Genetics & Patterns of Inheritance Reading PDF

MODULE 10 Introduction to Genetics and Patterns of Inheritance 1 Mendelian Genetics BEFORE YOU READ...

MODULE 10 Introduction to Genetics and Patterns of Inheritance 1 Mendelian Genetics BEFORE YOU READ WHAT YOU’LL LEARN Think about what you have learned about the scientific method. the law of segregation and On the lines below, list some of the steps Mendel might have used the law of independent assortment to learn about the natural world. In this lesson, you will learn about Gregor Mendel’s experiments. how to use a Punnett square READ TO LEARN ⊳ FOCUS How Genetics Began As you read this lesson, highlight any parts you do not Gregor Mendel, an Austrian Monk, lived in the 1800s. He understand. After you have experimented with pea plants in the monastery gardens. read the lesson, reread the parts you have highlighted. Pea plants usually reproduce by self-fertilization. This means that the female gamete is fertilized by a male gamete in the same Copyright © McGraw-Hill Education flower. Mendel discovered a way to cross-pollinate peas by hand. He removed the male gametes from a flower. He then fertilized the flower with the male gamete from a different flower. Through these experiments, Mendel made several hypotheses Get It? about how traits are inherited. In 1866, he published his findings. 1. Infer why it is important that Mendel’s experiments used That year marks the beginning of the science of genetics, the a true-breeding plant. science of heredity. Mendel is called the father of genetics. The Inheritance of Traits Mendel used true-breeding pea plants—plants whose traits stayed the same from generation to generation. Mendel studied seven traits—flower color, seed color, seed pod color, seed shape, seed pod shape, stem length, and flower position. Reading Essentials Introduction to Genetics and Patterns of Inheritance 113  What did Mendel find when he crossed pea plants with different traits? Mendel called the original plants the parent, or P, generation. The offspring were called the F1 generation. The offspring of the F1 plants were called the F2 generation. In one experiment, Mendel crossed yellow-seeded and green- seeded plants. All the F1 offspring had yellow seeds. The green- seed trait seemed to disappear. Mendel allowed the F1 plants to self-fertilize. He planted thousands of seeds from these plants. He saw that in these offspring, the F2 generation, three-fourths of the plants had yellow seeds and one-fourth had green seeds, a 3:1 ratio. Mendel performed similar experiments for other traits. Each time, he observed the same 3:1 ratio. TAKE A LOOK 2. Label Fill in the boxes with the name of each x generation of offspring. P Draw the peas you would expect to see in the empty green-seed plant yellow-seed plant pods. Use shading to indicate a green pea. x all yellow seeds 3/4 yellow seeds Copyright © McGraw-Hill Education 1/4 green seeds How did Mendel explain his results? Mendel proposed that there were two forms of each trait, and C10_002A_896205 THINK IT OVER each form was controlled by a factor, which is now called an 3. Apply In Mendel’s allele. An allele (uh LEEL) is a different form of a gene passed experiment with green and from generation to generation. Yellow-seed plants have a different yellow seeds, what was the allele than green-seed plants. dominant trait? Mendel proposed that each trait was controlled by two alleles. The dominant form is the version of the trait that appears in the F1 generation. The recessive form is the version that is hidden in the F1 generation. Reading Essentials Introduction to Genetics and Patterns of Inheritance 114  How does dominance work? When written, the dominant allele is represented by a capital letter. The recessive allele is represented by a lowercase letter. THINK IT OVER An organism is homozygous (hoh muh ZI gus) if both alleles 4. Predict What would be the for a trait are the same. The organism is heterozygous (heh tuh phenotype of a homozygous, recessive (yy) roh ZY gus) if the alleles for a trait are different. In heterozygous pea plant? organisms, only the dominant trait can be seen. Dominant alleles mask recessive alleles. How do genotype and phenotype differ? It is not always possible to know what alleles are present just by Get It? looking at an organism. A yellow-seed plant could be 5. Infer whether an individual homozygous (YY) or heterozygous (Yy). An organism’s allele pairs with a recessive phenotype for a trait