In Drosophila mdanogaster, ebony body ( e ) and rough eyes...

In Drosophila mdanogaster, ebony body ( $e$ ) and rough eyes $(r o)$ are encoded by autosomal recessive genes found on chromosome $3 ;$ they are separated by 20 m.u. The gene that encodes forked bristles $(f)$ is $X$ -linked recessive and assorts independently of $e$ and ro. Give the phenotypes of progeny and their expected proportions when a female of each of the following genotypes is test-crossed with a male. a. $\frac{e^{+}}{e} \quad \frac{r o^{+}}{r o} \quad \frac{f^{+}}{f}$ $\mathbf{b} \cdot \frac{e^{+}}{e} \frac{m}{r o^{+}} \frac{f^{+}}{f}$

In Drosophila mdanogaster, ebony body ( $e$ ) and rough eyes $(r o)$ are encoded by autosomal recessive genes found on chromosome $3 ;$ they are separated by 20 m.u. The gene that encodes forked bristles $(f)$ is $X$ -linked recessive and assorts independently of $e$ and ro. Give the phenotypes of progeny and their expected proportions when a female of each of the following genotypes is test-crossed with a male. a. $\frac{e^{+}}{e} \quad \frac{r o^{+}}{r o} \quad \frac{f^{+}}{f}$
$\mathbf{b} \cdot \frac{e^{+}}{e} \frac{m}{r o^{+}} \frac{f^{+}}{f}$

Key Concepts

Autosomal Recessive Inheritance

This concept refers to traits governed by genes located on autosomes (non-sex chromosomes) that require two copies of the recessive allele for the phenotype to be expressed. In problems dealing with autosomal recessive traits, individuals displaying the mutant phenotype are homozygous recessive, while heterozygous individuals are typically phenotypically normal carriers.

X-linked Inheritance

X-linked inheritance involves genes that reside on the X chromosome and display distinctive patterns of expression because of the difference in chromosome number between males and females. Since males have only one X chromosome, a single recessive allele on the X will express the mutant phenotype, while females require two copies of the recessive allele to exhibit the trait.

Genetic Linkage

Genetic linkage describes the tendency of genes that are located close together on the same chromosome to be inherited together rather than assorting independently. Linkage affects the expected ratios of offspring phenotypes because crossing-over between linked genes is less likely, leading to non-independent segregation.

Recombination Frequency and Map Units

Recombination frequency quantifies the likelihood of crossing-over between two genes during meiosis and is expressed in map units (centimorgans). A recombination frequency of 20 map units indicates that there is a 20% chance of a crossover event occurring between the two genes, which helps in constructing genetic maps and predicting progeny ratios.

Test Cross

A test cross is a genetic cross between an individual with a dominant phenotype (but an unknown genotype) and an individual that is homozygous recessive for the trait of interest. This method is used to determine the genotype of the dominant individual based on the phenotypic outcomes observed in the progeny.

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