Question

In this chapter, we focused on sex differentiation, sex chromosomes, 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, what answers would you propose to the following fundamental questions? (a) How do we know that specific genes in maize play a role in sexual differentiation? (b) How do we know whether or not a heteromorphic chromosome such as the Y chromosome plays a crucial role in the determination of sex? (c) 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? (d) How did we learn that, although the sex ratio at birth in humans favors males slightly, the sex ratio at conception favors them much more? (e) 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? (f) 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

Answer: The primary determinant of male development in humans is the presence of the sex-determining region Y (SRY) gene on the Y chromosome.

Step-by-step solution

a) Specific genes in maize role in sexual differentiation.

To know that specific genes in maize play a role in sexual differentiation, geneticists carried out experiments where they manipulated these genes and observed the resulting changes in the development of maize plants. They performed targeted mutations or gene knockouts in these specific genes, which then led to observable changes in the sexual differentiation of the mutated plants. Through these experiments, they were able to conclude that these genes indeed have essential roles in the sexual differentiation process in maize.

b) Role of the Y chromosome in sex determination.

To understand if the Y chromosome plays a crucial role in sex determination, researchers have carried out studies examining individuals with abnormal sex chromosome combinations. For example, individuals with Turner syndrome (XO) generally exhibit female characteristics, while those with Klinefelter syndrome (XXY) generally exhibit male characteristics. These observations, along with findings from other studies including Y chromosome translocation and gene deletions, suggest that the presence of a Y chromosome is essential for the development of male characteristics, and thus, plays a crucial role in the determination of sex.

c) X chromosomes and Y chromosome role in human sex determination.

Researchers have identified the sex-determining region Y (SRY) gene on the Y chromosome as the primary determinant of male development in humans. By studying individuals with sex reversal syndromes (such as XX males and XY females), they discovered that the presence or absence of the SRY gene determined the development of male or female characteristics, regardless of the individual's sex chromosome composition. This evidence indicates that the X chromosomes do not play a direct role in human sex determination, while the Y chromosome, with the presence of the SRY gene, causes maleness and its absence causes femaleness.

d) Sex ratio at conception and birth in humans.

Researchers have learned that the sex ratio at birth in humans slightly favors males, while the sex ratio at conception favors them much more, through large population studies. They analyzed data from pre-implantation genetic diagnosis (PGD) and amniocentesis procedures to determine the sex ratio at conception, which showed a significant bias towards males. These findings are then compared with sex ratio data at birth to understand the shift in the ratio as pregnancies progress.

e) Drosophila sex determination mechanism compared to mammals.

We know that Drosophila has a different sex determination mechanism than mammals through comparative genetic studies. In Drosophila, the ratio of X chromosomes to autosomes determines the sex of the organism. The presence of two X chromosomes (XX) results in a female, whereas one X chromosome (XY) results in a male. In contrast, mammals rely on the presence or absence of the Y chromosome to determine sex. The presence of similar sex-chromosome compositions does suggest a shared evolutionary history, but the mechanisms of sex determination are distinct in these organisms.

f) X chromosomal inactivation in mammalian females.

Researchers discovered that X chromosomal inactivation, either from the paternal or maternal homolog, is a random event during early development in mammalian females through experiments with mice. Using mice with genetically distinct X chromosomes, they observed the inactivation pattern in various tissues, finding that both the maternal and paternal X chromosomes were inactivated in roughly equal proportions, demonstrating the random nature of the process. This process is important to ensure dosage compensation between males and females as it equalizes the expression of X-linked genes.