is heterozygous or are called its genotype (JEE nuh tipe). The expression of an allele homozygous for the that trait. pair, or the way an organism looks or behaves, is called its phenotype (FEE nuh tipe). What is the law of segregation? Recall that the chromosome number is divided in half during meiosis. The gametes contain only one of the alleles. Mendel’s law of segregation states that the two alleles for each trait separate from each other during meiosis and then unite during fertilization. When parents with different forms of a trait are Get It? 6. Restate Mendel’s law of crossed, the offspring are heterozygous organisms known as segregation in your own hybrids (HI brudz). words. A cross which involves hybrids for a single trait is called a monohybrid cross. Mono means one. The offspring of the cross have a phenotypic ratio of 3:1. How are two or more traits inherited? Mendel also performed dihybrid crosses, crossing plants that Copyright © McGraw-Hill Education expressed two different traits. Mendel crossed yellow, round-seed plants with green, wrinkle-seed plants. Round seeds are dominant to wrinkled, just as yellow color is dominant to green. Get It? He wondered whether the two traits would be inherited together 7. Evaluate How can the or separately. Members of the F1 generation are dihybrids because random distribution of alleles result in a they are heterozygous for both traits. predictable ratio? Mendel found that the traits were inherited independently. Members of the F2 generation had the phenotypic ratio of 9:3:3:1—9 yellow round seeds, 3 green round, 3 yellow wrinkled, and 1 green wrinkled. From experiments with dihybrid crosses, Mendel developed the law of independent assortment, which states that alleles distribute randomly when gametes are made. Reading Essentials Introduction to Genetics and Patterns of Inheritance 115  Punnett Squares 1 Place the male alleles In the early 1900s, Dr. Reginald Punnett along developed a square to the top. predict possible offspring of a cross between two known 2 Place the genotypes. Punnett squares are useful for keeping track of female alleles THINK IT OVER genotypes in a cross. along the side. 8. Identify What is one purpose of a Punnett What information does a Punnett square contain? square? A Punnett square can help you predict the genotype and phenotype of the offspring. The genotype of one parent is written vertically, on the left side of the Punnett square. The genotype of the other parent is written horizontally, across the top. A Punnett square for a monohybrid cross contains four small squares. Each small square represents a possible combination of alleles in the children. The Punnett square below shows the results of Mendel’s experiment with seed color. 3 Copy The female Punnett alleles across square shows that four different genotypeseacharerow. possible—one Copy male allelesYY, two Yy, and one yy. down each column. Always The genotypic ratio is 1:2:1. list the dominant trait first. TAKE A LOOK 9. Define Circle the genotypes in the small squares that will give a yellow-seed phenotype. What will be the phenotypic ratio in the offspring? How is a Punnett square used for two traits? Copyright © McGraw-Hill Education Punnett squares also can be used to predict the results of a dihybrid cross. A Punnett square for a dihybrid cross is larger. It has 16 boxes to represent 16 allele combinations. PT-12A-MSS12 Probability Genetics follows the rules of probability, or chance. It is like flipping a coin. The probability of flipping heads is one out of two. Because of chance, if you flip a coin 100 times, it might not land heads exactly 50 times, but it will be close. It is the same in genetics. A cross might not give a perfect 3:1 or 9:3:3:1 ratio. The larger the number of offspring, the more closely the results will match the ratio predicted by the Punnett square. Reading Essentials Introduction to Genetics and Patterns of Inheritance 116  2 G  enetic Recombination and Gene Linkage BEFORE YOU READ WHAT YOU’LL LEARN Genetics is like a game of cards. In meiosis, chromosomes are how meiosis produces shuffled and sorted. On the lines below, explain the chances of a genetic recombination player getting the same cards two games in a row. In this lesson, how gene linkage is used to you will learn about the independent assortment of chromosomes make chromosome maps that occurs during meiosis. why polyploidy is important READ TO LEARN ⊳ FOCUS Genetic Recombination Highlight the main ideas under each heading. State each main During meiosis, genes are combined in new ways. Genetic point in your own words. recombination occurs when crossing over and independent assortment produce new combinations of genes. Recall that independent assortment occurs in meiosis when chromosomes separate randomly. The number of possible gene combinations due to independent assortment can be calculated using the formula 2n, where n equals the number of chromosome pairs. Pea plants have 7 pairs of chromosomes. The possible APPLYING MATH Copyright © McGraw-Hill Education combinations of these chromosomes would be 27, or 128. 