What Have You Learned?

1. Question 3.1

   How many pairs of chromosomes and how many genes does a person usually have?

   A person has 23 pairs of chromosomes and 18,000 to 23,000 genes.

2. Question 3.2

   What is the relationship among genes, base pairs, and alleles?

   Most genes have thousands of precise base pairs arranged in precise triplets, making the 20 types of amino acids needed for development into a human being. The codes for each particular gene can vary, although usually they do not. Some genes have alternate versions of base pairs, with transpositions, deletions, or repetitions of base pairs not found in other versions of the same gene. Each of these variations is called an allele of that gene. Most alleles cause only minor differences (such as the shape of an eyebrow); some seem inconsequential; some are notable.

3. Question 3.3

   What determines a person’s sex?

   Because the female’s 23rd pair of chromosomes is XX, every ovum contains either one X or the other—but always an X. And because a male’s 23rd pair is XY, half of his sperm carry an X chromosome and half a Y. The X chromosome is bigger and has more genes, but the Y chromosome has a crucial gene, called SRY, that directs the embryo to make male hormones and organs. Thus, the sex of the zygote depends on which sperm penetrates the ovum—a Y sperm with the SRY gene, creating a boy (XY), or an X sperm, creating a girl (XX). However, chromosomes and genes do not determine behavior. Every male–female difference is influenced by the culture and environment in which a child is raised.

4. Question 3.4

   What are the advantages and disadvantages of being a monozygotic twin?

   Twins come with a built-in constant companion, and monozygotic twins can donate an organ for surgical implantation in their twin without organ rejection. However, establishing two unique identities is a challenge for some monozygotic twins.

5. Question 3.5

   Why does in vitro fertilization increase the incidence of dizygotic twins?

   In this procedure, multiple zygotes are implanted to ensure success. As a result, more than one may grow into a healthy baby, thus increasing the numbers of multiple births in this country.

6. Question 3.6

   Why is a person’s genotype not usually apparent in the phenotype?

   The genotype underlies a person’s body and brain formation, but the person’s phenotype (the visible traits and behaviors) depends on many genes and on the environment.

7. Question 3.7

   What is the difference between an epigenetic characteristic and a multifactorial one?

   Epigenetic characteristics are those for which environmental factors affect genes and their expression. A multifactorial trait is a trait that is affected by many factors, both genetic and environmental. In both cases, the expression of genes is enhanced, halted, shaped, or altered, resulting in a phenotype that may differ markedly from the genotype.

8. Question 3.8

   Why do polygenic traits suggest that additive genes are more common than dominant–recessive ones?

   Polygenic traits are traits in the phenotype that are determined by many genes. One example is height, which is determined by the interaction of approximately 180 different genes acting together, all contributing a bit to the expression of the trait; these genes act additively. Hair curliness and skin color are also the result of additive genes. In the case of dominant–recessive genes, the expression of a trait in the phenotype depends on just one gene pair. An example is eye color. The brown-eye gene is dominant, and the blue-eye gene is recessive. The eye color of a child depends solely on the action of these two genes: a child who inherits two brown-eye genes or a brown-eye gene and a blue-eye gene will have brown eyes, whereas a child who inherits two blue-eye genes will have blue eyes. The sheer number of additive genes that can contribute to a single trait suggests that additive genes are more common than dominant–recessive ones.

9. Question 3.9

   What surprises came from the Human Genome Project?

   One of the surprises was that humans have far fewer than 100,000 genes, the number everyone believed throughout the twentieth century. The total number of genes in a person is between 18,000 and 23,000.

10. Question 3.10

    Regarding heritability, why is it important to know which population at what historical time provided the data?

    Heritability is a statistic that indicates the degree to which genes affected the variation in a particular trait within a particular population in a particular context and era. For example, the heritability of height is about 95 percent when children receive good medical care and nutrition, but only about 20 percent if children are malnourished. Thus, heritability is tied to context, and studying it outside of context would not be valid.

11. Question 3.11

    What nature and nurture reasons encourage one person to become an alcoholic and another not?

