A wild-type Drosophila female was mated to a wildtype male...

A wild-type Drosophila female was mated to a wildtype male that had been exposed to X-rays. One of the $\mathrm{F}_1$ females was then mated with a male that had the following recessive markers on the X chromosome: yellow body (y), crossveinless wings (cv), cut wings (ct), singed bristles ( sn ), and miniature wings ( $m$ ). These markers are known to map in the order $y-c v-c t-s n-m$. The progeny of this second mating were unusual in two respects. First, there were twice as many females as males. Second, while all of the males were wild type in phenotype, $1 / 2$ of the females were wild type, and the other $1 / 2$ exhibited the $c t$ and $s n$ phenotypes. a. What did the X -rays do to the irradiated male? b. Draw the X -chromosome pair present in a progeny female fly produced by the second mating that was phenotypically $c t$ and $s n$. c. If the $c t$ and $s n$ female fly whose chromosomes were drawn in part (b) was then crossed to a wildtype male, what phenotypic classes would you expect to find among the progeny males?

A wild-type Drosophila female was mated to a wildtype male that had been exposed to X-rays. One of the $\mathrm{F}_1$ females was then mated with a male that had the following recessive markers on the X chromosome: yellow body (y), crossveinless wings (cv), cut wings (ct), singed bristles ( sn ), and miniature wings ( $m$ ). These markers are known to map in the order $y-c v-c t-s n-m$. The progeny of this second mating were unusual in two respects. First, there were twice as many females as males. Second, while all of the males were wild type in phenotype, $1 / 2$ of the females were wild type, and the other $1 / 2$ exhibited the $c t$ and $s n$ phenotypes.
a. What did the X -rays do to the irradiated male?
b. Draw the X -chromosome pair present in a progeny female fly produced by the second mating that was phenotypically $c t$ and $s n$.
c. If the $c t$ and $s n$ female fly whose chromosomes were drawn in part (b) was then crossed to a wildtype male, what phenotypic classes would you expect to find among the progeny males?

Key Concepts

X-linked Inheritance

This concept describes the transmission of genes that are located on the X chromosome. In species like Drosophila, where sex is determined by the combination of X and Y chromosomes, traits on the X are inherited differently between males and females. Males typically have only one X chromosome, so any mutation or alteration on that chromosome often has a direct phenotypic effect, whereas females have two X chromosomes, allowing for potential masking of recessive mutations. This difference is key to interpreting unusual progeny ratios that arise from aberrant X chromosomes.

Chromosomal Aberrations Induced by Ionizing Radiation

Exposure to X?rays can cause breaks in DNA which may lead to structural changes in chromosomes, such as deletions, duplications, inversions, or translocations. Such chromosomal rearrangements can alter the normal gene order and integrity. When these aberrant chromosomes participate in subsequent matings, they can result in abnormal inheritance patterns, altered gene dosage, or even lethality in specific gamete or zygote types, which in turn affects the overall phenotypic ratios in the offspring.

Crossing Over and Recombination

During meiosis, homologous chromosomes exchange segments in a process called crossing over. This recombination is essential for genetic diversity but can have unusual outcomes when one of the chromosomes carries a structural rearrangement. In females, where recombination occurs, a rearranged X can produce recombinant chromosomes with segments that have been either duplicated or deleted. These recombinant chromosomes may have predictable phenotypes in progeny, especially when paired with a well?defined set of markers, and provide clues about the nature of the abnormality.

Gene Mapping and Phenotypic Analysis

Gene mapping involves determining the order and relative distances between genes on a chromosome by analyzing how frequently they recombine during meiosis. When multiple, closely linked markers are used, deviations from expected phenotypic ratios can indicate structural changes such as deletions or duplications. In genetic experiments, careful phenotypic analysis of progeny resulting from controlled crosses allows researchers to deduce the arrangement of genes and the nature of any chromosomal rearrangements that have occurred.

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