Question

In this chapter, we have focused on sex dif. ferentiation, sex chromosomes, and genetic mechanisms involved in sex determination. At the same time, we found many opportu- nities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, you should answer the following fundamental questions? (a) How do we know whether or not a heteromorphic chromosome such as the \(Y\) chromosome plays a crucial role in the deter- mination of \(\operatorname{sex} ?\) (b) How do we know that in humans the X chromosomes play no role in human sex determination, while the \(Y\) chromosome causes maleness and its absence causes femaleness? (c) How do we know that Drosophila utilizes a different sex. determination mechanism than mammals, even though it has the same sex-chromosome compositions in males and females? (d) How do we know that \(X\) chromosomal inactivation of either the paternal or maternal homolog is a random event during early development in mammalian females?

Short answer

Question: Explain the role of the Y chromosome in sex determination and how it compares to the mechanism in Drosophila. Answer: The Y chromosome plays a crucial role in sex determination. In humans, the presence of the Y chromosome, specifically the SRY gene, causes maleness while its absence causes femaleness. In Drosophila, on the other hand, sex is determined by the ratio between the number of X chromosomes and the number of sets of autosomes. Thus, even though both have XY males and XX females, their sex determination mechanisms are different.

Step-by-step solution

(a) Understanding the role of the Y chromosome in sex determination.

To determine whether a heteromorphic chromosome like the Y chromosome plays a crucial role in sex determination, scientists analyze chromosomal inheritance patterns in different organisms and observe the outcomes of these patterns. Usually, in species where the Y chromosome determines sex, males possess both X and Y chromosomes (XY), while females have two X chromosomes (XX). When a sex-linked trait is associated with the presence of the Y chromosome, this indicates that the Y chromosome plays a role in sex determination.

(b) Determining the roles of X and Y chromosomes in human sex determination.

In humans, researchers found that the presence of the Y chromosome causes maleness, while its absence causes femaleness. Through comparison of individuals with different chromosomal aberrations or mutations, it was discovered that male individuals bearing the SRY gene (Sex-determining Region Y) on their Y chromosome developed male characteristics, regardless of the number of X chromosomes they possessed. Conversely, the absence of the SRY gene led to the development of female characteristics. This evidence suggests that the Y chromosome (specifically the SRY gene) plays a crucial role in human sex determination, while X chromosomes do not contribute to this process.

(c) Comparing the sex determination mechanisms of Drosophila and mammals.

Drosophila and mammals have the same sex-chromosome compositions (XY in males and XX in females), but their sex determination mechanisms differ. In Drosophila, sex is determined by the ratio between the number of X chromosomes and the number of sets of autosomes (non-sex chromosomes). If the ratio is 1 (equal number of X chromosomes and sets of autosomes), the individual develops as a female; if the ratio is 0.5 (half as many X chromosomes as sets of autosomes), the individual develops as a male. This mechanism contrasts with the SRY gene system found in mammals, which is based on the presence or absence of the Y chromosome.

(d) Understanding X chromosomal inactivation during early development in mammalian females.

Mammalian females inherit one X chromosome from each parent. To prevent the overexpression of X-linked genes, one of the two X chromosomes in each cell is inactivated during early development. Studies of female mammals with X chromosomes carrying genetic markers from both parents have demonstrated that the inactivated X chromosome can come from either parent and that this inactivation occurs randomly in different cells. This evidence supports the idea that X chromosomal inactivation during early development in mammalian females is a random event.

Key concepts

Y Chromosome Role in Sex Determination

The Y chromosome serves as the pivotal genetic element in sex determination for many organisms, most notably in humans. This tiny chromosome harbors the Sex-determining Region Y (SRY) gene, which triggers a cascade of biological events leading to the development of male characteristics.

When a sperm cell with a Y chromosome fertilizes an egg, the resulting embryo carries an XY chromosome pair and typically develops male sexual organs. Without the Y chromosome (XX), the default developmental pathway is female. The presence or absence of the Y chromosome in this XY system therefore directly dictates the individual's sex, making the Y chromosome a critical player in sex determination.

X Chromosomal Inactivation

While having two X chromosomes could potentially lead to a 'double dose' of the genes they contain, nature has a fascinating solution—X chromosomal inactivation. This process ensures that females, like males, have only one functional set of X chromosome genes. Early in embryonic development, one of the two X chromosomes in each cell in female mammals gets randomly 'switched off'.

This silenced X chromosome becomes a Barr body, and its genes are mostly not expressed. However, some genes on the inactivated X chromosome can still be active, which can lead to subtle differences between females. The randomness of this inactivation means that different cells will inactivate different X chromosomes, whether maternal or paternal, resulting in a mosaic pattern of gene expression.

Drosophila Sex Determination Mechanism

Unlike in humans, Drosophila, or fruit flies, determine their sex through a clever counting mechanism. It doesn't just rely on the presence or absence of the Y chromosome but instead assesses the ratio between the number of X chromosomes and the sets of autosomes (non-sex chromosomes).

An X-to-autosome ratio of 1, which results from two X chromosomes (XX) and two sets of autosomes, specifies female development. Conversely, a ratio of 0.5, one X chromosome (XY or XO) and two sets of autosomes, specifies male development. This system is sensitive to the total number of chromosomes and ensures that each sex has a balanced gene expression from sex-linked genes, offering a different perspective on the complexities of genetic sex determination.

SRY Gene in Humans

Focusing on the key genetic determinant of maleness in humans, the SRY gene located on the Y chromosome is the critical switch that turns on the development of male characteristics. The presence of this gene propels the undifferentiated gonadal tissue of an embryo to form testes, initiating the pathway to male sexual development.

Even with an unusual number of X chromosomes, as long as the SRY gene is present, the individual will develop as male. Therefore, while the Y chromosome as a whole is important, it's this specific gene that wields the power in directing male sex differentiation. This knowledge comes from studying people with different chromosomal compositions, such as Turner syndrome (X0) and Klinefelter syndrome (XXY), helping clarify the singular role of the SRY gene in sex determination.