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636 Chapter 19 NEL

Crossing Over

Mendel had explained most of his observations by hypothesizing that the two genes were

both on the same chromosome. By the Punnett square analysis shown in Figure 2, only two

different phenotypes are predicted for these linked genes. This was not what Morgan observed.

In a small number of flies from the dihybrid cross, the offspring had a different combination

of traits than the parents.Table 2shows the numbers of the different phenotypes and their

predicted genotypes. Where did the new allele combinations come from? Where did the new

combinations of the two traits come from?

AB

ab

AB ab

“a” allele is linked

with “b” allele

“A” allele is linked

with “B” allele

AABB AaBb

AaBb aabb

Figure 2
Punnett square analysis, assuming
that all the gametes carry the same
alleles as the parent. The expected
phenotypic ratio is three wild-type
body-colour, straight wings to one
black body-colour, curved wings.

Table 2 Observed Progeny (F 2 ) ofAaBb×AaBbF 1 Parents

Phenotype Number Possible genotype

wild-type body-colour, 290 AABBorAaBb

straight wings

black body-colour, 92 Aabb

curved wings

wild-type body-colour, 9 AAbborAabb

curved wings indicated recombinations

black body-colour, straight wings 9 AaBBoraaBb

indicated recombinations

Table 1 Linked Genes Identified by Morgan’s Research on Drosophila

Trait Dominant/Recessive Location

wingless(wg) recessive lethal chromosome 2

(all wingless offspring are born dead)

curly wings (Cy) dominant chromosome 2

purple eyes (pr) recessive nonlethal chromosome 2

stubble bristles (Sb) dominant chromosome 3

ebony body (e) recessive nonlethal chromosome 3

miniature wings (m) sex-linked recessive chromosome 4

cut wings (ct) sex-linked recessive chromosome 4

white eyes (w) sex-linked recessive chromosome 4

vermillion eyes (v) sex-linked recessive chromosome 4

Morgan was able to find a number of linked genes. Some of these are shown in

Table 1.