1. Calculate The fruit fly has four chromosome pairs. Fertilization further increases the number of combinations. How many possible During fertilization, any possible male gamete can fertilize any combinations of possible female gamete. The number of combinations after chromosomes can be fertilization would be 2n × 2n. For peas, this number is 16,384, or produced by meiosis and fertilization? 128 × 128. In people, the possible combinations of chromosomes are 2 × 223—over 70 trillion. Crossing over increases genetic 23 recombination even more. Reading Essentials Introduction to Genetics and Patterns of Inheritance 117 Get It? 2. Analyze the effect of Gene Linkage crossing over on linked Chromosomes contain many genes. Genes that are located genes. close together on the same chromosome are said to be linked. This means they usually travel together during gamete formation. Linked genes do not segregate independently. They are an exception to Mendel’s law of independent assortment. Occasionally, linked genes separate due to crossing over. Crossing over occurs more frequently between genes that are far apart than between genes that are close together. THINK IT OVER What does a chromosome map show? 3. Explain What event causes linked genes to separate? The relationship between crossing over and chromosome distance is very useful. The distance between two genes can be estimated by the frequency of crossing over that occurs between them. Scientists use cross-over data to create a drawing of genes along a chromosome. The drawing, called a chromosome map, shows the order of genes on a chromosome. The first TAKE A LOOK chromosome maps were published in 1913 for fruit-fly crosses. 4. Identify Which two genes are not likely to cross over? One is shown in the figure below. (Circle your answer.) a. yellow body color and vermilion eye color b. white eye color and vermilion eye color yellow white vermilion miniature rudimentary body eyes eyes wings wings THINK IT OVER 5. Identify Name two y w v m r organisms that have polyploidy. Copyright © McGraw-Hill Education Polyploidy Most organisms have diploid cells—cells with two chromosomes in each cell. Some species have polyploid cells. Polyploidy (PA lih ploy dee) means the cells have one or more extra sets of all Get It? chromosomes. For instance, a triploid organism has three 6. Explain why plant growers often select for polyploid complete sets of chromosomes in each cell. It is designated 3n. plants. Polyploidy occurs in only a few animals, such as earthworms and goldfish. It is always lethal in humans. Polyploidy is common in flowering plants. Polyploid plants are often bigger and more vigorous. Many food plants, such as wheat (6n), oats (6n), and sugarcane (8n), are polyploid. Reading Essentials Introduction to Genetics and Patterns of Inheritance 118  3 Applied Genetics BEFORE YOU READ WHAT YOU’LL LEARN Imagine that you could design the perfect dog. What color would how inbreeding differs from it be? Would it be big or small? On the lines below, describe the hybridization traits your dog would have. In this lesson, you will learn how how to use test crosses and selective breeding produces certain traits. a Punnett square to find the genotypes of organisms READ TO LEARN ⊳ FOCUS Selective Breeding After you read this lesson, create a five-question quiz For thousands of years, people have been breeding animals from what you have learned. Then, exchange quizzes with and plants to have certain traits. For instance, some dogs, such another student. After taking as huskies, have been bred to be strong runners. Other dogs, the quizzes, review your such as Saint Bernards, have been bred to have a good sense of answers together. smell. People have also bred plants, such as tomatoes, apples, and roses, to taste better, resist disease, or produce fragrant flowers. Selective breeding is the process used to breed animals and plants to have desired traits. As a result of selective breeding, desired traits become more common. What is hybridization? A hybrid is an organism whose parents each have different Copyright © McGraw-Hill Education forms of a trait. For instance, a disease-resistant tomato plant can be crossed with a fast-growing tomato plant. The offspring THINK IT OVER of the cross would be a tomato plant that has both traits. 