In this chapter, we have 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, 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?

Step by step solution

01

(A) The Role of X and Y Chromosomes in Human Sex Determination

To answer this question, let's consider the experiments and observations that have provided evidence for the roles of the X and Y chromosomes in sex determination in humans: 1. Experiments involving the manipulation of sex chromosomes in laboratory animals, such as mice, have shown that the presence of the Y chromosome determines the development of male traits, while its absence results in female traits. These experiments are considered as valid models for humans due to the similarity in sex chromosomes. 2. Studies involving humans with sex chromosome abnormalities like Turner Syndrome (45,X) or Klinefelter Syndrome (47,XXY) show that individuals with only one X chromosome are female, while those with one or more Y chromosomes develop as males. These findings support the notion that the Y chromosome is responsible for maleness, whereas X chromosomes don't play a significant role in sex determination.

02

(B) Sex Ratio at Conception in Humans

To understand how the study of the sex ratio at conception has evolved over time, let's examine the approaches used in the late 1940s and in 2015: 1. In the late 1940s, researchers primarily relied on data from war-time rationing registries and vital statistics (births and deaths) to assess the sex ratio at conception in humans. These data sources had limitations in accuracy and could not account for early pregnancy losses or spontaneous abortions. 2. In 2015, modern technologies made it possible to study the sex ratio at conception more accurately. Techniques like pre-implantation genetic diagnosis (PGD) allowed scientists to determine the sex of embryos before implantation, providing more precise sex ratio estimations. Additionally, large-scale epidemiological studies and better access to medical records led to improved data quality and validity in sex ratio research.

03

(C) Random X Chromosomal Inactivation

Evidence for random X chromosomal inactivation in mammalian females comes primarily from studying cells and tissues in females who are heterozygous for X-linked genes. By observing the patterns of gene expression in these cells, scientists concluded that X chromosomal inactivation occurs randomly: 1. In some cells, the paternal X chromosome is inactivated, and the maternal X chromosome remains active. 2. In other cells, the maternal X chromosome is inactivated, and the paternal X chromosome remains active. These observations suggest that X chromosomal inactivation is a random event, as it does not follow a specific pattern for the paternal or maternal homolog.

04

(D) Different Sex Determination Mechanisms in Drosophila

To determine that Drosophila has a different sex determination mechanism than mammals, despite having the same sex-chromosome compositions, we can compare the mechanisms in both organisms: 1. In mammals, the presence or absence of the Y chromosome determines sex, as it carries the SRY gene responsible for testis development and male traits. 2. In Drosophila, the sex is determined by the ratio of X chromosomes to sets of autosomes (X/A ratio), where a low X/A ratio results in maleness and a high X/A ratio results in femaleness. The presence or absence of the Y chromosome in Drosophila does not influence sex determination, as it does not carry any essential genes for sex determination. By comparing these mechanisms, we can conclude that Drosophila utilizes a different sex determination system than mammals.

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