Chapter 9 Patterns of Inheritance

Transcription

1 Chapter 9 Patterns of Inheritance PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE TAYLOR SIMON DICKEY HOGAN Lecture by Edward J. Zalisko

2 Introduction The people of Tibet live and work at altitudes above 3,000 feet, where the amount of oxygen that reaches the blood is 40% less than at sea level. What makes the Tibetan people so able to tolerate their harsh surroundings? Over the last several thousand years, the Tibetan population has accumulated several dozen genetic mutations that affect their circulatory and respiratory systems.

3 Figure 9.0-

4 Figure Chapter 9: Big Ideas Mendel s Laws Variations on Mendel s Laws The Chromosomal Basis of Inheritance Sex Chromosomes and Sex-Linked Genes

5 MENDEL S LAWS

6 9. The study of genetics has ancient roots The Greek physician Hippocrates explained inheritance by proposing that particles called pangenes travel from each part of an organism s body to the eggs or sperm and characteristics acquired during the parents lifetime could be transferred to the offspring. Hippocrates s idea is incorrect in several respects. The reproductive cells are not composed of particles from somatic (body) cells. Changes in somatic cells do not influence eggs and sperm.

7 Figure 9.

8 9. The study of genetics has ancient roots The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was suggested in the 9th century by scientists studying plants but later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.

9 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 860s, deduced the principles of genetics by breeding garden peas, and relied upon a background of mathematics, physics, and chemistry.

10 Figure 9.2a

11 9.2 The science of genetics began in an abbey garden In 866, 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.

12 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.

13 Figure 9.2b Petal Carpel (contains eggs) Stamens (release sperm-containing pollen)

14 Figure 9.2c- Parents (P) Carpel Stamens

15 Figure 9.2c-2 Stamen removal 2 Pollen transfer Parents (P) Carpel Stamens 3 Carpel matures into pod

16 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.

17 Figure 9.2c-3 Stamen removal 2 Pollen transfer Parents (P) Carpel Stamens 3 Carpel matures into pod 4 Seeds from pod planted Offspring (F )

18 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 F generation. A cross of F plants produces an F 2 generation.

19 Figure 9.2d-0 Character Dominant Traits Recessive Flower color Purple White Flower position Seed color Seed shape Pod shape Axial Yellow Round Inflated Terminal Green Wrinkled Constricted Pod color Green Yellow Stem length Tall Dwarf

20 Figure 9.2d- Character Dominant Traits Recessive Flower color Purple White Flower position Seed color Seed shape Axial Yellow Round Terminal Green Wrinkled

21 Figure 9.2d-2 Character Pod shape Dominant Inflated Traits Recessive Constricted Pod color Green Yellow Stem length Tall Dwarf

22 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 F generation produced all plants with purple flowers. A cross of F plants with each other produced an F 2 generation with ¾ purple and ¼ white flowers.

23 Figure 9.3a- The Experiment P generation (true-breeding parents) Purple flowers White flowers

24 Figure 9.3a-2 The Experiment P generation (true-breeding parents) Purple flowers White flowers F generation All plants have purple flowers

25 Figure 9.3a-3 The Experiment P generation (true-breeding parents) Purple flowers White flowers F generation All plants have purple flowers Fertilization among F plants (F F ) F 2 generation 3 4 of plants have purple flowers 4 of plants have white flowers

26 9.3 Mendel s law of segregation describes the inheritance of a single character The all-purple F 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 F generation and why white color reappeared in one-quarter of the F 2 offspring.

27 9.3 Mendel s law of segregation describes the inheritance of a single character Mendel developed four hypotheses, described below using modern terminology.. 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.

28 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.

29 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.

30 9.3 Mendel s law of segregation describes the inheritance of a single character Mendel s hypotheses also explain the 3: ratio in the F 2 generation. The F hybrids all have a Pp genotype. A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine.

31 Figure 9.3b-0- The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p

32 Figure 9.3b-0-2 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p F generation (hybrids) All Pp Gametes 2 P Alleles segregate 2 p

33 Figure 9.3b-0-3 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p F generation (hybrids) All Pp Gametes 2 P Alleles segregate 2 p Fertilization F 2 generation Results: Sperm from F plant P p Phenotypic ratio 3 purple : white Eggs from F plant Genotypic ratio PP : 2 Pp : pp P p PP Pp Pp pp Results

34 Figure 9.3b- F 2 generation Results: Sperm from F plant P p Phenotypic ratio 3 purple : white Eggs from F plant Genotypic ratio PP : 2 Pp : pp P p PP Pp Pp pp Results

35 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.