1. Name an advantage The hybrid is disease resistant and grows quickly. of hybridization. Hybridization is the process of making a hybrid organism. Hybridization is expensive and takes a long time, but it is a good way to breed animals and plants with the right combination of traits. Reading Essentials Introduction to Genetics and Patterns of Inheritance 119 Get It? How is inbreeding used? 2. Describe the disadvantages Inbreeding is another example of selective breeding. associated with hybridization and inbreeding. Inbreeding occurs when two closely related organisms that both display the desired trait are bred. Inbreeding can be used to ensure that the desired trait is passed on. Inbreeding can also eliminate traits that are not desired. Purebred animals are created by inbreeding. Clydesdale horses are an example of a purebred animal. Clydesdale horses were first bred in Scotland hundreds of years ago. They were bred for use as farm horses that could pull heavy loads. All Clydesdales have the traits of strength, agility, and obedience. A disadvantage of inbreeding is that harmful traits can be passed on. Harmful traits are usually carried on recessive THINK IT OVER genes. Both parents must pass on the recessive genes for the 3. Explain What is the harmful traits to appear in the offspring. Inbreeding increases purpose of a test cross? the chance that both parents carry the harmful traits. Test Cross Breeders need a way to determine the genotype of the organisms they want to cross before creating a hybrid. They use test crosses to find out the genotype of an organism. In a test cross, an organism whose genotype for a desired trait is unknown is crossed with an organism that has two recessive genes for the trait. TAKE A LOOK 4. Label Fill in the phenotype When are test crosses performed? with the word white or red An orchard owner might use a test cross to find out the for each genotype. genotype of a white-grapefruit tree. In grapefruits, white color is a dominant trait and red color is a recessive trait. A red-grapefruit tree has two recessive genes (ww). A white- Copyright © McGraw-Hill Education grapefruit tree might have two dominant genes (WW), or it might have one dominant gene and one recessive gene (Ww). Genotype Phenotype Homozygous dominant (WW) Homozygous recessive (ww) Heterozygous (Ww) Reading Essentials Introduction to Genetics and Patterns of Inheritance 120  How does a test cross reveal the genotype? TAKE A LOOK The orchard owner decides to do a test cross to find out the 5. Evaluate If you planted 100 seeds from this test cross, genotype of a white grapefruit tree. The white grapefruit tree is about how many would crossed with a red grapefruit tree. The orchard owner uses a be white? How many would Punnett square to understand the results of the cross. be red? The figure below shows a Punnett square for the test cross if the white grapefruit tree is homozygous, meaning it has two dominant genes (WW) for white fruit. All the offspring from the test cross will be heterozygous, meaning they will have one dominant and one recessive gene (Ww). All the offspring of the test cross are white grapefruit trees. Homozygous white grapefruit W W red grapefruit w Ww Ww Homozygous w Ww Ww What if the test cross involved a heterozygous tree? The figure below shows a Punnett square for the test cross if the white grapefruit tree is heterozygous (Ww). Half the offspring from the test cross will be white (Ww). Half the TAKE A LOOK offspring from the test cross will be red (ww). 