E. It can be observed that A is linked to B and C and that C and D are linked to E, but that no linkage is observed between A and E in breeding experiments (Fig. 1.22). Given that A is on the same DNA molecule as B and that B is on the same DNA molecule as C etc., it can be deduced that A, B, C, D and E must all be on the same chromosome. In genetic terminology, it is said that A, B, C, D and E are all in the same linkage group. Even though the most distant members of a linkage group may not directly show linkage to each other, their relationship can be deduced from their mutual linkage to intervening genes.
It is often important to know the precise location of a gene. For example, this is true of the genes responsible for hereditary defects. In the past, geneticists measured the recombination frequencies of genes in order to estimate how far apart they were on the chromosome. Nowadays, physical methods are used to measure the distances between genes in terms of the length of the DNA molecule upon which they are carried.
Using simpler organisms has allowed more detailed analysis of gene structure and function.
Bacteria are smaller and multiply faster than flies. Bacterial cultures contain many millions of individuals for analysis. Indeed a typical culture of a vigorous bacterium such as E. coli may contain as many as 5 x 109 cells per ml—roughly the same number as linkage group A group of alleles carried on the same DNA molecule (that is, on the same chromosome)
FIGURE 1.22 Groups
If genes A, B, C, D and E are all on the same chromosome, they will show linkage. The extent of linkage depends primarily on their distance from each other on the chromosome. For example, the alleles of two genes close to each other may be inherited together 90% of the time, whereas the alleles of more distant genes will stay together less often. These percentages are somewhat deceptive since alleles on different chromosomes will accompany each other 50% of the time due to random segregation. Thus 50% is the lowest possible numerical value for "linkage" and does not in fact imply either the presence or absence of linkage.
Fruit flies of the species Drosophila melanogaster are raised in milk bottles for genetic research. The milk bottles are sterilized in an autoclave, then partly filled with nutritious growth medium plus a sheet of filter paper. The fly larvae eat the pale brown medium and pupate on the paper. When the flies need to be examined, they are knocked unconscious with ether, from which they recover in about 10 minutes. Courtesy of: Dr Jeremy Burgess, Science Photo Library.
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