In Drosophila, yellow body is due to an X-linked gene that...

In Drosophila, yellow body is due to an X-linked gene that is recessive to the gene for gray body. a. A homozygous gray female is crossed with a yellow male. The $F_{1}$ are intercrossed to produce $F_{2}$. Give the genotypes and phenotypes, along with the expected proportions, of the $\mathrm{F}_{1}$ and $\mathrm{F}_{2}$ progeny. b. A yellow female is crossed with a gray male. The $\mathrm{F}_{1}$ are intercrossed to produce the $\mathrm{F}_{2}$. Give the genotypes and phenotypes, along with the expected proportions, of the $\mathrm{F}_{1}$ and $\mathrm{F}_{2}$ progeny. c. A yellow female is crossed with a gray male. The $\mathrm{F}_{1}$ females are backcrossed with gray males. Give the genotypes and phenotypes, along with the expected proportions, of the $\mathrm{F}_{2}$ progeny. d. If the $F_{2}$ flies in part $b$ mate randomly, what are the expected phenotypic proportions of flies in the $\mathrm{F}_{3} ?$ (FIGURE CAN'T COPY)

In Drosophila, yellow body is due to an X-linked gene that is recessive to the gene for gray body.
a. A homozygous gray female is crossed with a yellow male. The $F_{1}$ are intercrossed to produce $F_{2}$. Give the genotypes and phenotypes, along with the expected proportions, of the $\mathrm{F}_{1}$ and $\mathrm{F}_{2}$ progeny.
b. A yellow female is crossed with a gray male. The $\mathrm{F}_{1}$ are intercrossed to produce the $\mathrm{F}_{2}$. Give the genotypes and phenotypes, along with the expected proportions, of the
$\mathrm{F}_{1}$ and $\mathrm{F}_{2}$ progeny.
c. A yellow female is crossed with a gray male. The $\mathrm{F}_{1}$ females are backcrossed with gray males. Give the genotypes and phenotypes, along with the expected proportions, of the $\mathrm{F}_{2}$ progeny.
d. If the $F_{2}$ flies in part $b$ mate randomly, what are the expected phenotypic proportions of flies in the $\mathrm{F}_{3} ?$
(FIGURE CAN'T COPY)

Key Concepts

Punnett Squares and Predicting Ratios

Punnett squares are a tool used to map out and calculate the possible genotypes and phenotypes resulting from a cross between two organisms. By listing all possible gametes from each parent, this method helps in deriving the expected proportions of offspring, which is essential for understanding and predicting inheritance patterns in complex crosses.

Genotype-Phenotype Relationships

The connection between an organism's genotype and its observable phenotype is fundamental in genetics. For X-linked traits, the combination of alleles—whether heterozygous or homozygous in females, or hemizygous in males—determines the expression of a trait. Understanding this relationship helps in predicting the outcomes of genetic crosses.

Genetic Crosses

Genetic crosses, such as intercrosses and backcrosses, are experimental methods used to study inheritance patterns. An intercross is a mating between individuals of the same generation (often heterozygotes), while a backcross involves mating an offspring with one of its parents or with an individual of a known genotype. These approaches allow geneticists to trace the inheritance of specific alleles and predict the frequency of genotypes and phenotypes in the progeny.

X-linked Inheritance

X-linked inheritance refers to the transmission of genes located on the X chromosome. Because males (XY) have only one X chromosome, a single recessive allele on it can result in the expression of the corresponding trait, whereas females (XX) may require both copies of the recessive allele to show the trait. This difference between sexes significantly affects the distribution of phenotypes in offspring.

Recessive Alleles

Recessive alleles are versions of a gene that do not produce a noticeable effect in the presence of a dominant allele. In the context of X-linked traits, the recessive allele will only manifest in a homozygous state in females or in a hemizygous state in males, where there is no allele on the corresponding Y chromosome to mask its expression.

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