Child Development From Infancy to Adolescence. Laura E. Levine
Mendelian Inheritance: Dominant and Recessive Genes
You may have been told that you look very much like your father or exactly like your mother. How can this be, when you received an equal number of chromosomes from each parent? When eggs and sperm cells are formed, the chromosomes in each “unzip” along the double helix so they each contain half the genetic material usually found in a cell. When egg and sperm combine at conception, chromosomes from each parent pair up and the genes from one parent are zipped up to similar genes from the other parent. Traditional Mendelian genetics tells us that each pair of genes is made up of some combination of dominant and recessive genes and the genes that are present at a particular location on a chromosome and code for a particular trait or traits are called the genotype. The information contained in the dominant genes is what is usually expressed in the person’s body. What we see when we look at a person’s bodily traits and characteristics is called the phenotype. The information from a recessive gene is usually not expressed in the phenotype unless the gene is paired with another recessive gene.
Dominant genes: Genes that are usually expressed in the phenotype.
Recessive genes: Genes that are generally not expressed in the phenotype unless paired with another recessive gene.
Genotype: The genes that are present at a particular location on a chromosome and are responsible for a particular trait or traits.
Phenotype: The genetically based characteristics that are actually shown in one’s body.
To use a simplified example, brown eye color is dominant over blue eye color. If your mother has brown eyes because she has two genes for brown eyes in her genotype, and your father has blue eyes because he must have two recessive genes for blue eyes, you will have brown eyes. Your mother can pass along only the dominant brown-eye genes to her children (because that is the only genetic information she has for this trait), and your father can pass on only recessive genetic information for blue eyes (because that is all he has for this trait). The only possible genetic combination you can inherit is one dominant gene for brown eyes and one recessive gene for blue eyes. Therefore, brown eyes will be expressed as the phenotype of all the children in your family. However, you will still carry in your genotype the one recessive gene for blue eyes you received from your father. And if you have a child with someone who has brown eyes but who also carries one recessive gene for blue eyes, you will have a blue-eyed child if those two recessive genes are paired in the child. See Figure 3.3 to better understand how this might happen.
T/F #3
Two parents with brown eyes can still have a child with blue eyes. True
Gene dominance. Do you look as similar to one of your parents as this girl does to her mother? Inheritance of dominant genes from one parent or the other can result in striking resemblances.
©iStockphoto/digitalskillet
Figure 3.3 Genetic transmission of eye color (dominant and recessive genes).
Although eye color is frequently used to illustrate the idea of dominant and recessive genes, you may have already realized that it isn’t that simple. People also have green eyes, gray eyes, and hazel eyes. Although brown as an eye color is dominant over any of these alternatives, green, gray, and hazel eyes have their own dominance hierarchies. Also, the color of some people’s eyes is bright and clear, and the color of other people’s eyes is soft and washed out. You may even know someone who has one blue eye and one brown eye. But while the genetic process is more complicated than our example indicates, understanding the way dominant and recessive genes work is still central to understanding genetic inheritance.
Transmission of eye color. Although this mother has brown eyes, she must carry the recessive gene for blue eyes in her genotype. When her daughter received this recessive gene from her mother and another recessive gene for blue eyes from her father, she ended up with blue eyes.
©iStockphoto.com/KarenMower
Whether you have blue eyes or brown eyes is not crucial to your future development. Other types of gene pairings are, however, because some genetic disorders are caused by two recessive genes pairing up with each other. One such disease is sickle-cell disease, which affects millions of people in different regions around the world, and is more prevalent in African Americans than other groups of people within the United States. Sickle-cell disease is a painful and destructive disease in which the shape of red blood cells is distorted. Normal red blood cells are smooth and round, but sickle cells look like the letter C (National Heart, Lung, and Blood Institute [NHLBI], n.d.). Normal red blood cells also have a large surface area, which enables them to transport oxygen throughout the body, but sickle cells are not able to do this effectively. They are hard and tend to clump together, restricting the flow of blood into smaller blood vessels, as shown in Figure 3.4. This failure to transport oxygen where it is needed results in pain and can eventually cause damage to the organs (NHLBI, n.d.).
Figure 3.4 Sickle-cell disease.
Source: National Heart, Lung, and Blood Institute.
You may wonder why such maladaptive genes have not disappeared from the human gene pool, but there is a good evolutionary reason. It turns out that, although having two such recessive genes is harmful, having one may be protective in certain environments. This realization came from the observation that the areas in Africa in which this gene is found in the population are almost identical to the areas in which malaria is a major problem. It appears that having one gene for sickle-cell disease is protective against malaria. With its protective advantages, individuals with the recessive gene are more likely to survive to pass it on to the next generation (Sabeti, 2008). Today the recessive gene for sickle-cell disease is carried in the genotype of about 1 in 13 Americans of African heritage (Centers for Disease Control and Prevention [CDC], 2017g). Similar to our example with blue eye color, if two people who each carry the recessive gene have children, there is a 1-in-4 chance their children will inherit two recessive genes and suffer from sickle-cell disease. For a better idea about how disorders can result from recessive genes, try to answer the questions in Active Learning: Understanding the Inheritance of Tay-Sachs Disease.
Active Learning: Understanding the Inheritance of Tay-Sachs Disease
Tay-Sachs is a genetic disease that results in progressive neurological deterioration and death, usually by age 5. The highest incidence occurs among Jews whose ancestors came from Eastern Europe, and elevated levels are also found in French Canadians living in Quebec and members of the Cajun community in Louisiana (NHGRI, 2011). A recessive gene is responsible for Tay-Sachs, and a simple blood test can identify it. Answer the questions below to enhance your understanding of how a recessive gene works:
1 A woman decides to be tested for the Tay-Sachs gene and finds she is a “carrier” of Tay-Sachs, meaning she has the gene for the disease. If she is a carrier, does she have to worry that she will develop Tay-Sachs herself? What, if anything, does she