16.1 Genes and Variation PDF

This group of ladybug beetles illustrates a population with a number of inherited traits. Darwin recognized such variation as the raw material for evolution. Inquiry Activity Does sexual reproduction change genotype ratios? Think About It 1. Calculating What was the genotype ratio of the Procedure 2...

This group of ladybug beetles illustrates a population with a number of inherited traits. Darwin recognized such variation as the raw material for evolution. Inquiry Activity Does sexual reproduction change genotype ratios? Think About It 1. Calculating What was the genotype ratio of the Procedure 2. Comparing and Contrasting Was the genotype ratio the same as the 1 : 2 : 1 genotype ratio for a cross between two heterozygotes (Aa x Aa)? Explain. 1. Put 33 red and 67 black beads in a large paper cup to represent two alleles of a certain gene in a population. 2. To model the genotype of an offspring, remove two beads. Record the genotype. Return the beads. 3. Repeat step 2 for a total of 1 0 offspring. Add your data to the class total. offspring? 3. Predicting If you repeated this activity over and over, would you expect the genotype ratios to change? Explain. 16-1 Genes and Variation 7 3.a. Students know both genetic variation and environmental factors are causes of evolution and diversity of organisms. Bl 7.c. Students know new mutations are constantly being generated in a gene pool. Bl 7.d. Students know variation within a species increases the likelihood that at least some members of a species will survive under changed environmental conditions. A s Darwin developed his theory of evolution, he worked under a serious handicap. He didn't know how heredity worked! Although Mendel's work on inheritance in peas was published during Darwin's lifetime, its importance wasn't recognized for decades. This lack of knowledge left two big gaps in Darwin's thinking. First, he had no idea how heritable traits pass from one generation to the next. Second, although variation in heritable traits was central to Darwin's theory, he had no idea how that variation appeared. Evolutionary biologists connected Mendel's work to Darwin's during the 1930s. By then, biologists understood that genes control heritable traits. They soon realized that changes in genes produce heritable variation on which natural selection can operate. Genes became the focus of new hypotheses and experiments aimed at understanding evolutionary change. Another revolution in evolutionary thought began with Watson and Crick's studies on DNA. Their model of the DNA molecule helped evolutionary biologists because it demonstrated the molecular nature of mutation and genetic variation. Today, molecular techniques are used to test hypotheses about how heritable variation appears and how natural selection operates on that variation. As you willleam in this chapter, fitness, adaptation, species, and evolutionary change are now defined in genetic terms. We understand how evolution works better than Darwin ever could, beginning with heritable variation. Guide for Reading llf'J------Key Concepts. What are the main sources of heritable variation in a population? How is evolution defined in genetic terms? What determines the numbers of phenotypes for a given trait? Vocabulary gene pool relative frequency single-gene trait polygenic trait Reading Strategy: Building Vocabulary Before you read, make a list of the vocabulary terms above. As you read, take notes about the meaning of each term. How Common Is Genetic Variation? We now know that many genes have at least two forms, or alleles. Animals such as horses, dogs, and mice often have several alleles for traits such as body size or coat color. Plants, such as peas, often have several alleles for flower color. All organisms have additional genetic variation that is "invisible" because it involves small differences in biochemical processes. In addition, an individual organism is heterozygous for many genes. An insect may be heterozygous for as many as 15 percent of its genes. Individual fishes, reptiles, and mammals are typically heterozygous for between 4 and 8 percent of ~heir genes. T Figure 16-1 ~ There are two main sources of genetic variation: mutations and the gene shuffling that results from sexual reproduction. Each of these babies has inherited a collection of traits. Some, such as hair color, are visible, while others, such as the ability to resist certain diseases, are not. Sample Population Frequency of Alleles allele for Variation and Gene Pools Genetic variation is studied in populations. A population is a group of individuals of the same species that interbreed. Because members of a population interbreed, they share a common group of genes called a gene pool. A gene pool