6. Calculate If you planted Copyright © McGraw-Hill Education Heterozygous 100 seeds from this test white grapefruit cross, about what W w percentage would be white? What percentage would be red? red grapefruit w Ww ww Homozygous w Ww ww Reading Essentials Introduction to Genetics and Patterns of Inheritance 121  4 Basic Patterns of Human Inheritance WHAT YOU’LL LEARN BEFORE YOU READ how to determine if an A family tree shows how people in a family are related. On the inherited trait is dominant or lines below, list people who might appear in a family tree. Then recessive read the lesson to learn how scientists trace inheritance through examples of dominant and several generations of a family. recessive disorders FOCUS ⊲ READ TO LEARN After you read this lesson, create a quiz based on what Pedigrees you have learned. Then be Review the table below and recall that a recessive trait is sure to answer the quiz questions. expressed when the person is homozygous recessive for that trait. A person with at least one dominant allele will not express the recessive trait. A person who is heterozygous for a recessive disorder is called a carrier. TAKE A LOOK Term Description 1. Identify Circle the term that describes the genotype of a Homozygous An organism with two of the same alleles for person who expresses a a particular trait is said to be homozygous for recessive trait. that trait. Heterozygous An organism with two different alleles for a particular trait is said to be heterozygous for that trait. When alleles are present in the heterozygous state, the dominant trait will be observed. Copyright © McGraw-Hill Education Scientists use a diagram called a pedigree to trace inheritance of a trait through several generations. A pedigree uses symbols to illustrate inheritance of the trait. A sample pedigree is shown in the figure on the next page. In the top row, the two symbols connected by a horizontal line are the parents. Their children are listed below them, oldest to youngest from left to right. Reading Essentials Introduction to Genetics and Patterns of Inheritance 122  Roman numerals are used to represent generations—I for the parents, II for the children, and so on. Arabic numbers are used to represent the individuals within a generation. Key to Symbols Example Pedigree Normal female Normal male I Female who expresses Male who expresses 1 2 the trait being studied the trait being studied Female who is a carrier Male who is a carrier for the particular trait for the particular trait II 1 2 3 4 Generation Roman numerals — Generations Parents Arabic numerals — Individuals in a certain generation Siblings Analyzing Pedigrees TAKE A LOOK 2. Calculate What percentage C10_024A-145262 The figure below is a pedigree showing the inheritance of the children in this family of Tay-Sachs disease, a recessive disorder. The pedigree shows inherited Tay-Sachs that two parents who do not have Tay-Sachs disease can have disease? a child who has the disorder. Carriers for Tay-Sachs I 1 2 Get It? 3. Explain how symbols are II used to represent 1 2 3 4 individuals in a pedigree. Tay-Sachs How is the inheritance of a dominant disorder shown on a pedigree? Copyright © McGraw-Hill Education The pedigree below shows the inheritance of the dominant C10_025A-145262 disorder, polydactyly (pah lee DAK tuh lee)—extra fingers and toes. A person with dominant disorders could be homozygous or heterozygous for the trait. A person who does not have polydactyly would be homozygous recessive for the trait. I 1 2 II TAKE A LOOK 1 2 3 4 5 6 7 4. Identify Do any of the grandchildren in this family have polydactyly? III 1 2 Reading Essentials Introduction to Genetics and Patterns of Inheritance 123 Get It? How are genotypes deduced? 5. Analyze What can be A pedigree can be used to learn the genotype of a person. determined about the genotypes of the parents of The genotype is determined by observing the phenotypes, an individual who expresses or physical traits, of a person. a recessive trait? Genetic counselors use pedigrees to determine if an inherited trait is dominant or recessive. Dominant traits are easy to recognize. Recessive traits are more difficult because people who carry the allele do not always show the trait. Can genetic disorders be predicted? Scientists can use pedigrees to predict whether a person in a family will get a genetic disorder. Scientists have to follow several people for many generations to accurately study a disorder. Good record keeping within a family can help scientists predict the inheritance of a disorder. Types of Recessive Genetic Disorders What causes albinism? Make a vocabulary book Albinism is a recessive disorder found in people and animals. and label each tab with the In humans, it is caused by the absence of the skin pigment name of a different genetic melanin in hair and eyes. People with albinism have white hair, disorder. Use it to organize your notes on genetic pale skin, and pink eyes. They need to protect their skin from the disorders. Sun’s ultraviolet rays. What causes galactosemia? Galactosemia (guh lak tuh SEE mee uh) is a recessive genetic disorder. It causes intolerance of the sugar galactose. Milk contains the sugar lactose. During digestion, lactose breaks down into galactose and glucose, the sugar used by the body for energy. Copyright © McGraw-Hill Education People with galactosemia lack the enzyme needed to break down galactose. C10_002A-145262 THINK IT OVER What is cystic fibrosis? 