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NEL Beyond Mendel 635

Gene Linkage and Crossover 19.219.2

It is often said that great science occurs when good questions are asked. Like Mendel,

Morgan asked great questions when he observed a few unexpected gene combinations when

he performed some dihybrid crosses with Drosophila. Morgan had found a number of

obvious mutations in Drosophila. He had noted a number of genes in Drosophilathat

had different alleles that were easy to observe, which he used in many genetic experi-

ments. When he carried out dihybrid crosses ofDrosophila, Morgan observed that in

some of the crosses, almost all the offspring had the same combination of traits as did the

parents. Morgan’s hypothesis to explain these observations, which he tested with further

experiments, gave further support to the theory that the genes are located on chromosomes.

Morgan first crossed Drosophilahomozygous for wild-type body-colour (AA) and

straight wings (BB) with Drosophilahomozygous for black body-colour (aa) and curved

wings (bb). The resulting F 1 generation was therefore heterozygous for both traits (AaBb).

When members of the F 1 generation mated among themselves, the F 2 generation showed

far less variability than expected. Since this was a dihybrid cross, Morgan had predicted

that the F 2 generation would have a 9:3:3:1 phenotypic ratio, as was observed in the

work of Mendel. Instead, nearly all the individuals with wild-type body-colour had

straight wings and nearly all those with black body-colour had curved wings.

Why did the observed ratios differ so much from the predicted ratio? From these

observations, Morgan concluded that the two genes must not have undergone inde-

pendent segregation. For this to be true, both genes would have to be located on the

same chromosome. In other words, the genes for body colour and wing shape must be

linked genes.

Figure 1illustrates what would happen to the alleles in this cross during meiosis, if

Morgan’s hypothesis was correct and the genes for body colour and wing shape were

linked genes.

When two gametes from this cross unite, the new individual is heterozygous for both

traits (AaBb). Remember that one parent carried the dominant alleles of the two linked

genes (Ais linked to B) and the other parent carried the recessive alleles (ais linked to

b). Morgan, therefore, predicted that the F 2 generation would have a 3:1 phenotypic

ratio (three flies with wild-type body-colour and straight wings to every one with black

body-colour and curved wings), as shown in Figure 2, on the next page.

parents

B B

A A

B

A

AaBb

two types of

gametes in

equal ratio

F 1 generation

a

b

a

b

a

b

Figure 1

During meiosis, homologous

chromosomes (represented as green

and red chromosomes) move to

opposite poles. One gamete carries

the ABalleles and the other carries

the aballeles.