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Chapter 5: Problem 5

Distinguish between (a) the concepts of sexual differentiation and sex determination and (b) Protenor and Lygaeus modes of sex determination.

Short Answer

Expert verified
The main difference between the Protenor and Lygaeus modes of sex determination is the sex that is heterogametic. In the Protenor mode, males have different sex chromosomes (XY), while females have the same sex chromosomes (XX). In the Lygaeus mode, it is the opposite: females have different sex chromosomes (ZW), and males have the same sex chromosomes (ZZ).

Step by step solution

01

Definition of Sexual Differentiation and Sex Determination

Sexual differentiation is the process by which an individual's sex-related characteristics, such as genitals, internal reproductive organs, and secondary sexual characteristics, develop during gestation, childhood, and puberty. This process is influenced by genetic, hormonal, and environmental factors. Sex determination, on the other hand, is the process that determines the sex of an individual organism during its development, usually determined by its genetic makeup. The sex of an individual can be determined by various methods, such as chromosomal, genic, or environmental factors.
02

Difference between Sexual Differentiation and Sex Determination

To differentiate, sexual differentiation refers to the development of sex-specific characteristics of an individual, whereas sex determination refers to the process of determining an individual's sex based on its genetic makeup or other factors. In other words, sex determination leads to sexual differentiation.
03

Definition of Protenor and Lygaeus Modes of Sex Determination

The Protenor and Lygaeus modes of sex determination are two different systems of chromosomal sex determination found in insects. Protenor mode: In this system, the male has a heterogametic sex chromosome system (XY) and the female has a homogametic sex chromosome system (XX). This mode of sex determination is similar to the one found in humans and most mammals. Lygaeus mode: In this system, the female is heterogametic (ZW), and the male is homogametic (ZZ). This mode of sex determination is found in some insects, birds, and reptiles.
04

Comparison of Protenor and Lygaeus Modes

The main difference between the Protenor and Lygaeus modes of sex determination is the sex that is heterogametic. In the Protenor mode, males have different sex chromosomes (XY), while females have the same sex chromosomes (XX). In the Lygaeus mode, it is the opposite: females have different sex chromosomes (ZW), and males have the same sex chromosomes (ZZ).

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Most popular questions from this chapter

In Drosophila, an individual female fly was observed to be of the \(\mathrm{XXY}\) chromosome complement (normal autosomal complement) and to have white eyes as contrasted with the normal red eye color of wild type. The female's father had red eyes, and the mother had white eyes. Knowing that white eyes are X-linked and recessive, present an explanation for the genetic and chromosomal constitution of the XXY, whiteeyed individual. It is important that you state in which parent and at what stage the chromosomal event occurred that caused the genetic and cytogenetic abnormality.

In chickens, a key gene involved in sex determination has recently been identified. Called \(D M R T 1\), it is located on the \(Z\) chromosome and is absent on the W chromosome. Like SRY in humans, it is male determining. Unlike \(S R Y\) in humans, however, female chickens (ZW) have a single copy while males (ZZ) have two copies of the gene. Nevertheless, it is transcribed only in the developing testis. Working in the laboratory of Andrew sinclair (a co-discoverer of the human SRY gene), Craig Smith and colleagues were able to "knock down" expression of \(D M R T 1\) in \(\mathrm{ZZ}\) embryos using RNA interference techniques (see Chapter 15 ). In such cases, the developing gonads look more like ovaries than testes [Nature 461: 267 (2009)]. What conclusions can you draw about the role that the DMRTI gene plays in chickens in contrast to the role the SRY gene plays in humans?

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?

An attached-X female fly, XXY (see the "Insights and Solutions" box), expresses the recessive X-linked white-eye phenotype. It is crossed to a male fly that expresses the X-linked recessive miniature wing phenotype. Determine the outcome of this cross in terms of sex, eye color, and wing size of the offspring.

Can the Lyon hypothesis be tested in a human female who is homozygous for one allele of the X-linked G6PD gene? Why, or why not?
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