Читать книгу Child Development From Infancy to Adolescence - Laura E. Levine - Страница 177

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

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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.).

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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 have to worry about?

2 Can you establish the likelihood that any child she has will inherit Tay-Sachs disease, or do you need other information to do this?

3 The woman’s husband decides to be tested and finds he does not carry the Tay-Sachs gene. What is the likelihood this couple will have a child with the disease?

4 If the husband is a carrier, what is the likelihood that a child of theirs will have Tay-Sachs?

Answers:

1 The woman will not develop the disease herself. Because the gene is recessive, the dominant partner in this gene pair will determine the outcome for the person carrying it. In this case, she “carries” the gene but does not experience its effects. However, she might worry about passing on the recessive gene to her children.

2 We cannot know how likely it is that a child will inherit a recessive gene disorder unless we know the genotype of both the mother and the father. If the father also is a carrier, he could pass a recessive gene for the condition on to his children. A baby must inherit the recessive Tay-Sachs gene from both the mother and the father to develop the disease.

3 If the husband is not a carrier, there is no chance the child will have Tay-Sachs because the baby must have two Tay-Sachs genes, one from the mother and one from the father. Any children from this couple will inherit one dominant gene from the father that will protect them from having this condition.

4 Look at the chart below (referred to as a Punnett square), which shows the possible pairings of a mother’s and a father’s genes, to see what the likelihood is of a child having Tay-Sachs disease when both parents are carriers:

* This is the only combination that will result in the child having Tay-Sachs because both parents are contributing a recessive gene for the disease. Therefore, each time this couple conceives a child, there will be a 1-in-4 (or 25%) chance the child will have the disease.

Pair bonding. This type of vole mates for life. Researchers have found that a single gene can make the difference between a vole who is monogamous and one who isn’t. A similar gene may contribute to human infidelity as well.

Larry Young

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As we have shown, a single gene pair can be responsible for deadly disorders. At least in animals, a single gene pair also can be linked with behaviors that appear quite complex. For example, in a small animal called a vole, one particular gene governs whether the animal is monogamous. While the prairie vole chooses a partner for life, the meadow vole, whose gene is slightly different, mates with whoever is available. Scientists discovered that the gene that produces the hormone vasopressin differs in these two types of voles. When they switched that gene between the two types, the monogamous prairie vole became a wanderer and the wandering meadow vole immediately began to direct his mating energies toward one female only and gave up his wandering ways (Lim et al., 2004). It is unlikely a single gene would govern such complex behavior in human beings, but research in Finland found that both men and women with a certain form of the vasopressin gene are more likely than others to have had multiple romantic relationships in the previous year (Zietsch, Westberg, Santtila, & Jern, 2015). Clearly, human genes interact with cultural expectations to shape practices such as monogamy, so these findings point to an interaction with genetic predispositions in determining complex behaviors such as these.