36 Figure 9.4 P Gene loci a B Dominant allele Homologous chromosomes Genotype: P PP Homozygous for the dominant allele a aa Homozygous for the recessive allele b Bb Recessive allele Heterozygous, with one dominant and one recessive allele

37 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 F generation: all plants with round yellow seeds F 2 generation: 9/6 had round yellow seeds 3/6 had wrinkled yellow seeds 3/6 had round green seeds /6 had wrinkled green seeds

38 Figure 9.5a-0 P generation RRYY rryy Gametes RY ry F 2 generation Eggs 2 RY ry 2 Sperm RY F generation The hypothesis of dependent assortment Not actually seen; hypothesis refuted 2 ry Eggs RrYy RY ry Ry ry 4 RY Sperm RRYY RrYY RRYy RrYy RrYY rryy RrYy rryy RRYy RrYy 4 ry RrYy rryy Ry RRyy Rryy Rryy rryy The hypothesis of independent assortment Actual results; hypothesis supported 4 4 ry Results: Yellow round Green round Yellow wrinkled Green wrinkled

39 Figure 9.5a- P generation RRYY rryy Gametes RY ry F generation RrYy

40 Figure 9.5a-2 F generation RrYy 2 Sperm RY 2 ry F 2 generation 2 Eggs 2 RY ry The hypothesis of dependent assortment Not actually seen; hypothesis refuted

41 Figure 9.5a-3 F generation RrYy Sperm 4 RY 4 ry 4 Ry 4 ry 4 4 Eggs 4 4 RY ry Ry ry RRYY RrYY RRYy RrYy RrYY rryy RrYy rryy RRYy RrYy RrYy rryy RRyy Rryy Rryy rryy The hypothesis of independent assortment Actual results; hypothesis supported Results: Yellow round Green round Yellow wrinkled Green wrinkled

42 9.5 The law of independent assortment is revealed by tracking two characters at once Mendel needed to explain why the F 2 offspring had new nonparental combinations of traits and had a 9:3:3: 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.

43 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.

44 Figure 9.5b-0 Blind Blind Phenotypes Genotypes Black coat, normal vision Black coat, blind (PRA) Chocolate coat, normal vision Chocolate coat, blind (PRA) B_N_ B_nn bbn_ bbnn Mating of double heterozygotes (black coat, normal vision) BbNn BbNn Blind Blind Phenotypic ratio of the offspring 9 Black coat, normal vision 3 Black coat, blind (PRA) 3 Chocolate coat, normal vision Chocolate coat, blind (PRA)

45 Figure 9.5b- Blind Phenotypes Genotypes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Blind Phenotypes Genotypes Chocolate coat, normal vision bbn_ Chocolate coat, blind (PRA) bbnn

46 Figure 9.5b-2 Mating of double heterozygotes (black coat, normal vision) BbNn BbNn Blind Phenotypic ratio of the offspring 9 Black coat, normal vision 3 Black coat, blind (PRA) Blind Phenotypic ratio of the offspring 3 Chocolate coat, normal vision Chocolate coat, blind (PRA)

47 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.

48 Figure 9.6 What is the genotype of the black dog? Testcross Genotypes B_? bb Two possibilities for the black dog: BB or Bb Gametes B B b b Bb b Bb bb Offspring All black black : chocolate

49 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. An event that is certain to occur has a probability of and certain not to occur has a probability of 0.

50 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.

51 Figure 9.7 F genotypes Bb female Bb male Formation of eggs Formation of sperm 2 2 B 2 ( ) 2 Sperm b 2 B B B B b F 2 genotypes Eggs b b B b b 4 4

52 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.

53 Figure 9.8- Straight hairline Widow s peak

54 Figure Al Mating Beth KEY Female Male Widow s peak hairline trait Straight hairline trait ST GENERATION Charles H: widow s peak allele h: straight allele Debbie Children 2ND GENERATION Evelyn Frank Gary Henry Isabel Juliana Straight hairline 3RD GENERATION Widow s peak Kristin Lori

55 Figure KEY Female Male Widow s peak hairline trait Straight hairline trait H: widow s peak allele h: straight allele Al Beth ST GENERATION Charles Debbie Hh Hh hh Hh 2ND GENERATION Evelyn Frank Gary Henry hh hh Hh A parent with a widow s peak who has a child with a straight hairline must be Hh. Isabel Juliana Hh hh Straight hairline 3RD GENERATION Widow s peak Kristin hh Kristin has a straight hairline but her parents do not, so straight hairline must be homozygous recessive (hh). Lori

56 Figure KEY Female Male Widow s peak hairline trait Straight hairline trait H: widow s peak allele h: straight allele Al Beth ST GENERATION Charles Debbie Hh Hh hh Hh 2ND GENERATION Evelyn Frank Gary Henry HH or Hh hh hh Straight hairline Hh 3RD GENERATION Isabel Juliana Hh hh Widow s peak Kristin hh Not all genotypes can be determined. Lori could be HH or Hh and there is no way to know (unless she has some children and the pedigree is extended). Lori HH or Hh

57 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.

