2.1: Heredity

As your recall from chapter one, nature refers to the contribution of genetics to one’s development. The basic building block of the nature perspective is the gene. Genes are recipes for making proteins, while proteins influence the structure and functions of cells. Genes are located on the chromosomes and there are an estimated 20,500 genes for humans, according to the Human Genome Project (NIH, 2015). See Box 2.2 at the end of this section for more details on the Human Genome Project.
A gradual zoom-in from cell, nucleus, chromosone, to DNA.
Figure 2.1: Genes are located on chromosomes.

Normal human cells contain 46 chromosomes (or 23 pairs; one from each parent) in the nucleus of the cells. After conception, most cells of the body are created by a process called mitosis. Mitosis is defined as the cell’s nucleus making an exact copy of all the chromosomes and splitting into two new cells. However, the cells used in sexual reproduction, called the gametes (sperm or ova), are formed in a process called meiosis. In meiosis the gamete’s chromosomes duplicate, and then divide twice resulting in four cells containing only half the genetic material of the original gamete. Thus, each sperm and egg possesses only 23 chromosomes and combine to produce the normal 46. See Figure 2.2 for details on both mitosis and meiosis. Given the amount of genes present and the unpredictability of the meiosis process, the likelihood of having offspring that are genetically identical (and not twins) is one in trillions (Gould & Keeton, 1997).

The process of mitosis and meiosis, each simplified by using 2 chromosomes as the starting point. In Mitosis, the 2 chromosomes make a duplicate copy and two identical cells are created. This is the process used by all cells, except those used in sexual reproduction. In Meiosis, the process used in sexual reproduction, involve 4 steps: Step 1 – Exchange of genes between the 2 chromosomes (crossing over), Step 2 – Chromosomes make a duplicate, Step 3 – First cell division, Step 4 – Second cell division.
Figure 2.2: Mitosis vs. Meiosis.

Of the 23 pairs of chromosomes created at conception, 22 pairs are similar in length. These are called autosomes. The remaining pair, or sex chromosomes, may differ in length. If a child receives the combination of XY the child will be genetically male. If the child receives the combination XX the child will be genetically female.


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Lifespan Development - A Psychological Perspective by Martha Lally and Suzanne Valentine-French is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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