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Chapter 5: Problem 5

What are the sexual phenotypes of the following genotypes in Drosopbila: XX, XY, XXY, XXX, XO?

Short Answer

Expert verified
XX: Female, XY: Male, XXY: Female, XXX: Female, XO: Male

Step by step solution

01

Understand Drosophila Sex Determination

Drosophila sex determination is based on the ratio of X chromosomes to sets of autosomes (X:A ratio). Typically, an X:A ratio of 1.0 results in a female, while a ratio of 0.5 results in a male.
02

Analyze Genotype XX

The genotype XX has two X chromosomes and two sets of autosomes. The X:A ratio is 1.0 (2 X chromosomes to 2 sets of autosomes), resulting in a female phenotype.
03

Analyze Genotype XY

The genotype XY has one X chromosome and one Y chromosome, with two sets of autosomes. The X:A ratio is 0.5 (1 X chromosome to 2 sets of autosomes), resulting in a male phenotype.
04

Analyze Genotype XXY

The genotype XXY has two X chromosomes, one Y chromosome, and two sets of autosomes. The X:A ratio is 1.0 (2 X chromosomes to 2 sets of autosomes), resulting in a female phenotype despite the presence of the Y chromosome.
05

Analyze Genotype XXX

The genotype XXX has three X chromosomes and two sets of autosomes. The X:A ratio is 1.5 (3 X chromosomes to 2 sets of autosomes), resulting in a female phenotype. Some abnormalities in development may occur.
06

Analyze Genotype XO

The genotype XO has one X chromosome and no Y chromosome, with two sets of autosomes. The X:A ratio is 0.5 (1 X chromosome to 2 sets of autosomes), resulting in a male phenotype, although they may be sterile.

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Key Concepts

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

X:A Ratio
The X:A ratio is a crucial concept in understanding sex determination in Drosophila. It refers to the ratio of X chromosomes to the sets of autosomes in an organism. This ratio helps determine whether the Drosophila will develop as male or female.
  • When the ratio is 1.0, which means there are equal numbers of X chromosomes and sets of autosomes, the organism develops as a female. For instance, the XX genotype has an X:A ratio of 1.0, leading to a female phenotype.
  • If the ratio is 0.5, the organism develops as male. This occurs when there is only one X chromosome for every two sets of autosomes, as seen in the XY genotype.
The X:A ratio is integral to predicting sexual phenotypes, as it translates genetic information into observable characteristics.
Genotype Analysis
Analyzing the genotypes is essential for determining the sex of Drosophila based on their chromosomal compositions. Let's consider several genotypes to see how the X:A ratio applies to them.
  • **XX Genotype**: It consists of two X chromosomes. With two sets of autosomes, the X:A ratio is 1.0, making it a female.
  • **XY Genotype**: This genotype has one X and one Y chromosome, which still results in an X:A ratio of 0.5, thus, a male phenotype.
  • **XXY Genotype**: Despite the presence of a Y chromosome along with two X chromosomes, the X:A ratio remains 1.0, indicating a female phenotype.
  • **XXX Genotype**: With three X chromosomes, the X:A ratio becomes 1.5. This typically results in a female phenotype, but could also lead to developmental abnormalities due to the abnormal ratio.
  • **XO Genotype**: Lacking a second sex chromosome but having two sets of autosomes results in an X:A ratio of 0.5, often leading to a sterile male phenotype.
Understanding these genotypes helps decipher the complex mechanisms that control sex determination in Drosophila.
Sexual Phenotypes
Sexual phenotypes in Drosophila are determined largely by the X:A ratio. The phenotype reflects the observable characteristics that result from the genotype. Here's a closer look at how different X:A ratios result in varying sexual phenotypes:
  • **Female Phenotype**: A ratio of 1.0 or higher usually results in a female phenotype. Drosophila with XX, XXY, and XXX genotypes, all having a ratio of at least 1.0, develop as females. However, the XXX genotype may exhibit some developmental issues.

  • **Male Phenotype**: A ratio of 0.5 produces a male phenotype. This is seen in both the XY and XO genotypes. It's notable that despite the missing Y chromosome, the XO genotype still manifests as male but may experience fertility issues.
The understanding of sexual phenotypes is fundamental in studies of genetics and developmental biology.
Chromosomal Abnormalities
Chromosomal abnormalities play a significant role in affecting the normal development and function of organisms. In Drosophila, these abnormalities can alter the expected phenotype and lead to various developmental concerns.:
  • **Abnormal Ratios**: Ratios that deviate from the typical 0.5 or 1.0 indicate possible complications. For example, the XXX genotype with an X:A ratio of 1.5 is an anomaly, which could lead to reproductive or developmental challenges.

  • **Influence on Phenotype**: Despite the presence of abnormal ratios, certain genotypes, such as XXY, manifest as females, although the presence of a Y chromosome generally suggests male traits. Such conditions might introduce minor phenotypic variations.
  • **XO Genotype**: Lacking a second sex chromosome, these Drosophila often have fertility issues, highlighting another way chromosomal abnormalities impact the organism.
Exploring these abnormalities offers insights into genetic mechanisms and how they might affect phenotype and viability.

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Most popular questions from this chapter

In Drosophila, a recessive mutation called chocolate (c) causes the eyes to be darkly pigmented. The mutant phenotype is indistinguishable from that of an autosomal recessive mutation called brown ( \(b w\) ). A cross of chocolateeyed females to homozygous brown males yielded wildtype \(\mathrm{F}_{1}\) females and darkly pigmented \(\mathrm{F}_{1}\) males. If the \(\mathrm{F}\) flies are intercrossed, what types of progeny are expected, and in what proportions? (Assume the double mutant combination has the same phenotype as either of the \(\sin\) gle mutants alone.)

A Drosopbila male carrying a recessive X-linked mutation for yellow body is mated to a homozygous wild-type female with gray body. The daughters of this mating all have uniformly gray bodies. Why aren't their bodies a mosaic of yellow and gray patches?

In 1908 F. M. Durham and D. C. E. Marryat reported the results of breeding experiments with canaries. Cinnamon canaries have pink eyes when they first hatch, whereas green canaries have black eyes. Durham and Marryat crossed cinnamon females with green males and observed that all the \(\mathrm{F}_{1}\) progeny had black eyes, just like those of the green strain. When the \(\mathrm{F}_{1}\) males were crossed to green females, all the male progeny had black eyes, whereas all the female progeny had either black or pink eyes, in about equal proportions. When the \(\mathrm{F}_{1}\) males were crossed to cinnamon females, four classes of progeny were obtained: females with black eyes, females with pink eyes, males with black eyes, and males with pink eyes- all in approximately equal proportions. Propose an explanation for these findings.

A Drosophila female heterozygous for the recessive X-linked mutation \(w\) (for white eyes) and its wild-type allele \(w^{+}\) is mated to a wild-type male with red eyes. Among the sons, half have white eyes and half have red eyes. Among the daughters, nearly all have red eyes; however, a few have white eyes. Explain the origin of these white-eyed daughters.

In humans, a recessive X-linked mutation, \(g\), causes greendefective color vision; the wild-type allele, \(G,\) causes normal color vision. A man (a) and a woman (b), both with normal vision, have three children, all married to people with normal vision: a color-defective son (c), who has a daughter with normal vision (f); a daughter with normal vision (d), who has one color-defective son (g) and two normal sons \((\mathrm{h}) ;\) and a daughter with normal vision (e), who has six normal sons (i). Give the most likely genotypes for the individuals (a to i) in this family.
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