    We now know that biochemistry makes some people more vulnerable to alcoholism and drug use than others, so various proposed cures do not apply equally to everyone. Each person’s biochemistry reacts to alcohol differently, causing sleep, nausea, aggression, joy, relaxation, forgetfulness, sexiness, or tears. How their bodies metabolize alcohol allows some people to “hold their liquor” and therefore abuse alcohol, while others sweat and become red-faced after just a few sips, a response that may lead to abstinence. Alcoholism is inherited via psychological as well as biochemical tendencies. Some inherited personality traits (a quick temper, sensation seeking, high anxiety) encourage drinking and the use of drugs. Furthermore, some contexts, such as fraternity parties, make it hard to avoid alcohol; other contexts, such as a church social in a “dry” county, make it hard to find anything stronger than lemonade.

12. Question 3.12

    What nature and nurture reasons make one person nearsighted and another not?

    A study of British twins found that the Pax6 gene, which governs eye formation, has many alleles that make people somewhat nearsighted. This research found heritability of almost 90 percent, which means that if one monozygotic twin was nearsighted, the other twin was almost always nearsighted, too. Visual problems may also be caused by the environment. Nutrition, age, and other confounding variables such as culture and recreational habits may impact those with a genetic vulnerability more than those without a genetic vulnerability.

13. Question 3.13

    What can be learned about the nature-nurture connection from Mickey Mantle’s life?

    Ignoring the nature–nurture connection can be deadly. Most of Mantle’s male relatives were alcoholics and died before middle age, including his father, who died of Hodgkin disease (a cancer) at 39. Mantle thought that a family history of early mortality meant he would die young and took up drinking. At age 46 Mantle said, “If I knew I was going to live this long, I would have taken better care of myself.” He never developed Hodgkin disease, and if he had, chemotherapy developed since his father’s death would likely have saved him—an example of environment prevailing over genes. Instead, his drinking destroyed his liver, which caused his death.

14. Question 3.14

    Why does this textbook on normal development include information about abnormal development?

    Studying abnormal development provides us with three things: 1) Insight into the complexities of nature and nurture; 2) Knowledge of disorders’ origins to help limit their negative effects; and 3) Information to combat prejudice by demonstrating that difference is not always deficit.

15. Question 3.15

    What usually happens when a zygote has fewer or more than 46 chromosomes?

    Usually, each sperm and each ovum have 23 chromosomes, and the zygote they create has 46. However, some gametes have more or less than 46 chromosomes. About 5 to 10 percent of all zygotes have more or less than 46 chromosomes. Few of these zygotes develop to birth (less than 1 percent), primarily because most such zygotes never duplicate, divide, and differentiate. Many of the rest are aborted spontaneously or by choice. Birth itself is hazardous; about 5 percent of stillborn babies have 47 chromosomes. Once in about every 200 births, a newborn survives with 45, 47, or, rarely, 48 or 49 chromosomes. Each abnormality leads to a recognizable syndrome, a cluster of distinct characteristics that tend to occur together. Usually the cause is three chromosomes at a particular location instead of the usual two (a condition called a trisomy).

16. Question 3.16

    What are the consequences if a newborn is born with trisomy-21?

    No individual born with trisomy-21, or Down syndrome, is identical to another, but most have specific facial characteristics including a thick tongue, round face, and slanted eyes. Many infants with trisomy-21 also have hearing problems, heart abnormalities, muscle weakness, and short stature. They are usually slower to develop intellectually, especially in language, though their eventual intellect varies, in part because of epigenetics and partly because of family support.

17. Question 3.17

    Why are so few genetic conditions dominant?

    Severe dominant disorders are rare because children with such disorders rarely live long enough to pass on the gene.

18. Question 3.18

    Why are a few recessive traits (such as sickle-cell) quite common?

    The reason is evolutionary—carriers of certain recessive disorders were protected from deadly diseases. For example, sickle-cell carriers are unlikely to die from malaria, which was a major killer of those living in Africa.

19. Question 3.19

    What are the advantages and disadvantages of genetic testing?

    Genetic counseling relieves some worries by providing facts and helping prospective parents discuss sensitive issues. However, counselors must be carefully trained to communicate clearly, because many people misinterpret words such as “risks” and “probability.” Even doctors do not always understand genetics.

20. Question 3.20

    Why do people need genetic counselors rather than fact sheets about genetic conditions?

    The counselor’s job is to make sure the person understands the facts and treatment options as well as possible outcomes of not treating a condition. It allows them to understand the difference between probabilities and certainties and to make decisions accordingly.