Early embryonic gonads can become either testes or ovaries. A particular gene on the Y chromosome induces the embryonic gonads to become testes. Females lack a Y chromosome, and the absence of this gene causes the development of ovaries.The embryonic testes secrete testosterone, which induces the development of male accessory sex organs and external genitalia.The absence of testes (rather than the presence of ovaries) in a female embryo causes the development of the female accessory sex organs.
"A chicken is an egg's way of making another egg." Phrased in more modern terms, genes are "selfish." Genes, according to this view, do not exist in order to make a well-functioning chicken (or other organism). The organism, rather, exists and functions so that the genes can survive beyond the mortal life of individual members of a species. Whether or not one accepts this rather cynical view, it is clear that reproduction is one of life's essential functions. The incredible complexity of structure and function in living organisms could not be produced in successive generations by chance; mechanisms must exist to transmit the blueprint (genetic code) from one generation to the next. Sexual reproduction, in which genes from two individuals are combined in random and novel ways with each new generation, offers the further advantage of introducing great variability into a population. This diversity of genetic constitution helps to ensure that some members of a population will survive changes in the environment over evolutionary time.
In sexual reproduction, germ cells, or gametes (sperm and ova), are formed within the gonads (testes and ovaries) by a process of reduction division, or meiosis (chapter 3). During this type of cell division, the normal number of chromosomes in human cells—forty-six—is halved, so that each gamete receives twenty-three chromosomes. Fusion of a sperm cell and ovum
■ Figure 20.1 The human life cycle. Numbers in parentheses indicate the haploid state (twenty-three chromosomes) or diploid state (forty-six chromosomes).
(egg cell) in the act of fertilization results in restoration of the original chromosome number of forty-six in the zygote, or fertilized egg. Growth of the zygote into an adult member of the next generation occurs by means of mitotic cell divisions, as described in chapter 3. When this individual reaches puberty, mature sperm or ova will be formed by meiosis within the gonads so that the life cycle can be continued (fig. 20.1).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.