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Chapter 9 Patterns of Inheritance PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE • TAYLOR • SIMON • DICKEY • HOGAN © 2015 Pearson Education, Inc. Lecture by Edward J. Zalisko MENDEL’S LAWS © 2015 Pearson Education, Inc. 9.2 The science of genetics began in an abbey garden • Heredity is the transmission of traits from one generation to the next. • Genetics is the scientific study of heredity. • Gregor Mendel • began the field of genetics in the 1860s, • deduced the principles of genetics by breeding garden peas, and • relied upon a background of mathematics, physics, and chemistry. © 2015 Pearson Education, Inc. Figure 9.2a © 2015 Pearson Education, Inc. 9.2 The science of genetics began in an abbey garden • In 1866, Mendel • correctly argued that parents pass on to their offspring discrete “heritable factors” and • stressed that the heritable factors (today called genes) retain their individuality generation after generation. • A heritable feature that varies among individuals, such as flower color, is called a character. • Each variant for a character, such as purple or white flowers, is a trait. © 2015 Pearson Education, Inc. 9.2 The science of genetics began in an abbey garden • Perhaps the most important advantage of pea plants as an experimental model was that Mendel could strictly control matings. • The petals of the pea flower almost completely enclose the reproductive organs: the stamens and carpel. • Consequently, pea plants usually are able to selffertilize in nature. © 2015 Pearson Education, Inc. Figure 9.2b Petal Carpel (contains eggs) Stamens (release sperm-containing pollen) © 2015 Pearson Education, Inc. Figure 9.2c-1 Parents (P) © 2015 Pearson Education, Inc. Stamens Carpel Figure 9.2c-2 1 Stamen removal Parents (P) 2 Pollen transfer Stamens Carpel 3 Carpel matures into pod © 2015 Pearson Education, Inc. 9.2 The science of genetics began in an abbey garden • When Mendel wanted to cross-fertilize plants, he • prevented self-fertilization by cutting off the immature stamens of a plant before they produced pollen and • dusted its carpel with pollen from another plant to cross-fertilize the stamenless flower. • After pollination, the carpel developed into a pod, containing seeds (peas) that he later planted. © 2015 Pearson Education, Inc. Figure 9.2c-3 1 Stamen removal Parents (P) 2 Pollen transfer Stamens Carpel 3 Carpel matures into pod 4 Seeds from pod planted Offspring (F1) © 2015 Pearson Education, Inc. 9.2 The science of genetics began in an abbey garden • True-breeding varieties result when self-fertilization produces offspring all identical to the parent. • The offspring of two different varieties are hybrids. • The cross-fertilization is a hybridization, or genetic cross. • True-breeding parental plants are the P generation. • Hybrid offspring are the F1 generation. • A cross of F1 plants produces an F2 generation. © 2015 Pearson Education, Inc. Figure 9.2d-1 Character Traits Recessive Dominant Flower color Purple White Axial Terminal Yellow Green Round Wrinkled Flower position Seed color Seed shape © 2015 Pearson Education, Inc. Figure 9.2d-2 Character Traits Recessive Dominant Pod shape Inflated Constricted Green Yellow Tall Dwarf Pod color Stem length © 2015 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character • A cross between two individuals differing in a single character is a monohybrid cross. • Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers. • The F1 generation produced all plants with purple flowers. • A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers. © 2015 Pearson Education, Inc. Figure 9.3a-1 The Experiment P generation (true-breeding parents) Purple flowers © 2015 Pearson Education, Inc. White flowers Figure 9.3a-2 The Experiment P generation (true-breeding parents) Purple flowers F1 generation © 2015 Pearson Education, Inc. White flowers All plants have purple flowers Figure 9.3a-3 The Experiment P generation (true-breeding parents) Purple flowers F1 generation White flowers All plants have purple flowers Fertilization among F1 plants (F1 × F1) F2 generation 3 4 1 of plants of plants 4 have purple flowers have white flowers © 2015 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character • The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis. • Mendel needed to explain • why white color seemed to disappear in the F1 generation and • why white color reappeared in one-quarter of the F2 offspring. © 2015 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character • Mendel developed four hypotheses, described below using modern terminology. 