consists of all genes, including all the different alleles, that are present in a population. The relative frequency of an allele is the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Relative frequency is often expressed as a percentage. For example, in the mouse population in Figure 16-2, the relative frequency of the dominant B allele (black fur) is 40 percent, and the relative frequency of the recessive b allele (brown fur) is 60 percent. The relative frequency of an allele has nothing to do with whether the allele is dominant or recessive. In this particular mouse population, the recessive allele occurs more frequently than the dominant allele. Gene pools are important to evolutionary theory, because evolution involves changes in populations over time. ~ In genetic terms, evolution is any change in the relative frequency of alleles in a population. For example, if the relative frequency of the B allele in the mouse population changed over time to 30 percent, the population is evolving. _. Figure 16-2 When scientists determine whether a population is evolving, they may look at the sum of the population's alleles, or its gene pool. This diagram shows the gene pool for fur color in a population of mice. Calculating Here, in a total of 50 alleles, 20 alleles are 8 (black), and 30 are b (brown). How many of each allele would be present in a total of 100 alleles? For: Links on population genetics Visit: www.Scilinks.org Web Code: cbn-5161 394 Chapter 16 Cf iNKS Sources of Genetic Variation Biologists can now explain how variation is produced. ~ The two main sources of genetic variation are mutations and the genetic shuftling that results from sexual reproduction. Mutations A mutation is any change in a sequence of DNA. Mutations can occur because of mistakes in the replication of DNA or as a result of radiation or chemicals in the environment. Mutations do not always affect an organism's phenotype. For example, a DNA codon altered from GGA to GGU will still code for the same amino acid, glycine. That mutation has no effect on phenotype. Many mutations do produce changes in phenotype, however. Some can affect an organism's fitness, or its ability to survive and reproduce in its environment. Other mutations may have no effect on fitness. Gene Shuffling Mutations are not the only source of heritable variation. You do not look exactly like your biological parents, even though they provided you with all your genes. You probably look even less like any brothers or sisters you may have. Yet, no matter how you feel about your relatives, mutant genes are not primarily what makes them so different from you. Most heritable differences are due to gene shuffling that occurs during the production of gametes. Recall that each chromosome of a homologous pair moves independently during meiosis. As a result, the 23 pairs of chromosomes found in humans can produce 8.4 million different combinations of genes! Another process, crossing-over, also occurs during meiosis. Crossing-over further increases the number of different genotypes that can appear in offspring. Recall that a genotype is an organism's genetic makeup. When alleles are recombined during sexual reproduction, they can produce dramatically different phenotypes. Thus, sexual reproduction is a major source of variation within many populations. Sexual reproduction can produce many different phenotypes, but it does not change the relative frequency of alleles in a population. To understand why, compare a population's gene pool to a deck of playing cards. Each card represents an allele found in the population. The exchange of genes during gene shuffling is similar to shuffling a deck of cards. Shuffling leads to different types ofhands, but it can never change the relative numbers of aces, kings, or queens in the deck. The probability of drawing an ace off the top of the deck will always be 4 in 52, or one thirteenth (4/52 = 1/13). No matter how many times you shuffle the deck, this probability will remain the same. Similarly, sexual reproduction produces many different combinations of genes, but in itself it does not alter the relative frequencies of each type of allele in a population. Single-Gene and Polygenic Traits Word Origins Gene comes from the Greek word gignesthai, meaning "to be born," and refers to factors that produce an organism. The prefix polycomes from the Greek word polys, meaning "many," so polygenic means "having many genes." The prefix mono- means "one." What do you think the term monogenic means? Figure 16-3 In humans, a single gene with two alleles controls whether a person has a widow's peak (left) or does not have a widow's peak (right). As a result, only two phenotypes are possible. ~ The number of phenotypes a given trait has is