Question

In this chapter, we have focused on sex differentiation, sex chro- mosomes, and genetic mechanisms involved in sex determination. At the same time, we found many opportunities 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 that in humans the X chromosomes play no role in sex determination, while the Y chromosome causes maleness and its absence causes femaleness? (b) How did we originally (in the late 1940 s) analyze the sex ratio at conception in humans, and how has our approach to studying this issue changed in \(2015 ?\) (c) 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? (d) How do we know that Drosophila utilizes a different sexdetermination mechanism than mammals, even though it has the same sex-chromosome compositions in males and females?

Short answer

#tag_title# Short Answer #tag_content# The X and Y chromosomes are the sex chromosomes in humans. The presence of the Y chromosome determines maleness due to the SRY gene. X chromosomal inactivation in mammalian females ensures a similar dosage of X-linked genes as males, occurring randomly during early development. In contrast, Drosophila's sex determination is based on the ratio of X chromosomes to sets of autosomes (X:A ratio), differing from the Y chromosome-dependent mechanism in mammals. Modern approaches to studying sex ratios include genetic analysis and ultrasound imaging, providing more accurate information compared to historical methods.

Step-by-step solution

(a) Role of X and Y chromosomes in human sex determination

The X and Y chromosomes are the sex chromosomes in humans. In humans, the presence of the Y chromosome determines maleness, while its absence causes femaleness. This is because the Y chromosome contains the SRY gene, which is responsible for maleness. When present, the SRY gene initiates the development of male sex organs and inhibits the formation of female sex organs. In the absence of the SRY gene (and hence the Y chromosome), female sex organs develop. Therefore, it is the Y chromosome that plays the role in sex determination, while the X chromosomes do not play a direct role in this process.

(b) Analyzing sex ratio at conception and modern approaches

In the late 1940s, sex ratio at conception was studied by examining the number of male and female offspring in a population. Factors such as war and cultural preferences could potentially influence the observed sex ratio. In the 21st century, our approach to studying this issue has become more advanced and includes genetic analysis, ultrasound imaging, and other laboratory techniques to gather more accurate and detailed information about the sex ratio at conception.

(c) Random X chromosomal inactivation during early development

X chromosomal inactivation in mammalian females is a process that occurs during early development to ensure that female mammals have a similar dosage of X-linked genes as males. This inactivation is random, meaning that either the paternal or maternal X chromosome can be inactivated in each cell. The randomness of this event is supported by studies on cells from female mammals, which show that the inactivated X chromosome can come from either parent. Additionally, the pattern of inactivation is clonally inherited, meaning that all cells derived from the initially inactivated cell will have the same inactivation pattern.

(d) Drosophila's different sex-determination mechanism

Drosophila and mammals have the same sex-chromosome compositions (XX for females and XY for males). However, their mechanisms of sex determination are different. In Drosophila, the sex of an individual is determined by the ratio of the number of X chromosomes to the number of sets of autosomes (X:A ratio). A ratio of 1:2 (1 X chromosome to 2 sets of autosomes) results in a male, while a ratio of 1:1 (2 X chromosomes to 2 sets of autosomes) results in a female. This mechanism of sex determination is different from the Y chromosome-dependent mechanism found in mammals.