1. Alleles are alternative versions of genes that account for variations in inherited characters. 2. For each character, an organism inherits two alleles, one from each parent. The alleles can be the same or different. • A homozygous genotype has identical alleles. • A heterozygous genotype has two different alleles. © 2015 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character 3. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele. • The phenotype is the appearance or expression of a trait. • The genotype is the genetic makeup of a trait. • The same phenotype may be determined by more than one genotype. © 2015 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character 4. A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation. • The fusion of gametes at fertilization creates allele pairs once again. © 2015 Pearson Education, Inc. 9.3 Mendel’s law of segregation describes the inheritance of a single character • Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation. • The F1 hybrids all have a Pp genotype. • A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine. © 2015 Pearson Education, Inc. Figure 9.3b-0-1 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes © 2015 Pearson Education, Inc. All P All p Figure 9.3b-0-2 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P F1 generation (hybrids) All p All Pp Alleles segregate Gametes © 2015 Pearson Education, Inc. 1 2 P 1 2 p Figure 9.3b-0-3 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p F1 generation (hybrids) All Pp Alleles segregate Gametes 1 2 1 2 P Fertilization F2 generation Results: p Sperm from F1 plant P Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Eggs from F1 plant P PP Pp p Pp pp Results © 2015 Pearson Education, Inc. p Figure 9.3b-1 F2 generation Results: p Sperm from F1 plant P Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Eggs from F1 plant P PP Pp p Pp pp Results © 2015 Pearson Education, Inc. 9.4 Homologous chromosomes bear the alleles for each character • A locus (plural, loci) is the specific location of a gene along a chromosome. • For a pair of homologous chromosomes (homologs), alleles of a gene reside at the same locus. • Homozygous individuals have the same allele on both homologs. • Heterozygous individuals have a different allele on each homolog. © 2015 Pearson Education, Inc. Figure 9.4 Gene loci P a B P a b Dominant allele Homologous chromosomes Genotype: © 2015 Pearson Education, Inc. PP Homozygous for the dominant allele aa Homozygous for the recessive allele Recessive allele Bb Heterozygous, with one dominant and one recessive allele 9.5 The law of independent assortment is revealed by tracking two characters at once • A dihybrid cross is a mating of parental varieties that differ in two characters. • Mendel performed the following dihybrid cross with the following results: • P generation: round yellow seeds wrinkled green seeds • F1 generation: all plants with round yellow seeds • F2 generation: • 9/16 had round yellow seeds • 3/16 had wrinkled yellow seeds • 3/16 had round green seeds • 1/16 had wrinkled green seeds © 2015 Pearson Education, Inc. Figure 9.5a-0 P generation RRYY rryy ry Gametes RY F1 generation RrYy Sperm 1 4 RY 1 4 rY 1 4 Ry 1 4 ry Sperm 1 2 RY 1 2 ry RY F2 generation Eggs 1 2 ry 1 4 1 4 © 2015 Pearson Education, Inc. RRYY 1 4 RrYY RRYy RrYy rY RrYY Eggs 1 4 The hypothesis of dependent assortment Not actually seen; hypothesis refuted RY rrYY RrYy rrYy Results: 9 16 3 16 Ry RRYy RrYy RRyy Rryy RrYy rrYy Rryy rryy ry 3 16 1 16 The hypothesis of independent assortment Actual results; hypothesis supported Yellow round Green round Yellow wrinkled Green wrinkled 9.5 The law of independent assortment is revealed by tracking two characters at once • Mendel needed to explain why the F2 offspring • had new nonparental combinations of traits and • had a 9:3:3:1 phenotypic ratio. • Mendel • suggested that the inheritance of one character has no effect on the inheritance of another, • suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and • called this the law of independent assortment. © 2015 Pearson Education, Inc. 9.5 The law of independent assortment is revealed by tracking two characters at once • The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers: • black versus chocolate color and • normal vision versus progressive retinal atrophy. © 2015 Pearson Education, Inc. Figure 9.5b-0 Blind Blind Phenotypes Genotypes