6. Explain Why are recessive traits difficult to study? Cystic fibrosis is a recessive genetic trait. Most chloride ions are not absorbed into cells but are excreted in sweat. Without the chloride ions in cells, very little water diffuses from cells. This causes the mucus secreted by many areas of the body to be abnormally thick. The mucus interferes with digestion, clogs ducts in the pancreas, and blocks air pathways in the lungs. Patients with cystic fibrosis often get infections because of excess mucus in their lungs. Treatment includes physical therapy, medicine, special diets, and replacement digestive enzymes. Genetic tests can determine if the recessive gene is present. Reading Essentials Introduction to Genetics and Patterns of Inheritance 124  What is Tay-Sachs disease? THINK IT OVER Tay-Sachs (TAY saks) disease is a recessive genetic disorder. 7. Explain Why do gangliosides build up Tay-Sachs disease (TSD) is more common among Jews whose in the brain of people with ancestors are from eastern Europe. Tay-Sachs disease? People with TSD are missing an enzyme needed to break down fatty acids called gangliosides. Normally, gangliosides are made and then destroyed as the brain develops. In people with TSD, gangliosides build up in the brain, causing mental deterioration. Children born with TSD usually die by age five. Currently, there is no cure. Dominant Genetic Disorders Not all genetic disorders are recessive. Some are caused by dominant alleles. People who do not have the disorder are always homozygous recessive, meaning they carry two recessive genes for the trait. What happens in Huntington’s disease? Get It? Huntington’s disease is a dominant genetic disorder that affects 8. Compare the chances of inheriting a dominant the nervous system. It is rare. Symptoms occur when the person disorder to the chances is between 30 and 50 years old. Symptoms are gradual loss of of inheriting a recessive brain function, uncontrollable movements, and emotional disorder if you have one disturbances. Genetic tests can tell people whether they have the parent with the disease. gene for Huntington’s disease, but there is currently no treatment or cure. What is achondroplasia? Achondroplasia (a kahn droh PLAY zhee uh) is a dominant genetic disorder that is also known as dwarfism. People with this disorder have a small body size and short limbs. They grow to an THINK IT OVER Copyright © McGraw-Hill Education adult height of about 1.2 m (4 ft). 9. Explain How can scientists determine if achondroplasia About 75 percent of people with achondroplasia have parents developed from a new of average size. Because the gene is dominant, parents who are mutation? average size do not have the gene. Therefore, when average-sized parents have a child with achondroplasia, the condition occurs because of a new mutation. Reading Essentials Introduction to Genetics and Patterns of Inheritance 125  5 Complex Patterns of Inheritance WHAT YOU’LL LEARN BEFORE YOU READ the difference between sex- Cats can look different from one another because of differences in linked and sex-limited their coats. On the lines below, describe differences you have seen inheritance in the coats of cats. Then read the lesson to learn more about how environment can complex inheritance patterns. influence a trait FOCUS ⊲ READ TO LEARN Highlight each question head. Then highlight the answer Incomplete Dominance to the question. Not all traits follow Mendel’s rules. Some traits are not dominant or recessive. Sometimes, the heterozygous organism has a mixed phenotype. Incomplete dominance occurs when the heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes. An example of incomplete dominance occurs in snapdragon flowers. Red-flowered snapdragons (CR CR) can be crossed with white-flowered snapdragons (CW CW) to produce offspring with pink flowers (CR CW). When heterozygous F1 generation snapdragon plants (CR CW) self-fertilize, the offspring have a 1:2:1 ratio of red, pink, and white flowers. Copyright © McGraw-Hill Education