Child Development From Infancy to Adolescence. Laura E. Levine
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
iStockphotos.com/skynesher
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.
One Behavior, Many Genes; One Gene, Many Effects
Disorders such as sickle-cell disease and Tay-Sachs disease are created by a single recessive gene pair. However, most human behaviors are unlikely to be the result of only one gene. Polygenic inheritance means many different genes may interact to promote any particular trait or behavior. In addition, they may interact with environmental experiences in ways we describe later in this chapter. Therefore, the occurrence of any trait or ability is likely to be multifactorial; that is, it depends on many factors. In addition, any one gene may influence a variety of outcomes. This is referred to as pleiotropic effects. For example, one gene has been implicated in both lung capacity and high blood pressure in African Americans and with risk of heart attack in European Americans (Tyler, Crawford, & Pendergrass, 2014).
Polygenic inheritance: Numerous genes may interact to promote any particular trait or behavior.
Pleiotropic effects: The many different influences any single gene may have.
T/F #4
Every gene in your body has one specific function. False
A specific type of pleiotropic effect occurs when some genes or combinations of genes seem to have a general effect on many related abilities. These have been labeled generalist genes. For example, genetic analyses have shown that the same genes that influence verbal abilities also influence nonverbal cognitive abilities (Trzaskowski, Shakeshaft, & Plomin, 2013). Researchers believe they will find a set of many genes, each having a small effect, that together exert a general effect on a range of cognitive abilities (Trzaskowski et al., 2013). Therefore, although scientists have identified the functions of some individual genes, we must be careful not to oversimplify the findings that emerge as research continues.
Generalist genes: Genes that affect many apparently distinct cognitive abilities.
Check Your Understanding
Knowledge Questions
1 What are chromosomes?
2 What is polygenic inheritance?
3 What are pleiotropic effects?
Critical Thinking
Prenatal genetic testing is currently carried out to diagnose genetic diseases such as sickle-cell anemia; however, whole genome sequencing, in which all of the fetus’ genes would be analyzed, is likely to become standard practice at some point in the future. What are the risks and benefits of using this technology prenatally?
Genetic Disorders
>> LQ 3.3 How do genetic disorders develop, and what role do genetic testing and counseling play in identifying, preventing, and treating these disorders?
We have discussed some aspects of human functioning in which genes play a role. Now we focus on situations in which genes contribute to disorders that interfere with the healthy functioning of the human mind and body. We describe three types of genetic disorders: single-gene disorders, chromosome disorders, and multifactorial inheritance disorders.
Single-Gene Disorders
Some genetically based disorders result from a single gene. You have already learned about sickle-cell disease and Tay-Sachs disease. Two other such diseases are phenylketonuria (PKU) and cystic fibrosis. Phenylketonuria is a condition in which the child cannot digest a common protein in the human diet. This condition can result in intellectual disability (U.S National Library of Medicine, 2017b). In cystic fibrosis the child’s body produces a thick, sticky mucus that clogs the lungs, making the child vulnerable to pulmonary infections. It is also associated with nutritional deficiencies (Elborn, 2016).
Single-gene disorders can occur in two ways: (1) An individual inherits a pair of recessive genes that carry the instructions for that disorder, or (2) mutations occur as cells divide so that some of the bases that give