91Ó°ÊÓ

Miniature wings in Drosophila result from an X-linked allele \(\left(\mathrm{X}^{m}\right)\) that is recessive to the allele for long wings \(\left(\mathrm{X}^{+}\right) .\) Sepia eyes are produced by an autosomal allele (s) that is recessive to an allele for red eyes \(\left(s^{+}\right)\). a. A female fly that has miniature wings and sepia eyes is crossed with a male that has normal wings and is homozygous for red eyes. The \(\mathrm{F}_{1}\) flies are intercrossed to produce the \(\mathrm{F}_{2}\). Give the phenotypes, as well as their expected proportions, of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) flies. b. A female fly that is homozygous for normal wings and has sepia eyes is crossed with a male that has miniature wings and is homozygous for red eyes. The \(\mathrm{F}_{1}\) flies are intercrossed to produce the \(\mathrm{F}_{2}\). Give the phenotypes, as well as their expected proportions, of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) flies.

Step by step solution

01

Understanding the Genetic Cross in Part (a) - F1 Generation

We begin by examining the initial parental cross. The female fly is described as X-linked recessive for miniature wings (X^m) and has sepia eyes (ss). The male fly possesses normal wings (X^+) and is homozygous for red eyes (s^+s^+). In the F1 generation, the female offspring will inherit X^+ from the father and X^m from the mother, while the male offspring will inherit X^m from the mother. All F1 offspring will have normal wings, as X^+ is dominant over X^m. For the eyes, since the male is s^+s^+, all F1 offspring will be s^+s, showing red eyes. Thus, all F1 flies will be phenotypically normal-winged and red-eyed.

02

Predicting the F2 Generation Phenotypes in Part (a)

In the F2 generation, the F1 offspring (normal-winged and red-eyed) are crossed. The genotypic possibilities for wings are X^+X^+, X^+X^m, X^mX^m for females and X^+Y, X^mY for males. For eye color, the possibilities are s^+s^+, s^+s, and ss. Using Punnett squares, we calculate the phenotypic ratios: normal wings with red eyes, miniature wings with red eyes, normal wings with sepia eyes, and miniature wings with sepia eyes. Since X^+ is dominant, females need at least one X^mX^m for miniature wings, and males require X^mY. For eye colors, ss gives sepia eyes. Resulting in the following proportions: 3/8 normal-winged red-eyed, 1/8 normal-winged sepia-eyed, 3/8 miniature-winged red-eyed, 1/8 miniature-winged sepia-eyed.

03

Understanding the Genetic Cross in Part (b) - F1 Generation

In part B, the female fly is X^+X^+ with sepia eyes (ss) and is crossed with a male having miniature wings (X^mY) and red eyes (s^+s^+). In the F1 generation, females inherit one X^+ from both parents, resulting in normal wings and s^+s from the male, showing red eyes. Males inherit X^+ from the mother and X^m from the father, giving normal wings and s^+ from the mother, also yielding red eyes. Therefore, all F1 offspring are phenotypically normal-winged and red-eyed.

04

Predicting the F2 Generation Phenotypes in Part (b)

For the F2 generation, the intercross of F1 flies (phenotypically normal-winged, red-eyed) produces combinations similar to those in part (a). Using Punnett squares for both traits: wing type (X^+X^+, X^+X^m, X^mX^m; and X^+Y, X^mY for males) and eye type (s^+s^+, s^+s, ss). The resulting proportions are the same as in part (a): 3/8 normal-winged red-eyed, 1/8 normal-winged sepia-eyed, 3/8 miniature-winged red-eyed, 1/8 miniature-winged sepia-eyed.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Drosophila genetics

Drosophila melanogaster, commonly known as the fruit fly, is a vital model organism in genetics due to its rapid life cycle and easily identifiable traits. The study of Drosophila genetics provides insights into how genes and chromosomes determine phenotypes. Fruit flies have four pairs of chromosomes, one of which is the sex chromosomes (X and Y), while the others are autosomal chromosomes. Each trait in Drosophila is represented by specific alleles located on these chromosomes. For example, wing shape and eye color are traits that can be studied easily due to distinct and observable differences. The use of Drosophila in experiments helps researchers understand genetic inheritance patterns, which are applicable to many organisms, including humans. By crossing Drosophila with different phenotypic traits, scientists can predict offspring traits and investigate the behavior of alleles in genetic combinations.

X-linked traits

X-linked traits are those whose associated genes reside on the X chromosome. The inheritance pattern for X-linked traits can differ significantly from autosomal traits because females have two X chromosomes (XX), while males have one X and one Y (XY). This difference means that males express X-linked recessive traits more easily, as they only need one copy of the recessive allele from their mother. In the given exercise, the wing shape trait is X-linked, where normal wings are dominant (X^{+}) and miniature wings are recessive (X^m). When a male Drosophila inherits the X^m allele from a carrier mother, he will show the miniature wing phenotype due to the absence of a second X chromosome that might carry a dominant allele. For females, both X chromosomes must have the X^m allele to exhibit the miniature wing phenotype. The study of X-linked traits in fruit flies provides an easy-to-understand example of sex-linked inheritance.

Autosomal recessive inheritance

Autosomal recessive inheritance involves traits associated with genes located on autosomes, which are chromosomes that do not determine sex. Traits controlled by these genes follow a Mendelian inheritance pattern. Recessive traits require two copies of the recessive allele (one from each parent) for their expression. In this exercise, sepia eyes result from autosomal recessive alleles (ss), while the dominant allele (s^{+}) results in red eyes. Both male and female fruit flies need to inherit the allele for sepia eyes from both parents to exhibit this phenotype. Autosomal recessive traits often skip generations, which can make their analysis interesting and sometimes complex. Moreover, by examining the offspring when crossing flies with dominant and recessive sepia eye traits, geneticists can predict the likelihood of future generations expressing these traits, enhancing the understanding of autosomal inheritance patterns.

Punnett square analysis

Punnett squares are a valuable tool in genetics used to predict the probabilities of offspring inheriting particular genotypes and phenotypes based on parental genetics. It is a grid system that represents all potential allele combinations from the parental gametes. In the exercise, Punnett square analysis is used to deduce proportions of phenotypes concerning wing shape (an X-linked trait) and eye color (an autosomal trait). By cross-referencing alleles of the parents, the Punnett square visually organizes possible offspring combinations:

• X-linked trait analysis: The Punnett square helps determine ratios of normal (X^+) versus miniature wings (X^m) in the offspring.
• Autosomal trait analysis: Similarly, it predicts eye color based on the s^+ and s alleles.

For genetic crosses, Punnett squares simplify visualizing how genes segregate and help see the phenotypic ratios expected in the progeny. This visualization aids students in comprehending Mendelian inheritance and the randomness associated with allele segregation.