THINK IT OVER Codominance 1. Define What is In Mendel’s experiments with pea plants, heterozygous pea codominance? plants expressed only the dominant allele. Codominance occurs when a heterozygous organism expresses both alleles. Sickle-cell anemia is an example of codominance. People who are heterozygous for the sickle-cell trait have both normal and sickle- shaped cells. Reading Essentials Introduction to Genetics and Patterns of Inheritance 126  What happens in sickle-cell disease? Sickle-cell disease is common in people of African descent. Sickle-cell disease affects red blood cells and their ability to transport oxygen. Changes in the protein in red blood cells cause those red blood cells to change from a normal disc shape to a sickle or C shape. THINK IT OVER Sickle-cell disease is a codominant trait. People who are 2. Describe What effect does heterozygous for the trait make both normal and sickle-shaped sickle-cell disease have on red blood cells? cells. The normal cells compensate for the sickle-shaped cells. How does sickle-cell disease relate to malaria? Sickle-cell disease is found in areas of Africa where malaria occurs. Scientists have discovered that people who are heterozygous for the sickle-cell trait are resistant to malaria. Because the sickle-cell gene helps people resist malaria, they are more likely to pass the sickle-cell trait on to their offspring. Get It? 3. Explain how the genetic Multiple Alleles traits carried on multiple So far you have learned about traits that result from a gene with alleles can lead to a wide range of characteristics in two alleles. Some traits are controlled by a gene that has multiple humans. alleles. Blood groups in humans is an example of a multiple allele trait. How are blood types produced? There are four blood types in people: A, AB, B, or O. The four types result from the interaction of three different alleles, as shown below. The allele IA produces blood type A. IB produces blood type B. The allele i is recessive and produces blood type O. Type O is the absence of AB alleles. People with one IA and one IB allele have blood type AB. Blood types are Copyright © McGraw-Hill Education examples of multiple alleles and codominance. Rh factors are also in blood. One factor is inherited from each parent. Rh factors are either positive or negative (Rh+ or Rh−); the Rh+ is dominant. Genotypes Resulting Phenotypes TAKE A LOOK 4. Evaluate What phenotype IAIA Type A IAi Type A results from a genotype IBIB Type B of IBi? IBi Type B IAIB Type AB ii Type O Reading Essentials Introduction to Genetics and Patterns of Inheritance 127  What genes control coat color in rabbits? The fur color of rabbits is another trait controlled by multiple alleles. In rabbits, four alleles control coat color: C, cch, ch, and c. THINK IT OVER The alleles are dominant in varying degrees. The hierarchy can 5. Evaluate What allele is be written as C > cch > ch > c. dominant over cch? Allele C is dominant to all other alleles and results in a dark a. ch gray coat color. Allele cch is dominant to ch, and ch is dominant b. c to c. Allele c is recessive and results in an albino when the c. C genotype is homozygous recessive. Multiple alleles increase the possible number of genotypes and phenotypes. Two alleles have three possible genotypes and two possible phenotypes. Four alleles have ten possible genotypes and can have five or more phenotypes. Epistasis Epistasis (ih PIHS tuh sus) occurs when one allele hides the effects of another allele. Coat color in Labrador retrievers is a trait controlled by epistasis. Labrador coats vary from yellow to black. Two different genes control coat color. The dominant allele E determines whether the coat will have dark pigment. A dog with genotype ee will not have any pigment. The dominant allele B determines how dark the pigment will be. If the genotype is EEbb or Eebb the coat will be chocolate. If the genotype is eebb, eeBb, or eeBB the coat will be yellow because the e allele hides the effects of the dominant B allele. THINK IT OVER 6. Identify A person has 22 Dosage Compensation pairs of autosomes and two There are two types of sex chromosomes—X and Y. A person’s X chromosomes. What is gender is determined by the sex chromosomes present in the egg Copyright © McGraw-Hill Education the person’s gender? and sperm cell. Females inherit two X chromosomes. Males inherit one X and one Y chromosome. In humans, the X chromosome carries genes needed by males and females. The Y