Info

(b) Environment people born in the developing world (Figure 4.13). We know that this difference is not genetic because when families move from nonindustrialized to industrialized countries, their life spans quickly increase to that typical of their new country. The variation in life expectancies among countries is almost entirely due to differences in their environments—primarily access to unpolluted water and basic health care and the prevalence of infection with HIV, the virus that causes AIDS.

Most traits that show continuous variation are influenced by both genes and the effect of differing environmental factors. Skin color in humans is an example of this type of trait. The shade of an individual's skin is dependent upon the amount of melanin present near the skin's surface. A number of genes have an effect on this phenotype—both those that influence melanin production and those that affect the distribution of melanin in the skin. However the environment, particularly the amount of exposure to the sun, also influences the skin color of individuals (Figure 4.14): melanin production increases, and any melanin that is present darkens in sun-exposed skin. In climates with warm summers and cool winters, an individual's skin color changes over the course of a year. After many years of intensive sun exposure, skin may become permanently darker. Among people with light skin, the effect of the sun on skin color can be quite dramatic.

Both genetic factors and environmental factors influence most traits that are of interest to women choosing sperm donors. Women choosing Doctorate-category sperm donors from Fairfax Cryobank are presumably interested in having smart, successful children, but intelligence has both a genetic and an environmental component. Intelligence partly depends on brain structure and function, and many al-leles that interfere with brain structure and function—and thus intelligence—have been identified; but intelligence also depends on environmental factors. For example, if a developing baby is exposed to high levels of cigarette by-products or alcohol before birth, its brain will develop differently, and it may have delayed or diminished intellectual development.

Predicting the Inheritance of Quantitative Traits Unlike qualitative traits, where the relationship between genes and traits is very clear, the inheritance of quantitative traits is difficult to understand. For instance, if the variation among individuals could be a result of many different genes, a variety of environmental effects on phenotype, or (most likely) the interaction of genes and environment, how can we predict if the child of a father with a doctorate will also be capable of earning a doctorate? The most common way in which scientists approach this question is to attempt to determine the relative importance of different alleles in determining variation in phenotype among individuals.

(b) Environment

Figure 4.14 Skin color is influenced by genes and environment. (a) The difference in skin color between these two women is primarily a result of differences in several alleles that control skin pigment production. (b) The difference in color between the sun-protected and sun-exposed portions of the older man in this picture is entirely the result of environmental effects.

Figure 4.14 Skin color is influenced by genes and environment. (a) The difference in skin color between these two women is primarily a result of differences in several alleles that control skin pigment production. (b) The difference in color between the sun-protected and sun-exposed portions of the older man in this picture is entirely the result of environmental effects.

Researchers working with domestic animals and crop plants were the first to develop the scientific model used to measure the importance of genes in determining the value of quantitative traits. These researchers were trying to find the best way to improve production in various agricultural species. For example, farmers who wish to increase their dairy herd's milk production have two basic strategies for doing so: Change the herd's environment by changing the way the cows are reared, housed, and fed; or change the herd genetically by choosing only the offspring of the best milk producers for the next-generation herd. The technique of controlling the reproduction of individual organisms to influence the phenotype of the next generation is known as artificial selection (Figure 4.15). Artificial selection is similar to natural selection, which is described more thoroughly in Chapter 9.

If milk production in cows is strongly influenced by genes—in other words, it has high heritability—then artificial selection is an effective way to boost

Figure 4.15 Artificial selection increases milk production in cows. Cows that produce exceptional amounts of milk are bred to produce the next generation of dairy cattle. In this example, the female calves of the cow that produces 3.6 gallons of milk daily produce an average of 3.2 gallons of milk per day, 23% more than the previous herd.

Figure 4.15 Artificial selection increases milk production in cows. Cows that produce exceptional amounts of milk are bred to produce the next generation of dairy cattle. In this example, the female calves of the cow that produces 3.6 gallons of milk daily produce an average of 3.2 gallons of milk per day, 23% more than the previous herd.

milk output. In fact, heritability of milk production can easily be measured by how well a herd responds to artificial selection. If milk production increases in a herd of cows as a result of artificial selection, it is because alleles for proteins that increase milk production in an individual (for instance, alleles for genes that control the size of the udder, or the activity of milk-producing cells) have become more common—that is, the trait is strongly influenced by genes. If milk production does not increase as a result of artificial selection, then the alleles the high-production cows possess must not be as important in determining milk output—that is, the trait must be more strongly influenced by the environment than by genes. Scientists have calculated an average heritability of milk production in dairy cattle of 0.30. This means that, in a typical dairy herd, about 30% of the variation among cows in milk production is due to differences in their genes; the remainder of their production variation is due to differences among the cows in their environmental conditions.