Black coat, normal vision B_N_ Black coat, blind (PRA) Chocolate coat, normal vision B_nn bbN_ Chocolate coat, blind (PRA) bbnn Mating of double heterozygotes (black coat, normal vision) BbNn × BbNn Blind Blind Phenotypic ratio of the offspring © 2015 Pearson Education, Inc. 9 Black coat, normal vision 3 Black coat, blind (PRA) 3 Chocolate coat, normal vision 1 Chocolate coat, blind (PRA) 9.6 Geneticists can use the testcross to determine unknown genotypes • A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual. • A testcross can show whether the unknown genotype includes a recessive allele. • Mendel used testcrosses to verify that he had truebreeding varieties of plants. • The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes. © 2015 Pearson Education, Inc. Figure 9.6 What is the genotype of the black dog? Testcross Genotypes bb B_? Two possibilities for the black dog: BB Gametes B b Offspring © 2015 Pearson Education, Inc. Bb or Bb All black b B b Bb bb 1 black : 1 chocolate 9.7 Mendel’s laws reflect the rules of probability • Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected • the segregation of allele pairs during gamete formation and • the re-forming of pairs at fertilization. • The probability scale ranges from 0 to 1. An event that is • certain to occur has a probability of 1 and • certain not to occur has a probability of 0. © 2015 Pearson Education, Inc. 9.7 Mendel’s laws reflect the rules of probability • The probability of a specific event is the number of ways that event can occur out of the total possible outcomes. • Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event. • The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition. © 2015 Pearson Education, Inc. Figure 9.7 F1 genotypes Bb male Bb female Formation of eggs Formation of sperm 1 2 1 2 B b Sperm ( 12 ×12 ) 1 2 B B 1 2 b b 1 4 B b 1 4 © 2015 Pearson Education, Inc. B 1 4 Eggs F2 genotypes B b b 1 4 9.8 VISUALIZING THE CONCEPT: Genetic traits in humans can be tracked through family pedigrees • The inheritance of human traits follows Mendel’s laws. • A pedigree • shows the inheritance of a trait in a family through multiple generations, • demonstrates dominant or recessive inheritance, and • can also be used to deduce genotypes of family members. © 2015 Pearson Education, Inc. Figure 9.8-2 KEY Female Male Widow’s peak hairline trait Straight hairline trait H: widow’s peak allele h: straight allele Mating 1ST GENERATION Al Beth Charles Children Evelyn Frank Gary 2ND GENERATION Henry Straight hairline Isabel Juliana Widow’s peak 3RD GENERATION Kristin © 2015 Pearson Education, Inc. Debbie Lori Figure 9.8-3 KEY Female Male Widow’s peak hairline trait Straight hairline trait H: widow’s peak allele h: straight allele 1ST GENERATION Al Beth Charles Debbie Hh Hh hh Hh 2ND GENERATION Evelyn Frank Gary Henry hh hh Hh Straight hairline Isabel Juliana Hh hh 3RD GENERATION Kristin hh © 2015 Pearson Education, Inc. A parent with a widow’s peak who has a child with a straight hairline must be Hh. Kristin has a straight Lori hairline but her parents do not, so straight hairline must be homozygous recessive (hh). Widow’s peak Figure 9.8-4 KEY Female Male Widow’s peak hairline trait Straight hairline trait H: widow’s peak allele h: straight allele 1ST GENERATION Al Beth Charles Debbie Hh Hh hh Hh 2ND GENERATION Evelyn Frank Gary Henry Isabel Juliana HH hh hh Hh Hh hh or Hh Straight hairline Widow’s peak 3RD GENERATION Not all genotypes can be determined. Lori could be Kristin HH or Hh and there is no Lori way to know (unless she hh has some children and the HH or pedigree is extended). Hh © 2015 Pearson Education, Inc. 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene • Because a trait is dominant does not mean that it is • “normal” or • more common than a recessive trait. • Wild-type traits are • those most often seen in nature and • not necessarily specified by dominant alleles. © 2015 Pearson Education, Inc. Figure 9.9a-0 Dominant Traits Recessive Traits Freckles No freckles Normal pigmentation Albinism © 2015 Pearson Education, Inc. Key Wild-type (more common) trait Mutant (less common) trait 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene • The genetic disorders listed in Table 9.9 are known to be inherited as dominant or recessive traits controlled by a single gene. • These human disorders therefore show simple inheritance patterns like the traits Mendel studied in pea plants. • The genes discussed in this module are all located on autosomes. © 2015 Pearson Education, Inc. Table 9.9 © 2015 Pearson Education, Inc. 