Question

What specific observations (evidence) support the conclusions about sex determination in Drosophila and humans?

Short answer

Answer: In Drosophila, sex determination is based on the ratio of X chromosomes to autosomes, with the crucial gene Sxl controlling sexual development. In humans, sex determination relies on the presence or absence of the Y chromosome, specifically the SRY gene, which initiates male sexual development. Evidence supporting these conclusions includes mutations in Sxl leading to intersex or sex-transformed individuals in Drosophila, gene dosage experiments affecting sex determination, and the direct manipulation of Sxl expression. In humans, the presence of XYY males, cases of XX males and XY females, and experiments involving the addition of the SRY gene to XX mice, all support the role of the SRY gene in sex determination.

Step-by-step solution

Basic introduction to sex determination

Sex determination is the process by which an organism's sex is established. It can be a genetic or environmental process. In most animals, including Drosophila and humans, sex determination is based on the presence or absence of specific sex chromosomes.

Sex determination in Drosophila

In Drosophila melanogaster, sex determination is based on the ratio of X chromosomes to autosomes. Males have one X chromosome and one Y chromosome (XY), while females have two X chromosomes (XX). A critical gene on the X chromosome, called "Sex-lethal" (Sxl), controls this process. When Sxl is active, the female sexual development occurs, and when it is inactive, the male sexual development occurs.

Evidence for Drosophila sex determination

Several observations and experiments support this conclusion about sex determination in Drosophila: 1. Mutations in the Sxl gene can lead to intersex or sex-transformed individuals. 2. Gene dosage experiments showed that increasing or decreasing the number of X chromosomes can alter the sex of the individual. 3. Manipulation of Sxl expression through transgenic techniques can lead to the transformation of one sex into another, providing direct evidence for its role in sex determination.

Sex determination in humans

In humans, sex determination is based on the presence or absence of the Y chromosome. Males have one X chromosome and one Y chromosome (XY), while females have two X chromosomes (XX). The SRY (Sex-determining Region Y) gene, located on the Y chromosome, is responsible for initiating male sexual development.

Evidence for human sex determination

There is numerous evidence supporting the conclusion about sex determination in humans: 1. Individuals with an extra Y chromosome (XYY) are male, supporting the idea that the Y chromosome contains the essential genes for male development. 2. Cases of XX males and XY females show that the presence or absence of the SRY gene can determine the individual's sex, regardless of the chromosome composition. 3. Experiments involving the addition of the SRY gene to XX mice resulted in male sexual development, providing direct evidence for the role of the SRY gene in sex determination. By analyzing the evidence and observations in both Drosophila and humans, it is clear that the mechanisms of sex determination vary significantly in these species. The presence or absence of key sex-determining genes, such as Sxl and SRY, plays a crucial role in determining the sexual development of these organisms.

Key concepts

Drosophila Genetics

Drosophila, commonly known as fruit flies, have been a cornerstone in the field of genetics, particularly in the study of sex determination. In these flies, the sex of an individual is determined by the ratio of X chromosomes to autosomes. If you have one X chromosome (as in XY), the organism is male, while two X chromosomes (XX) lead to a female. The Sxl (Sex-lethal) gene found on the X chromosome is pivotal. It acts as a genetic switch.

• When Sxl is "on," the organism develops as female.
• If Sxl is "off," male development occurs.

Mutations in the Sxl gene can cause unusual development patterns such as intersex, where an organism exhibits both male and female characteristics. Gene dosage experiments, where the number of X chromosomes is altered, reinforce the idea that the X chromosome ratio plays a critical role in determining sex. Additionally, techniques that change Sxl expression can convert one sex into another, highlighting the gene's central role in sex determination.
Given these discoveries, Drosophila has become a model organism to understand genetic regulatory mechanisms.

Human Genetics

In humans, sex determination is significantly linked to the presence or absence of the Y chromosome. Males are typically denoted as XY, having one X and one Y chromosome, whereas females are XX, possessing two X chromosomes. The critical genetic element here is the SRY gene found on the Y chromosome. It essentially acts as a trigger for male development. The absence of this gene naturally leads to female sexual development.

• Individuals with an additional Y chromosome, forming an XYY composition, remain male, underscoring the Y chromosome's importance.
• Notably, XX males exist due to the revelation that they acquired the SRY gene through translocation, meaning it moved to an X chromosome.

Experiments and observations, such as introducing the SRY gene into XX mice leading to their development as males, further corroborate the gene's pivotal role. These scientific findings have formed the foundation of our understanding of sex determination in humans.

Chromosome Theory

The chromosome theory of inheritance is a fundamental principle in genetics that connects the physical structure of chromosomes with Mendel's laws of inheritance. It essentially posits that genes are located on chromosomes, which are the vehicles of genetic information. This theory provides a framework for understanding how sex determination genes, like Sxl in Drosophila and SRY in humans, influence sexual development. Chromosomes carry different alleles for such genes, and their arrangement determines the resulting traits in an organism.

• The movement and segregation of chromosomes during cell division (meiosis) explain how traits are inherited.
• Chromosomal abnormalities often result in disorders, illustrating their crucial role in development.

The theory further clarifies why certain traits, including those involved in sex determination, are hereditary, as they are transmitted through chromosomes from generation to generation. By understanding the chromosome theory, one can appreciate the vast and intricate hereditary network operating within every cell, linking directly to how sex chromosomes determine traits such as gender.