58 Figure 9.9a-0 Dominant Traits Recessive Traits Freckles No freckles Key Wild-type (more common) trait Mutant (less common) trait Normal pigmentation Albinism

59 Figure 9.9a- Freckles

60 Figure 9.9a-2 No freckles

61 Figure 9.9a-3 Normal pigmentation

62 Figure 9.9a-4 Albinism

63 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.

64 Table 9.9

65 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.

66 Figure 9.9b Parents Normal Aa Normal Aa A Sperm a Offspring A Eggs a AA Normal Aa Normal (carrier) Aa Normal (carrier) aa Albinism

67 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 in 3 Americans. Cystic fibrosis is characterized by an excessive secretion of very thick mucus from the lungs and other organs and most common in Caucasians.

68 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.

69 Figure 9.9c

70 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.

71 9.0 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.

72 9.0 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.

73 Video: Ultrasound of Human Fetus

74 Figure 9.0a-0 Ultrasound transducer Amniocentesis Needle extracts amniotic fluid. Ultrasound transducer Chorionic Villus Sampling (CVS) Suction tube extracts tissue from chorionic villi. Fetus Placenta Uterus Cervix Fetus Placenta Chorionic villi Uterus Centrifugation Cervix Amniotic fluid Fetal cells Cultured cells Several hours Several weeks Biochemical and genetic tests Fetal cells Several hours Several weeks Several hours Karyotyping

75 Figure 9.0a- Karyotyping

76 9.0 CONNECTION: New technologies can provide insight into one s genetic legacy Blood tests on the mother at 5 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.

77 Figure 9.0b-0

78 Figure 9.0b-

79 Figure 9.0b-2

80 9.0 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.

81 VARIATIONS ON MENDEL S LAWS

82 9. 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 F hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance.

83 Figure 9.a-0 P generation Red RR Gametes R r White rr F generation Pink hybrid Rr Gametes 2 R 2 r F 2 generation 2 R Sperm 2 r 2 R RR rr Eggs 2 r Rr rr

84 Figure 9.a- P generation Red RR Gametes R r White rr

85 Figure 9.a-2 F generation Pink hybrid Rr Gametes 2 R 2 r

86 Figure 9.a-3 F 2 generation Sperm 2 R 2 r 2 R RR rr Eggs 2 r Rr rr

87 9. 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.

88 Figure 9.b HH Homozygous for ability to make LDL receptors LDL Genotypes Hh Heterozygous Phenotypes hh Homozygous for inability to make LDL receptors LDL receptor Cell Normal Mild disease Severe disease

89 9.2 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.

90 9.2 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.

91 Figure Blood Group (Phenotype) Genotypes Carbohydrates Present on Red Blood Cells Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB A I A I A or I A i Carbohydrate A Anti-B B I B I B or I B i Carbohydrate B Anti-A AB I A I B Carbohydrate A and Carbohydrate B None O ii Neither Anti-A Anti-B No reaction Clumping reaction

92 Figure 9.2- Blood Group (Phenotype) Genotypes Carbohydrates Present on Red Blood Cells A I A I A or I A i Carbohydrate A B I B I B or I B i Carbohydrate B AB I A I B Carbohydrate A and Carbohydrate B O ii Neither

93 Figure Blood Group (Phenotype) Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB A Anti-B B Anti-A AB None O Anti-A Anti-B

94 9.3 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.

95 Figure 9.3a

96 Figure 9.3b 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) Other effects Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections

97 9.4 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.

98 Figure P generation aabbcc (very light) AABBCC (very dark) F generation AaBbCc (medium shade) AaBbCc (medium shade) Sperm F 2 generation Eggs Fraction of population Skin color

99 Figure 9.4- P generation aabbcc (very light) AABBCC (very dark) F generation AaBbCc (medium shade) AaBbCc (medium shade)

100 Figure Sperm F 2 generation Eggs

101 Figure Fraction of population Skin color

102 9.5 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.

103 Figure 9.5a

104 Figure 9.5b

105 THE CHROMOSOMAL BASIS OF INHERITANCE

106 9.6 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.

107 9.6 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.

