossing-over | 10 Natural Selection and Evolution | 11 Origin of Species by Natural Selection | 12 Non-Inheritance of Acquired Characters | 13 Human Inheritance | Figures

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 9 Crossing-over

If the linkage were never broken we should expect to find that groups of characters would be inherited together. There would be as many such groups of characters as there are pairs of chromosomes. To a certain extent this is true, but the study of the inheritance of two or more characters in the same linkage group has revealed a further fact of great interest, namely, that there takes place an interchange at times between the two members of the same linkage group, and, it may be added, only between members of the same linkage group and never between different linkage groups. This interchange gives rise to a new phenomenon in inheritance that is called crossing over, which may be illustrated by a few typical examples from Drosophila.


There are two mutant characters, black body color and vestigial wings, whose genes lie in the second chromosome. If a fly having these two characters is crossed to a wild type fly with normal color and long wings (fig. 49) the offspring are like wild type flies, because normal dominates black and long wing dominates vestigial wing.


If one of the daughters (i^i) from this cross is now mated to a male with black color and vestigial wings (both recessive characters) the offspring are of four kinds. Two of these are like the grandparents, one black-vestigial, the other normal-long, but the other two kinds have, as it were, interchanged these characteristics. One kind is black-long, the other normal-vestigial.

Morgan 1925 fig49.jpg

Fig. 49. Diagram to illustrate crossing-over. The two mutant characters, black and vestigial, are linked, as are their normal allelomorphs, gray and long. A black vestigial male is mated to a gray long female. The F1 female is back -crossed to the double recessive type, black vestigial. Four kinds of offspring are produced.


These four kinds do not appear in equal numbers, but 83 per cent are like the grandparents and 17 per cent are the cross-overs. The result may be stated in another way. The mutant characters that went in together, black-vestigial and normal-long, have remained together (linked) in 83 per cent of the grandchildren, while in 17 per cent of the grandchildren there has been an interchange or crossing-over.

The results may be represented in terms of chromosomes as follows. The gene for black is represented by b and its normal partner by B ; the gene for vestigial by V and its partner by V (fig. 49) . Black (b) and vestigial (v) are represented in the figure as contained in the same chromosome (here a rod) in one parent, and gray (B) and long wing (F) by the corresponding chromosome in the other parent, here by the all-black chromosome. The daughter contains one of each of these two chromosomes. When her eggs mature these two chromosomes separate and some of the eggs contain one, some the other chromosome. These two kinds of eggs as the sequel shows represent 83 per cent of all the eggs. But in 17 per cent of the cases there has occurred in some way an interchange between these two chromosomes with the result that the genes for black and long come to be in one chromosome and normal and vestigial in the other. The genes for black and normal may be said to have crossed over.

When such a female is mated to a black-vestigial male, whose spermatozoa contain the chromosome with black and vestigial, four combinations are formed. Remembering that normal dominates black, and long dominates vestigial, it will be seen (fig. 49) that four kinds of offspring are expected.

A second case of crossing over, here in the X chromosome, is illustrated in figure 50.

If a female with white eyes and yellow wings is crossed to a wild male with red eyes and gray wings, the sons are yellow and have white eyes and the daughters are gray and have red eyes. If two Fi flies are mated they will produce the following classes :

Morgan 1925 Chapter 9 cross diagram.jpg

Not only have the two grandparental combinations reappeared, but in addition two new combinations, viz., gray-white and yellow-red. The two original combinations far exceed in numbers the new or exchange combinations. If we follow the history of the A^-chromosomes we find that the large classes of grandchildren can be explained if A^-chromosomes are transmitted in their entirety from one generation to the next.


The smaller classes of grandchildren, the exchange combinations or cross-overs, can be explained by an interchange taking place between the chromosomes in the hybrid (2^^i) female. This is indicated in the diagram.

Morgan 1925 fig50.jpg

Fig. 50. Cross of a female vinegar fly that has white eyes and yellow wings to a wild type male with red eyes and gray wings, illustrating crossing-over.

The explanation of crossing-over rests on the assumption that members of the same pair of chromosomes may at times interchange. If the chromosomes are the bearers of the genes there can be no doubt from the genetic evidence that such an interchange takes place. When we turn to the known behavior of the chromosomes in the ripening of the germ cells we find certain stages where such a process may seem possible.

At the ripening period of the germ cell the members of each pair of chromosomes come together. In several forms they have been described as meeting at one end and then progressively coming to lie side by side as shown in figure 51. At the completion of the process they appear to have united along their length. It is always a maternal and a paternal chromosome that meet in this way and always two of the same kind. It has been observed that as the members of a pair come together they occasionally twist aromid each other (fig. 51). If where they overlap they should break and the ends uHite with the corresponding ends of the opposite chromosomes (fig. 52), the conditions of crossing-over would be fulfilled.

Morgan 1925 fig51.jpg

Fig. 51. Conjugation of the chromosomes in Batracoceps. (After Janssens.)

Morgan 1925 fig52.jpg

Fig. 52. Diagram to illustrate crossing-over of two conjugating threads.

Unfortunately the evidence that crossing-over takes place at the time of maturation, as the result of overlapping of the chromosomes, is very meagre and by no means conclusive, nevertheless, as far as it goes, this evidence is favorable for such an interpretation of genetic crossing-over as that given above.


From the genetic evidence for crossing-over it is possible to determine the relative location of the genes in the chromosomes. The method can not be given here in detail but the general point of view may be stated. If the genes lie along the length of the chromosomes and if crossing-over is as likely to occur at one level as at another, then, the nearer together two genes lie the less likely is a break between them, or conversely the further apart in the chromosome they lie the more likely is crossing-over to take place. In other words the percentage of crossovers is an index of the distance apart of the genes. On this basis the location of the genes, as shown in figure 38, has been determined. From such a chart one is enabled to calculate what the inheritance of any gene will be with respect to any other gene in its group provided its relation to two other genes is known.


The theory of crossing-over enables the geneticist to predict the results of a given experiment with the same precision that Mendel's two laws allow prediction for a single pair of characters in the same chromosome pair, or for two or more pairs of characters in different chromosome pairs.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Morgan (1925) Evolution and Genetics: 1 Different Kinds of Evolution | 2 Four Great Historical Speculations | 3 Evidence for Organic Evolution | 4 Materials of Evolution | 5 Mendel's Two Laws of Heredity | 6 Chromosomes and Mendel’s Two Laws | 7 Linkage Groups and the Chromosomes | 8 Sex-Linked Inheritance | 9 Crossing-over | 10 Natural Selection and Evolution | 11 Origin of Species by Natural Selection | 12 Non-Inheritance of Acquired Characters | 13 Human Inheritance | Figures