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Essential Reproduction
Martin H. Johnson
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eBook - ePub
Essential Reproduction
Martin H. Johnson
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About This Book
Essential Reproduction provides an accessible account of the fundamentals of reproduction within the context of cutting-edge knowledge and examples of its application. The eighth edition of this internationally best-selling title provides a multidisciplinary approach integrating anatomy, physiology, genetics, behaviour, biochemistry, molecular biology and clinical science, to give thorough coverage of the study of mammalian reproduction.
Key features:
- Contains discussion of the latest on conceptual, informational and applied aspects of reproduction
- New pedagogical features such as clinical case studies at the end of each chapter
- Better use of boxed material to improve separation of narrative text from ancillary information
- Highlighted key words for ease of reference relate to summary of key points
- Introduction now split into two sections
- Expanded content in Fetal challenges, and Society and reproduction
- Substantial rearrangement and updating in Making sperm, Controlling fertility, and Restoring fertility
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Part 1
Introduction
CHAPTER 1
What is reproduction?
The ability to reproduce is a defining feature of all living organisms. Through reproduction, we pass our genes to a new generation. Each new generation in turn reproduces or dies out. The survivors are âselectedâ, by disease resistance and by successful competition for resources and mates, for their âfitnessâ to live and to reproduce. In this way, the gene pool of surviving species is constantly adapting to the prevailing environment to provide the best available âfitâ. Thus, reproduction has been central to our evolution as the species Homo sapiens.
However, humans transmit more than simply their genes across generations. Humans have evolved high levels of sociability through which cultures are formed. Cultural practices are also transmitted across generations, and reproduction itself lies at the very heart of many of our cultural practices and taboos (see Chapters 5, 6, 20 and 23). Human society, by influencing socially and/or medically who survives to reproduce and with whom, is itself now part of the âselectionâ process. This pivotal position of reproduction in our culture makes it a sensitive subject for study. Indeed, scientific enquiry into human reproduction was relatively late to the modern research scene and even today can provoke hostility, embarrassment or distress.
In this opening chapter, human reproduction is introduced and contextualized: in relation to other species â reproductive strategies, and in relation to time â the reproductive life cycle.
Reproductive strategies
Most organisms reproduce asexually (or vegetatively). For example, many unicellular organisms reproduce themselves mitotically, just like the individual cells of our body (Figure 1.1). Mitotic divisions generate two offspring that are genetically identical to each other and to their single parent. Among multicellular organisms, some shed cells or even body parts from which another genetically identical individual can be generated â a process called regeneration. Others, including some complex vertebrates such as lizards, reproduce themselves by setting aside a special population of egg cells that can differentiate into conceptuses in the absence of a fertilizing spermatozoon. This type of asexual reproduction is called parthenogenesis, and generates a completely new organism with the same gene complement as its parent.