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene • Thousands of human genetic disorders—ranging in severity from relatively mild, such as albinism, to invariably fatal, such as cystic fibrosis—are inherited as recessive traits. • Most people who have recessive disorders are born to normal parents who • are both heterozygotes, carriers of the recessive allele for the disorder, but • are phenotypically normal. © 2015 Pearson Education, Inc. Figure 9.9b Normal Aa Parents Normal Aa A Sperm a A Offspring Aa Normal (carrier) aa Albinism Eggs a © 2015 Pearson Education, Inc. AA Normal Aa Normal (carrier) 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene • The most common lethal genetic disease in the United States is cystic fibrosis (CF). The CF allele is • recessive and • carried by about 1 in 31 Americans. • Cystic fibrosis is • characterized by an excessive secretion of very thick mucus from the lungs and other organs and • most common in Caucasians. © 2015 Pearson Education, Inc. 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene • Dominant human disorders include • Huntington’s disease, a degenerative disorder of the nervous system and • achondroplasia, a form of dwarfism in which • the head and torso of the body develop normally but • the arms and legs are short. © 2015 Pearson Education, Inc. Figure 9.9c © 2015 Pearson Education, Inc. 9.9 CONNECTION: Many inherited traits in humans are controlled by a single gene • Until relatively recently, the onset of symptoms was the only way to know if a person had inherited the Huntington’s allele. • A genetic test is now available that can detect the presence of the Huntington’s allele in an individual’s genome. • This is one of several genetic tests currently available. © 2015 Pearson Education, Inc. 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy • Modern technologies offer ways to obtain genetic information • before conception, • during pregnancy, and • after birth. • Genetic testing can identify prospective parents who are heterozygous carriers for certain diseases. © 2015 Pearson Education, Inc. 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy • Several technologies can be used for detecting genetic conditions in a fetus. • Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits • karyotyping to detect chromosomal abnormalities such as Down syndrome and • biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease. • Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests. © 2015 Pearson Education, Inc. Figure 9.10a-0 Amniocentesis Chorionic Villus Sampling (CVS) Needle extracts amniotic fluid. Ultrasound transducer Fetus Ultrasound transducer Fetus Placenta Chorionic villi Uterus Centrifugation Placenta Uterus Cervix Amniotic fluid Fetal cells Cervix Fetal cells Several hours Cultured cells Several weeks Biochemical and genetic tests Several weeks Several hours Several hours Karyotyping © 2015 Pearson Education, Inc. Suction tube extracts tissue from chorionic villi. 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy • Blood tests on the mother at 15–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects. • Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus. • Newborn screening can detect diseases that can be prevented by special care and precautions. © 2015 Pearson Education, Inc. 9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy • New technologies raise ethical considerations that include • the confidentiality and potential use of results of genetic testing, • time and financial costs, and • the determination of what, if anything, should be done as a result of the testing. © 2015 Pearson Education, Inc. VARIATIONS ON MENDEL’S LAWS © 2015 Pearson Education, Inc. 9.11 Incomplete dominance results in intermediate phenotypes • Mendel’s pea crosses always looked like one of the two parental varieties, a situation called complete dominance. • For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance. © 2015 Pearson Education, Inc. Figure 9.11a-0 P generation Red RR White rr Gametes r R F1 generation Pink hybrid Rr Gametes 1 2 1 2 R F2 generation Sperm 1 2 1 2 R 1 2 r R RR rR r Rr rr Eggs 1 2 © 2015 Pearson Education, Inc. r Figure 9.11a-2 F1 generation Pink hybrid Rr Gametes © 2015 Pearson Education, Inc. 1 2 R 1 2 r Figure 9.11a-3 F2 generation Sperm 1 2 1 2 R 1 2 r R RR rR r Rr rr Eggs 1 2 © 2015 Pearson Education, Inc. 9.11 Incomplete dominance results in intermediate phenotypes • One example of incomplete dominance