2.1: Heredity

Genetic Foundations of Development

As your recall from Chapter 1, nature refers to the contribution of genetics to one's development. The basic building block of the nature perspective is the gene. Genes are specific sequence of nucleotides and are recipes for making proteins. Proteins are responsible for influencing 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 Note \(\PageIndex{1}\) at the end of this section for more details on the Human Genome Project.

Learning Objectives: Heredity

By the end of the chapter, you will be able to: 

• Define genes
• Distinguish between mitosis and meiosis, genotype and phenotype, homozygous and heterozygous, and dominant and recessive
• Describe some genetic disorders, due to a gene defect, and chromosomal disorders
• Define polygenic and incomplete dominance
• Describe the function of genetic counseling and why individuals may seek genetic counseling
• Define behavioral genetics, describe genotype-environment correlations and genotype-environmental interactions, and define epigenetics

Figure \(\PageIndex{1}\): DNA is organized into chromosomes, which are located within the nucleus of each cell. (CC BY-SA 4.0; via Wikipedia).

Figure \(\PageIndex{1}\) illustrates the relationship between a cell, its nucleus, chromosomes, and DNA. On the left is a cell containing various internal structures. An arrow points to a magnified view of the cell's nucleus. A second arrow points from the nucleus to a chromosome. A highlighted section of the chromosome is enlarged to show the DNA double helix. The figure demonstrates that DNA is packaged into chromosomes, and chromosomes are found within the nucleus of a cell. 

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 each 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 \(\PageIndex{2}\) 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).

Figure \(\PageIndex{2}\): Mitosis produces two genetically identical cells, whereas meiosis produces four genetically unique sex cells. (CC BY-SA 4.0; via Pressbooks).

Figure \(\PageIndex{2}\) compares mitosis and meiosis using simplified diagrams with two chromosomes, one blue and one pink.

In the top section, mitosis is shown. A parent cell contains two chromosomes. The chromosomes duplicate, creating identical copies. The cell then divides once, producing two daughter cells. Each daughter cell contains the same chromosome set as the original parent cell. The result of mitosis is two genetically identical cells.

In the lower section, meiosis is shown. The process begins with a parent cell containing two chromosomes. In Step 1, corresponding chromosomes exchange genetic material through crossing over, creating chromosomes with segments of both colors. In Step 2, the chromosomes duplicate. In Step 3, the first cell division separates homologous chromosome pairs into two cells. In Step 4, a second cell division separates the duplicated chromosomes, resulting in four cells. Each resulting cell contains one chromosome from each pair and has a different combination of genetic material because of crossing over.

The figure emphasizes that mitosis involves one cell division and produces two identical cells, whereas meiosis includes crossing over and two cell divisions, producing four genetically unique cells used in sexual reproduction.

Note: The diagram uses only two chromosomes to simplify the processes.

Of the 23 pairs of chromosomes created at conception, 22 pairs are similar in length. These are called autosomes. The remaining pair, the 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.

Note \(\PageIndex{1}\)

In 1990 the Human Genome Project (HGP), an international scientific endeavor, began the task of sequencing the 3 billion base pairs that make up the human genome. In April of 2003, more than two years ahead of schedule, scientists gave us the genetic blueprint for building a human. Since then, using the information from the HGP, researchers have discovered the genes involved in over 1800 diseases. In 2005 the HGP amassed a large data base called HapMap that catalogs the genetic variations in 11 global populations. Data on genetic variation can improve our understanding of differential risk for disease and reactions to medical treatments, such as drugs. Pharmacogenomic researchers have already developed tests to determine whether a patient will respond favorably to certain drugs used in the treatment of breast cancer, lung cancer or HIV by using information from HapMap (NIH, 2015).

Future directions for the HGP include identifying the genetic markers for all 50 major forms of cancer (The Cancer Genome Atlas), continued use of the HapMap for creating more effective drugs for the treatment of disease, and examining the legal, social and ethical implications of genetic knowledge (NIH, 2015).

From the outset, the HGP made ethical issues one of their main concerns. Part of the HGP’s budget supports research and holds workshops that address these concerns. Who owns this information, and how the availability of genetic information may influence healthcare and its impact on individuals, their families, and the greater community are just some of the many questions being addressed (NIH, 2015).