PSY106Chap2 - CHAPTER 2 As stated in Chapter 1 the study of...

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CHAPTER 2 As stated in Chapter 1, the study of development across the life span begins at the very beginning of the life span, at fertilization , the process by which two gametes , one sperm and one ovum, fuse to form a single new cell, called a zygote. This single cell grows by a process of cell division, eventually resulting in an organism consisting of over 100 trillion cells with specialized functions. There are 2 types of cell division: 1. Mitosis : a process whereby the 23 pairs (46) chromosomes of a cell split and replicate to produce two identical sets of chromosomes, after which the cell divides. The result is two daughter cells, each with the full complement of 46 chromosomes of the original cell. 2. Meiosis : a process whereby the 23 pairs (46) chromosomes of a cell split and replicate, similar to mitotic cell division; however, the pairs of chromosomes in each of the would-be daughter cells split apart, as do the would-be daughter cells. This second meiotic division results in the production of four gametes produced by the two-step process, each gamete containing just 23 chromosomes--one chromosome from each of the 23 pairs of chromosomes in the original cell. It is during the replication of chromosomes and the subsequent splitting off of the chromosomes to form gametes that genetic shuffling occurs which results in familial genetic diversity. Genetic diversity in a species is then greatly expanded when a sperm gamete with its 23 chromosomes fertilizes an ovum gamete with its 23 chromosomes. The resulting zygote contains the full 46 chromosomes needed to direct the development of a new human. The new human formed is a unique combination of 100,000 genes, which have gone through extensive mixes and combinations over all the generations of human existence. Unique, that is, unless the new human is an identical twin, in which case, they have someone who is an exact duplicate of them genetically. There are two ways multiple births occur: 1. Monozygotic multiples (identical): happen when the ovum divides after fertilization. These babies are genetically identical and are always the same sex. 2. Dizygotic multiples (fraternal): happen when multiple eggs are released and fertilized by different sperm cells. These babies are no more alike genetically than siblings that are born at different times, and can be of the same or of different sexes. Fraternal twins are more common than identical twins.
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Sex Determination The sex of the new human is determined at conception. 22 pairs of chromosomes are unrelated to the sex of the new life, but the 23rd pair of chromosomes determines the sex of the new person, depending on whether they received an X or a Y chromosome from their father. When this 23rd pair of chromosomes are both X chromosomes ( one X from mom, one X from dad), then the
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PSY106Chap2 - CHAPTER 2 As stated in Chapter 1 the study of...

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