determined by how many genes control the trait. Heritable variation can be expressed in a variety of ways. The number of phenotypes produced for a given trait depends on how many genes control the trait. Among humans, a widow's peak-a downward dip in the center of the hairline-is a single-gene trait. It is controlled by a single gene that has two alleles. The allele for a widow's peak is dominant over the allele for a hairline with no peak. As a result, variation in this gene leads to only two distinct phenotypes, as shown in Figure 16-3. Single-Gene Trait As you can see, the frequency of phenotypes caused by this single gene is represented on the bar C1) a. 100 graph. This graph shows that the presence of a ~ 80 s::::: widow's peak may be less common in a population C1).c than the absence of a widow's peak, even though the c.- 60 allele for a widow's peak is the dominant form. In 0~ >-- 40 real populations, phenotypic ratios are determined u s::::: C1) by the frequency of alleles in the population as well 20 ::I cas by whether the alleles are in the dominant or ~ LL 0 recessive form. Allele frequencies may not match Widow's peak No widow's peak Mendelian ratios. Phenotype Evolution of Populations 395 Figure 16-4 The graph below shows the distribution of phenotypes that would be expected for a trait if many genes contributed to the trait. The photograph shows the actual distribution of heights of a group of young men. Using Tables and Graphs What does the shape of the graph indicate about height in humans? 0 > u c: CD ::s C"...CD u. - - Phenotype (height)~ 1. ~ Key Concept In genetic terms, what indicates that evolution is occurring in a population? 2. 3. ~ Key Concept What two processes can lead to inherited variation in populations? ~ Key Concept How does the range of phenotypes differ between single-gene traits and polygenic traits? 396 Chapter 16 Many traits are controlled by two or more genes and are, therefore, called polygenic traits. Each gene of a polygenic trait often has two or more alleles. As a result, one polygenic trait can have many possible genotypes and phenotypes. Height in humans is one example of a polygenic trait. You can sample phenotypic variation in this trait by measuring the height of all the students in your class. You can then calculate the average height of this group. Many students will be just a little taller or shorter than average. Some of your classmates, however, will be very tall or very short. If you graph the number of individuals of each height, you may get a graph similar to the one in Figure 16-4. The symmetrical bell-like shape of this curve is typical of polygenic traits. A bell-shaped curve is also called a normal distribution. 4. What is a gene pool? How are allele frequencies related to gene pools? 5. Critical Thinking Evaluating Evaluate the significance of mutations to the process of biological evolution. (Hint: How does mutation affect genetic variation?) Information and Heredity How does the process known as independent assortment relate to the genetic variation that results from sexual reproduction? Hint: Refer to Chapter 11. Name____________________________ Class __________________ Date __________ Gene Pools A homozygous black mouse has two alleles for black fur. A heterozygous black mouse has one allele for black fur and one allele for brown fur. A homozygous brown mouse has two alleles for brown fur. Sample Population 12 individuals heterozygous black 4 individuals homozygous black 9 individuals homozygous brown Each rectangle represents one mouse. Each mouse has two alleles, represented by circles, for fur color. Use the graph to color the gene pool of the sample population. Color alleles for black fur black and alleles for brown fur brown. Gene Pool Use the diagram to answer the questions. 1. How many black alleles are in the gene pool? 2. How many brown alleles are in the gene pool? © Pearson Education, Inc., publishing as Pearson Prentice Hall. 146 Name____________________________ Class __________________ Date __________ Polygenic Traits Follow the prompts to make a graph showing the frequency of different heights in a group of students. Draw one bar for each height range. The bar should show how many students have heights in that range. Draw a curve connecting the tops of the bars. Height in cm 155–159 160–164 165–169 170–174 175–179 180–184 185–189 190–194 Number of Students 1 2 6 10 10 6 2 1 Student Heights Number of Students 12 10 8 6 4 2 0 155–159 160–164 165–169 170–174 175–179 180–184 185–189 Height (cm) Use the graph to answer the questions. 1. What shape is the curve you drew? 2. What type of trait is height? Circle the correct answer. single-gene polygenic © Pearson Education, Inc., publishing as Pearson Prentice Hall. 147 190–194