in humans is hypercholesterolemia, in which • dangerously high levels of cholesterol occur in the blood and • heterozygotes have intermediately high cholesterol levels. © 2015 Pearson Education, Inc. Figure 9.11b Genotypes HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors Phenotypes LDL LDL receptor Cell Normal © 2015 Pearson Education, Inc. Mild disease Severe disease 9.12 Many genes have more than two alleles in the population • Although each individual carries, at most, two different alleles for a particular gene, in cases of multiple alleles, more than two possible alleles exist in a population. © 2015 Pearson Education, Inc. 9.12 Many genes have more than two alleles in the population • Human ABO blood group phenotypes involve three alleles for a single gene. • The four human blood groups, A, B, AB, and O, result from combinations of these three alleles. • The A and B alleles are both expressed in heterozygous individuals, making both alleles codominant. © 2015 Pearson Education, Inc. Figure 9.12-0 Blood Carbohydrates Present Group Genotypes on Red Blood Cells (Phenotype) A I AI A or I Ai Carbohydrate A Carbohydrate B B I BI B or I Bi AB I AI B O ii Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O AB A B Anti-B Anti-A Carbohydrate A and Carbohydrate B None Anti-A Neither Anti-B No reaction © 2015 Pearson Education, Inc. Clumping reaction 9.13 A single gene may affect many phenotypic characters • Pleiotropy occurs when one gene influences multiple characters. • Sickle-cell disease is a human example of pleiotropy. • This disease affects the type of hemoglobin produced and the shape of red blood cells and causes anemia and organ damage. • Sickle-cell and nonsickle alleles are codominant. • Carriers of sickle-cell disease have increased resistance to malaria. © 2015 Pearson Education, Inc. Figure 9.13a © 2015 Pearson Education, Inc. Figure 9.13b An individual homozygous for the sickle-cell allele Produces sickle-cell (abnormal) hemoglobin The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Damage to organs Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis) © 2015 Pearson Education, Inc. Other effects Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections 9.14 A single character may be influenced by many genes • Many characters result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes on a single phenotypic character. • Human skin color is an example of polygenic inheritance. © 2015 Pearson Education, Inc. Figure 9.14-0 P generation aabbcc (very light) AABBCC (very dark) F1 generation AaBbCc (medium shade) AaBbCc (medium shade) Sperm 1 8 F2 generation 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 Fraction of population Eggs 20 64 1 8 1 8 1 8 1 8 1 8 1 6 64 64 © 2015 Pearson Education, Inc. 15 64 20 64 15 64 6 64 1 64 15 64 6 64 1 64 Skin color Figure 9.14-1 P generation aabbcc (very light) AABBCC (very dark) F1 generation AaBbCc (medium shade) © 2015 Pearson Education, Inc. AaBbCc (medium shade) Figure 9.14-2 Sperm 1 8 F2 generation Eggs 1 64 © 2015 Pearson Education, Inc. 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 6 64 15 64 20 64 15 64 6 64 1 64 Figure 9.14-3 Fraction of population 20 64 15 64 6 64 1 64 Skin color © 2015 Pearson Education, Inc. 9.15 The environment affects many characters • Many characters result from a combination of heredity and the environment. For example, • skin color is affected by exposure to sunlight and • heart disease and cancer are influenced by genes and the environment. • Identical twins show that a person’s traits are the results of • genetics and • the environment. © 2015 Pearson Education, Inc. Figure 9.15a © 2015 Pearson Education, Inc. Figure 9.15b © 2015 Pearson Education, Inc. THE CHROMOSOMAL BASIS OF INHERITANCE © 2015 Pearson Education, Inc. 9.16 Chromosome behavior accounts for Mendel’s laws • The chromosome theory of inheritance states that • genes occupy specific loci (positions) on chromosomes and • chromosomes undergo segregation and independent assortment during meiosis. © 2015 Pearson Education, Inc. 9.16 Chromosome behavior accounts for Mendel’s laws • Mendel’s laws correlate with chromosome separation in meiosis. • The law of segregation • states that pairs of alleles separate from each other during gamete formation via meiosis and • depends on separation of homologous chromosomes in anaphase I. • The law of independent assortment • states that each pair of alleles sorts independently of other pairs of alleles during gamete formation and • depends on alternative orientations