chromosome mainly carries genes needed to develop male characteristics. Does this mean that females get a double dose of the X chromosome? No, because in females, one of the X chromosomes in every body cell stops working. This is called dosage compensation or X-inactivation. Reading Essentials Introduction to Genetics and Patterns of Inheritance 128  How is coat color determined in calico cats? Get It? The coat color of calico cats is controlled by the random 7. Summarize dosage compensation and its inactivation of X chromosomes. Orange patches are formed effects. when an X chromosome carrying the allele for black coat color is turned off. Black patches are formed when an X chromosome carrying the allele for orange coat color is turned off. What are Barr bodies? Canadian scientist Murray Barr first observed inactivated X chromosomes, now known as Barr bodies. Barr bodies appear as dark objects in the cell nuclei of female mammals. Sex-Linked Traits Traits controlled by genes on the X chromosome are called sex-linked traits or X-linked traits. Males have only one X chromosome, so they are affected more than females by THINK IT OVER recessive sex-linked traits. Females are less likely to express 8. Draw Conclusions Why is a recessive sex-linked trait a recessive sex-linked trait because one X chromosome may mask less likely to occur in the effect of the recessive trait on the other X chromosome. females than in males? How is red-green color blindness inherited? The trait for red-green color blindness is a recessive sex-linked trait. People who are color blind cannot see the colors red and green. About 8 percent of males XB = in Normal the United States are red-green color blind. Examine the Punnett square below X = Red-green b to see color blind how red-green color blindness is inherited. Y = Y chromosome XB Y XB = XB XBXB X BY TAKE A LOOK Copyright © McGraw-Hill Education Normal Xb = Red-green color blind 9. Predict Circle the genotype Y = Y chromosome that represents a color-blind person. XB Y Xb XBXb X bY XB XBXB X BY How is hemophilia inherited? Normally, when a person is cut, the bleeding stops quickly. Hemophilia Xb XisBX a recessive b Xbsex-linked Y disorder that slows blood clotting. Hemophilia is more common in males. Until the discovery of clotting factors in the twentieth century, most men with hemophilia died at an early age. Safe methods of treating the disorder now allow for a normal life span. Reading Essentials Introduction to Genetics and Patterns of Inheritance 129  Polygenic Traits So far you have learned about traits that are controlled by one gene with different alleles. Polygenic traits develop from the interaction of multiple pairs of genes. Many traits in humans are polygenic, including skin color, height, eye color, and fingerprint THINK IT OVER pattern. 10. List an example of a polygenic trait. Environmental Influences The environment influences many traits. Factors such as sunlight, temperature, and water can affect an organism’s phenotype. For example, the gene that codes for the production of color pigment in Siamese cats functions only under cooler conditions. Cooler parts of the cat’s body, such as the ears, nose, feet, and tail, are darker. The warmer parts of the body, where pigment production is inhibited, are lighter. Environmental factors also include an organism’s actions. Heart disease can be inherited, but diet and exercise also strongly influence the disease. An organism’s actions are considered part of the environment because they do not come from genes. Twin Studies Scientists can learn about inheritance patterns by studying twins. Twin studies often reveal how genes and the environment affect phenotype. Identical twins have identical genes. If a trait is inherited, both identical twins will have the trait. Scientists presume that traits that are different in identical twins are strongly influenced by the environment. The percentage of identical twins who both have Copyright © McGraw-Hill Education the same trait is called a concordance rate, as shown in the graph below. The higher the concordance rate, the stronger the genetic influence. TAKE A LOOK 11. Evaluate Circle the trait Concordance Rates that shows the strongest Identical twins Fraternal twins genetic influence. 100% Concordance 80% 60% 40% 20% 0% Alcoholism Alcoholism Alzheimer’s Blood Depression Reading in females in males disease types disability Traits Reading Essentials Introduction to Genetics and Patterns of Inheritance 130