Studies of the relative influence of genes and environment that use response to artificial selection cannot be performed in human populations. It is ethically and socially unacceptable to design breeding programs to produce people with various traits, or to select the men and women who will produce the next generation. An alternative strategy is for scientists to compare the value of a trait in children with that of their parents. This comparison takes the form of a correlation, where researchers estimate, for instance, how accurately we can predict the height of children if we know the height of their parents.

The correlation between the intelligence of parents and their children helps us determine how important the intelligence of a donor may be to the mental capacity of his children. Intelligence is often measured by performance on an IQ test. Alfred Binet, a French psychologist, developed the intelligence quotient (or IQ), in the early 1900s to more efficiently identify Paris schoolchildren who were in need of remedial help in school. The IQ is not an absolute score on a test, it represents a comparison between an individual and his peers in test performance. The average IQ was arbitrarily set at 100 with a standard deviation of 15 points, and IQ tests are designed to produce a normal distribution in scores taken from a sample population.

Binet's IQ test was not based on any theory of intelligence and was not meant to comprehensively measure mental ability, but the tests remain a common way to measure innate, or "natural" intelligence. Even if IQ tests do not really measure general intelligence, IQ scores have been correlated with academic success—meaning that individuals at higher academic levels usually have higher IQs. So, even without knowing their IQ scores, the expectation that donors in the Doctorate category have higher IQs than other available sperm donors is reasonable.

The average correlation between IQs of parents and their children is 0.42— in other words, 42% of IQ variation among individuals is apparently the result of their genes. However, children are typically raised in a similar social and economic environment as their parents. Does the IQ correlation above still clearly tell us the role of genes in determining IQ? Since parents and children are similar in genes and environment, comparisons between the two groups do not allow researchers to definitively determine the relative importance of each factor on a given trait. This is the problem found in most arguments about "nature versus nurture"—do children resemble their parents because they are "born that way," or because they are "raised that way"?

The impossibility of using traditional selection studies, and the difficulty of separating genetic and environmental influences in most families compels researchers interested in the heritability of traits in humans to use natural experiments. These are situations in which unique circumstances allow a hypothesis test without prior intervention by researchers. Human twins are one source of a natural experiment to test hypotheses about the heritability of quantitative traits in humans.

Estimating the Heritability of Intelligence By comparing monozygotic twins to dizygotic twins, researchers can begin to separate the effects of shared genes from the effects of shared environments. Twins raised in the same family presumably have similar childhood experiences, unlike nontwin siblings, whose social and family environments differ. Any unique social factors that may be associated with being a twin are probably common to both identical and non-identical twins. Thus, some scientists argue that the only real difference between monozygotic and dizygotic twins is the percentage of genes they share. Recall that monozygotic twins share all of their alleles, while dizygotic twins share, on average, only 50% of their alleles.

Using a formula that includes the correlation of IQ between pairs of monozy-gotic twins, the correlation of IQ between pairs of dizygotic twins, and relative genetic similarity, the average heritability of IQ calculated from a number of different studies is about 0.52. According to these studies, 52% of the variability in IQ among humans is due to differences in genotypes. It is somewhat surprising that this value is even higher than the 42% calculated from the correlation between parents and children.

However, the heritability value arrived at through twin studies has been criticized by other scientists. One major criticism is that monozygotic twins and dizygotic twins do differ in more than just genotype. In particular, identical twins are treated more alike than nonidentical twins. This occurs both because of the greater similarity in appearance of monozygotic twins, and because of the expectation by parents, relatives, friends, and teachers that identical twins are identical in all respects. In fact, a common way scientists determine whether a twin pair is monozygotic is to ask the twins if other people often have trouble telling them apart or comment on their similarities. If monozygotic twins are expected to be more alike than dizygotic twins, their IQ scores may be similar because they are continually encouraged to have the same experiences and to achieve at the same level.

Since this difference in treatment of the types of twins cannot be eliminated, researchers studying the heritability of IQ are especially interested in twins that have been raised apart. By comparing identical twins raised in different environments with nonidentical twins that have also been raised apart, the problem of differential treatment of the two types of twins is minimized because no one would know that the individual members of a pair have a twin.

The frequency of early twin separation is extremely rare and is becoming even rarer as the policy of keeping adoptive siblings together has become more standard. This infrequency makes the few identical-twin pairs known to be separated especially valuable. Researchers have estimated the heritability of IQ at a remarkable 0.72 in this small sample of twins raised apart. These studies support the hypothesis that differences in our genes explain much of the variation in IQ among people. Table 4.1 summarizes the estimates of IQ heri-tability and previews the cautions discussed in the next section of this chapter.

0 0

Post a comment