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this Class arises this class arises this class arises from a break, between frcm a break between from a break between a and c c and b a and c and c and b

this Class arises this class arises this class arises from a break, between frcm a break between from a break between a and c c and b a and c and c and b

FIC U R E 1 '7 The use of three-factor crosses to assign gene order. The least frequent pair of reciprocal recombinants must ansp from .a double crossover The percentages listed for the various classes are the theoretical values expected for an infinitely targe sample. When finite numbers of progeny are recorded, the exact values will be subject to random statistical fluctuations.

accurate mapping is often disturbed by interference phenomena, which can either increase or decrease the probability of correlated crossovers.

Using such reasoning, the Columbia University group headed by Morgan had by 1915 assigned locations to more than 85 mutant genes in Drosophila [Table 1-iJ, placing each of them al distinct spots on one of the four linkage groups, or chromosomes. Most importantly, all the genes on a given chromosome were located on a line. The gene arrangement was strictly linear and never branched. The genetic map of one of the chromosomes of Drosophila is shown in Figure 1-8. Distances between genes on such a map are measured in map units, which are related to the frequency of recombination between the genes, Thus, if the frequency of recombination between two genes is found to be 5%, the genes are said to be separated by five map units. Because of the high probability of double crossovers between widely spaced genes, such assignments of map units can be considered accurate only if recombination between closely spaced genes is followed.

Even when two genes are at the far ends of a very long chromosome, they assorl together at least 50% of the time because of multiple crossovers. The two genes will be separated if an odd number of crossovers occurs bntween them, but they will end up together if an even number occurs between them. Thus, in the beginning of the genetic analysis of Dmsophila, it was often impossible to determine whether two genes were on different chromosomes or at the opposite ends of one long chromosome, (inly after large numbers of genes had been mapped was it possible to demonstrate convincingly that the number of linkage groups equalled the numbeT of cytologic ally visible chromosomes. In 1915, Morgan, with his students Alfred H. Sturtevant, Hermann ]. Muller, and Calvin B. Bridges, published their definitive book The Mechanism of Mendeiian Heredity, which first announced the general validity of the chromosomal basis of heredity. We now rank this concept, along with the theories of evolution and the cell, as a major achievement in our quest to understand the nature of (he living world.

TABLE 1*1 Tfie 85 Mutant Genes Reported in Drosophila melanogaster in 191S*

Region Affected Name

Name

Group 1 Abnormal Bar Bifid Bow Cherry Chrome Cleft Club

Depressed Dotted

Eosin

Facet

Forked

Furrowed

Fused

Green

Jaunty

I cmon

Group 2 Antlered Apterous Arc

Balloon

Black

Blistered

Comma

Confluent

Cream II

Curved

Dachs

Extra vein

Fringed

Group 3 Band Beaded Cieam III Deformed Dwarf Ebony Giant Kidney l ow crossing over

Maroon

Peach

Croup 4 Bent

Abdomen Eye

Venation Wing Eye colot Body color Venal ion Wing Wing Thorax Eye coior Omnr.atidia Spine Eye

Venation Body color Wing

Body color

Wing

Wing

Wing

Venation

Body color

Wing

I horax mark Venation bye color Wing

Venation Wing

Pattern Wing Eye coior Eye

Size of body Body color Si2e of body Eye

Chromosome 3 Eye color Eye color

Wing

Lethal, 13 Miniature Notch

Reduplicated Ruby

Rudimentary

Sable

Shitted

Short

Skee

Spoon

Spot

Truncate Vermilion White Yellow

Jaunty Limited

I ittle crossover

Morula

Olive

Plexus

Purple

Speck

Strap

Streak

Trefoil

Truncate

Vesttgtai

Pink

Rough

Safrantn

Septa

Sooty

Spineless

Spread

Trident

Truncate

Whitehead

White ocelli

Eyeless

Region Affected

Body, death

Wing

Venation

Eye color

Wing

Body color

Venation

Wing

Wing

Wing

Body color Antenna Wing Eye Color Fye color Body color

Wing

Abdominal band

Chromosome 2

Ornrr.atidia

Body color

Venation

Eye ooior

Thorax mark

Wing

Pattern

Pattern

Wing

Wing

Eye color Eye

Eye coloi Eye color Body color Spine Wirig Pattern Wing Pattern Simple eye

"The mutations fell into lour linkage groups. Since four Chromosomes were cytotoijicalty observed Ihis indicated thai the gerBS are Situated on the chromosomes. Police that ffiulatior-.s in various genes can act to aller a single character such as body color in different ways.

Tile Origin of Genetic Variability through Mutations 15

red straight eyes wings

104 99.2 bw a w brown arc bent eyes wtngs straight long wir>gs wings

normal red eyes gray body ft

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