In Drosophila, the recessive d p allele of the dampy gene...

In Drosophila, the recessive $d p$ allele of the dampy gene produces short, curved wings, while the recessive allele bw of the brown gene causes brown eyes. In a testcross using females heterozygous for both of these genes, the following results were obtained:$$ \begin{array}{ll} \text { uild-type uings, wild-type eyes } & 178 \\ \text { uild-type uing, brown eys } & 185 \\ \text { dumpy wings, uild-type eyes } & 172 \\ \text { dumpy ings, brown eyes } & 181 \end{array} $$ In a testcross using males heterozygous for both of these genes, a different set of results was obtained: wild-type wings, wild-type eyes 247 dumpy wings, brown eyes 242 a. What can you conclude from the first testeross? b. What can you conclude from the second testeross? c. How can you reconcile the data shown in parts a and $b$ ? Can you exploit the difference between these two sets of data to devise a general test for synteny in Drosophila? d. The genetic distance between dumpy and brown is $91.5 \mathrm{~m} . \mathrm{u}$. How could this value be measured?

In Drosophila, the recessive $d p$ allele of the dampy gene produces short, curved wings, while the recessive allele bw of the brown gene causes brown eyes. In a testcross using females heterozygous for both of these genes, the following results were obtained:$$
\begin{array}{ll}
\text { uild-type uings, wild-type eyes } & 178 \\
\text { uild-type uing, brown eys } & 185 \\
\text { dumpy wings, uild-type eyes } & 172 \\
\text { dumpy ings, brown eyes } & 181
\end{array}
$$
In a testcross using males heterozygous for both of these genes, a different set of results was obtained:
wild-type wings, wild-type eyes
247
dumpy wings, brown eyes
242
a. What can you conclude from the first testeross?
b. What can you conclude from the second testeross?
c. How can you reconcile the data shown in parts a and $b$ ? Can you exploit the difference between these two sets of data to devise a general test for synteny in Drosophila?
d. The genetic distance between dumpy and brown is $91.5 \mathrm{~m} . \mathrm{u}$. How could this value be measured?

Key Concepts

Testcross

A testcross involves mating an individual heterozygous for one or more traits with an individual homozygous recessive for those traits. This allows researchers to observe the segregation of alleles and to determine the genotype of the heterozygote based on the offspring's phenotype distribution.

Recombination

Recombination, or crossing over, occurs during meiosis and involves the exchange of genetic material between homologous chromosomes. This process produces new allele combinations in gametes and is a basis for mapping the distance between genes based on the frequency of recombinants observed.

Sex-Specific Recombination

In Drosophila, recombination is a sex-specific phenomenon occurring only during female meiosis, while male meiosis does not exhibit crossing over. This leads to differences in the patterns of offspring phenotypes when heterozygous females are used in crosses compared to heterozygous males.

Genetic Mapping and Map Units

Genetic mapping utilizes recombination frequencies to estimate the distance between genes along a chromosome. One map unit (or centiMorgan) corresponds to a 1% chance of recombination between two genes. The observed recombinant frequencies allow researchers to generate gene maps and assess linkage relationships.

Synteny

Synteny refers to the occurrence of two or more genes on the same chromosome. The observed differences in recombination patterns in males versus females can be exploited to determine whether genes are syntenic. Since recombination in Drosophila males does not occur, the presence or absence of recombinant phenotypes in distinct crossing schemes can serve as a general test for synteny.

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