In Drosophila , the gene for cinnabar eye color is on...

In Drosophila , the gene for cinnabar eye color is on chromosome $2,$ and the gene for scarlet eye color is on chromosome 3. A fly homozygous for both recessive cinnabar and scarlet alleles $(\mathrm{cn} / \mathrm{cn} ; \mathrm{st} / \mathrm{st})$ is white-eyed. a. If male flies (containing chromosomes with the normal gene order) heterozygous for $c n$ and $s t$ alleles are crossed to white-eyed females homozygous for the $c n$ and $s t$ alleles, what are the expected phenotypes and their frequencies in the progeny? b. One unusual male heterozygous for $c n$ and $s t$ alleles, when crossed to a white-eyed female, produced only wild-type and white-eyed progeny. Explain the likely chromosomal constitution of this male. c. When the wild-type $F_{1}$ females from the cross with the unusual male were backcrossed to normal $c n / c n$ $s t / s t$ males, the following results were obtained: Diagram a genetic event at metaphase I that could produce the rare cinnabar or scarlet flies among the progeny of the wild-type $F_{1}$ females.

In Drosophila , the gene for cinnabar eye color is on chromosome $2,$ and the gene for scarlet eye color is on chromosome
3. A fly homozygous for both recessive cinnabar and scarlet alleles $(\mathrm{cn} / \mathrm{cn} ; \mathrm{st} / \mathrm{st})$ is white-eyed.
a. If male flies (containing chromosomes with the normal gene order) heterozygous for $c n$ and $s t$ alleles are crossed to white-eyed females homozygous for the $c n$ and $s t$ alleles, what are the expected phenotypes and their frequencies in the progeny?
b. One unusual male heterozygous for $c n$ and $s t$ alleles, when crossed to a white-eyed female, produced only wild-type and white-eyed progeny. Explain the likely chromosomal constitution of this male.
c. When the wild-type $F_{1}$ females from the cross with the unusual male were backcrossed to normal $c n / c n$ $s t / s t$ males, the following results were obtained:
Diagram a genetic event at metaphase I that could produce the rare cinnabar or scarlet flies among the progeny of the wild-type $F_{1}$ females.

Key Concepts

Chromosomal Rearrangements

Chromosomal rearrangements, such as translocations or inversions, can alter the normal arrangement of genes along a chromosome. These rearrangements may lead to atypical patterns of inheritance by changing recombination frequencies, which can result in the production of only certain types of gametes in genetic crosses.

Recessive Alleles and Phenotypic Expression

Recessive alleles typically only manifest in an organism’s phenotype when present in a homozygous state. Understanding how recessive traits are expressed, particularly in homozygous mutants, is key to analyzing the outcomes of genetic crosses and inferring underlying genotypes.

Recombination and Crossing Over

Recombination through crossing over is a crucial process during meiosis wherein homologous chromosomes exchange portions of their genetic material. This exchange creates new allele combinations (recombinants) that are essential for genetic diversity and explains the occurrence of non-parental phenotypes in progeny.

Mendelian Inheritance

Mendelian inheritance is the fundamental concept that explains how alleles segregate and assort independently during gamete formation. This principle governs the distribution of dominant and recessive traits from parents to offspring and forms the basis for understanding phenotypic ratios in genetic crosses.

Gene Localization on Chromosomes

Gene localization refers to the identification of specific positions for genes on chromosomes. By knowing on which chromosome a gene resides, one can predict patterns of inheritance, especially when genes are unlinked and segregate independently, or when they are linked and co-segregate more frequently than by chance.

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