of chromosomes in metaphase I. © 2015 Pearson Education, Inc. Figure 9.16-1-1 F1 generation R r y All yellow round seeds (RrYy) Y R Y © 2015 Pearson Education, Inc. r r Two equally probable y Y arrangements of chromosomes at metaphase I R y Figure 9.16-1-2 F1 generation R r y All yellow round seeds (RrYy) Y R Y R r Y y Two equally probable y Y arrangements of chromosomes at metaphase I Anaphase I R r Y y © 2015 Pearson Education, Inc. r r Metaphase II R y r R Y y r R Y y Figure 9.16-1-3 F1 generation R r y All yellow round seeds (RrYy) Y R Y R r Y y r r Two equally probable y Y arrangements of chromosomes at metaphase I Anaphase I R r Y y Metaphase II R Meiosis I y r R Y y r R Y y Meiosis II Gametes Y y Y R R 1 4 RY r Y Y r r r 1 1 4 ry 4 rY Fertilization among the F1 plants F2 generation 9 © 2015 Pearson Education, Inc. y : 3 :3 :1 y y R R 1 4 Ry Figure 9.16-2 Sperm 1 RY 4 1 4 RY 1 rY 4 Eggs 1 Ry 4 1 ry 4 1 rY 4 1 1 Ry ry 4 4 RRYY RrYY RRYy RrYy RrYY rrYY RrYy rrYy RRYy RrYy RRyy Rryy RrYy rrYy 9 Yellow 16 round Rryy rryy 3 Green 16 round 3 Yellow 1 Green 16 wrinkled 16 wrinkled © 2015 Pearson Education, Inc. 9.17 SCIENTIFIC THINKING: Genes on the same chromosome tend to be inherited together • Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which • are located close together on the same chromosome and • tend to be inherited together. © 2015 Pearson Education, Inc. Figure 9.17-0 The Experiment Purple flower PpLl PpLl Observed offspring 284 21 21 55 Phenotypes Purple long Purple round Red long Red round Long pollen Prediction (9:3:3:1) 215 71 71 24 The Explanation: Linked Genes PL Parental diploid cell pl PpLl Meiosis Most gametes pl PL Fertilization Sperm pl PL PL PL PL pl pl pl PL pl PL Most offspring Eggs pl 3 purple long : 1 red round Not accounted for: purple round and red long © 2015 Pearson Education, Inc. Figure 9.17-1 The Experiment Purple flower PpLl Phenotypes Purple long Purple round Red long Red round © 2015 Pearson Education, Inc. PpLl Observed offspring 284 21 21 55 Long pollen Prediction (9:3:3:1) 215 71 71 24 Figure 9.17-2 The Explanation: Linked Genes PL Parental diploid cell PpLl pl Meiosis Most gametes pl PL Fertilization Sperm PL PL PL Most PL offspring Eggs pl PL pl pl pl PL pl pl 3 purple long : 1 red round Not accounted for: purple round and red long © 2015 Pearson Education, Inc. 9.18 Crossing over produces new combinations of alleles • Crossing over between homologous chromosomes produces new combinations of alleles in gametes. • Linked genes can be separated by crossing over, forming recombinant gametes. • The percentage of recombinant offspring is the recombination frequency. © 2015 Pearson Education, Inc. Figure 9.18a P L p l PL Parental gametes p l p L Tetrad (pair of homologous chromosomes) © 2015 Pearson Education, Inc. P l Crossing over Recombinant gametes Figure 9.18b © 2015 Pearson Education, Inc. Figure 9.18c-0 The Explanation The Experiment Gray body, long wings (wild type) Black body, vestigial wings GgLl ggll Female Male GgLl Female GL g l g l g l Crossing over g l GL Gl gl gL Sperm Eggs Offspring Gray long Black vestigial Gray vestigial Offspring Black long GL g l Gl gL g l g l g l g l Parental 965 944 Parental phenotypes 206 185 Recombinant phenotypes Recombination = 391 recombinants = 0.17 or 17% 2,300 total offspring frequency © 2015 Pearson Education, Inc. ggll Male Recombinant Figure 9.18c-1 The Experiment Gray body, long wings (wild type) Black body, vestigial wings GgLl ggll Female Male Offspring Gray long Black vestigial Gray vestigial Black long 965 944 206 185 Parental phenotypes Recombinant phenotypes Recombination = 391 recombinants = 0.17 or 17% frequency 2,300 total offspring © 2015 Pearson Education, Inc. Figure 9.18c-2 The Explanation GgLl Female GL gl gl gl ggll Male Crossing over gl GL Gl gL Sperm Eggs GL gl Gl gL gl gl gl gl Parental © 2015 Pearson Education, Inc. gl Recombinant 9.19 Geneticists use crossover data to map genes • When examining recombinant frequency, Alfred H. Sturtevant, one of Morgan’s students, found that the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur. • Recombination frequencies can thus be used to map the relative position of genes on chromosomes. • Such a diagram of relative gene locations is called a linkage map. © 2015 Pearson Education, Inc. Figure 9.19a Section of chromosome carrying linked genes 17% 9% 9.5% Recombination frequencies © 2015 Pearson Education, Inc. Figure 9.19b Mutant (less common) phenotypes Short aristae Black body (g) Long aristae Gray (appendages body on head) (G) Cinnabar Vestigial eyes wings (c) (l) Red eyes (C) Normal wings (L) Brown eyes Red eyes Wild-type (more common) phenotypes © 2015 Pearson Education, Inc. SEX CHROMOSOMES AND SEX-LINKED GENES © 2015 Pearson Education, Inc. 9.20 Chromosomes determine sex in many species • Many animals have a pair of sex chromosomes, designated X and Y, that determine an individual’s sex. • Among humans and other mammals, • individuals with one X chromosome and one Y chromosome are males, and • XX individuals are females. • In addition, human males and females both have 44 autosomes (nonsex chromosomes). © 2015 Pearson Education, Inc. 9.20 Chromosomes determine sex in many species • In mammals (including humans), • the Y chromosome has a crucial gene, SRY, for the development of testes, and • an absence of the SRY gene directs ovaries to develop. © 2015 Pearson Education, Inc. Figure 9.20b-0 Parents (diploid) Gametes (haploid) Offspring (diploid) © 2015 Pearson Education, Inc. 22 + X Male Female 44 + XY 44 + XX 22 + Y Sperm 22 + X 44 + XX Female Egg 44 + XY Male 9.20 Chromosomes determine sex in many species • Grasshoppers, roaches, and some other insects have an X-O system, in which • O stands for the absence of a sex chromosome, • females are XX, and • males are XO. • In certain fishes, butterflies, and birds, • the sex chromosomes are Z and W, • males are ZZ, and • females are ZW. © 2015 Pearson Education, Inc. 9.20 Chromosomes determine sex in many species • Some organisms lack sex chromosomes altogether. • In most ants and bees, sex is determined by chromosome number. • Females develop from fertilized eggs and thus are diploid. • Males develop from unfertilized eggs. Males are thus • fatherless and • haploid. © 2015 Pearson Education, Inc. Table 9.20-0 © 2015 Pearson Education, Inc. 9.20 Chromosomes determine sex in many species • In some animals, environmental temperature determines the sex. • For some species of reptiles, the temperature at which the eggs are incubated during a specific period of embryonic development determines whether the embryo will develop into a male or female. • Global climate change may therefore impact the sex ratio of such species. © 2015 Pearson Education, Inc. 9.21 Sex-linked genes exhibit a unique pattern of inheritance • Sex-linked genes are located on either of the sex chromosomes. • The X chromosome carries many genes unrelated to sex. • The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait. © 2015 Pearson Education, Inc. Figure 9.21a-0 © 2015 Pearson Education, Inc. Figure 9.21c Female Male X RX r X RY Sperm XR Y XR X RX R X RY Xr X rX R X rY Eggs © 2015 Pearson Education, Inc. R = red-eye allele r = white-eye allele Figure 9.21d Female Male X RX r X rY Sperm Xr Y XR X RX r X RY Xr X rX r X rY Eggs © 2015 Pearson Education, Inc. R = red-eye allele r = white-eye allele 9.22 CONNECTION: Human sex-linked disorders affect mostly males • Most sex-linked human disorders are • due to recessive alleles and • seen mostly in males. • A male receiving a single X-linked recessive allele from his mother will have the disorder. • A female must receive the allele from both parents to be affected. © 2015 Pearson Education, Inc. 9.22 CONNECTION: Human sex-linked disorders affect mostly males • Recessive and sex-linked human disorders include • hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting, • red-green colorblindness, a malfunction of lightsensitive cells in the eyes, and • Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination. © 2015 Pearson Education, Inc. Figure 9.22-0 Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Female Male Hemophilia Carrier Alexis © 2015 Pearson Education, Inc. Normal 9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution • The Y chromosome provides clues about human male evolution because • Y chromosomes are passed intact from father to son and • mutations in Y chromosomes can reveal data about recent shared ancestry. © 2015 Pearson Education, Inc. 9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution • In 2003, geneticists discovered that about 8% of males currently living in central Asia have Y chromosomes of striking genetic similarity. • Further analysis traced their common genetic heritage to a single man living about 1,000 years ago. • In combination with historical records, the data led to the speculation that the Mongolian ruler Genghis Kahn may be responsible for the spread of the telltale chromosome to nearly 16 million men living today © 2015 Pearson Education, Inc.