108 Figure F generation R r Y y All yellow round seeds (RrYy) R Y r Two equally y probable arrangements of chromosomes at metaphase I r Y R y

109 Figure F generation R r Y y All yellow round seeds (RrYy) R r R Y r Two equally y probable arrangements of chromosomes at metaphase I r Y R y r R Y y Anaphase I Y y R r Metaphase II r R Y y Y y

110 Figure F generation R r Y y All yellow round seeds (RrYy) R r R Y r Two equally y probable arrangements of chromosomes at metaphase I r Y R y r R Meiosis I Y y Anaphase I Y y R r Metaphase II r R Gametes Y y Y y Y Y y y Y Y y y R R r r r r R R Meiosis II 4 RY 4 ry ry 4 Ry Fertilization among the F plants 4 F 2 generation 9 : 3 : 3 :

111 Figure Eggs RY ry Ry ry 4 RY 4 Sperm ry 4 Ry 4 ry RRYY RrYY RRYy RrYy RrYY rryy RrYy rryy RRYy RrYy RRyy Rryy RrYy rryy Rryy rryy 9 6 Yellow round 3 6 Green round 3 6 Yellow wrinkled 6 Green wrinkled

112 9.7 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: ratio in the F 2 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.

113 Figure The Experiment Purple flower Phenotypes Purple long Purple round Red long Red round PpLl Observed offspring The Explanation: Linked Genes Parental diploid cell PpLl PL pl PpLl Meiosis Long pollen Prediction (9:3:3:) Most gametes PL pl Fertilization Most offspring PL Eggs Sperm PL pl PL PL pl pl PL pl 3 purple long : red round Not accounted for: purple round and red long PL pl pl

114 Figure 9.7- The Experiment Purple flower Phenotypes Purple long Purple round Red long Red round PpLl Observed offspring PpLl Long pollen Prediction (9:3:3:)

115 Figure The Explanation: Linked Genes Parental diploid cell PpLl PL pl Meiosis Most gametes PL pl Fertilization Most offspring PL Eggs pl Sperm PL pl PL PL pl PL PL 3 purple long : red round Not accounted for: purple round and red long pl pl pl

116 9.8 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.

117 Figure 9.8a P L p l P L p l Tetrad (pair of homologous chromosomes) Crossing over Parental gametes p L P l Recombinant gametes

118 Figure 9.8b

119 Figure 9.8c-0 The Experiment The Explanation Gray body, long wings (wild type) GgLl Black body, vestigial wings ggll GgLl Female G L g l Crossing over g l g l ggll Male Female Male G L g l G l g L g l Offspring Eggs Sperm Gray long Black vestigial Gray vestigial Black long G L g l Offspring G l g L g l g l g l g l Parental Recombinant Parental phenotypes Recombinant phenotypes Recombination frequency = 39 recombinants = 0.7 or 7% 2,300 total offspring

120 Figure 9.8c- The Experiment Gray body, long wings (wild type) GgLl Female Black body, vestigial wings ggll Male Gray long Black vestigial Offspring Gray vestigial Black long Parental phenotypes Recombinant phenotypes Recombination frequency = 39 recombinants 2,300 total offspring = 0.7 or 7%

121 Figure 9.8c-2 The Explanation GgLl Female GL g l g l g l ggll Male Crossing over GL g l G l g L g l Eggs Sperm GL g l G l g L g l g l g l g l Parental Recombinant

122 9.9 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.

123 Figure 9.9a Section of chromosome carrying linked genes 7% 9% 9.5% Recombination frequencies

124 Figure 9.9b Short aristae Mutant (less common) phenotypes Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Long aristae (appendages on head) Gray body (G) Red eyes (C) Normal wings (L) Wild-type (more common) phenotypes Red eyes

125 SEX CHROMOSOMES AND SEX-LINKED GENES

126 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).

127 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.

128 Figure 9.20a X Y

129 Figure 9.20b-0 Male Female Parents (diploid) 44 + XY 44 + XX Gametes (haploid) 22 + X 22 + Y Sperm 22 + X Egg Offspring (diploid) 44 + XX Female 44 + XY Male

130 Figure 9.20b-

131 Figure 9.20b-2

132 Figure 9.20b-3

133 Figure 9.20b-4

134 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.

135 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.

136 Table

137 Table X-O system

138 Table Z-W system

139 Table Chromosome number determines sex

140 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.

141 9.2 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.

142 Figure 9.2a-0

143 Figure 9.2a-

144 Figure 9.2a-2

145 Figure 9.2b Female Male X R X R X r Y X r Sperm Y Eggs X R X R X r X R Y R = red-eye allele r = white-eye allele

146 Figure 9.2c Female Male X R X r X R Y X R Sperm Y X R X R X R X R Y Eggs X r X r X R X r Y R = red-eye allele r = white-eye allele

147 Figure 9.2d Female Male X R X r X r Y X r Sperm Y X R X R X r X R Y Eggs X r X r X r X r Y R = red-eye allele r = white-eye allele

148 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.

149 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.

150 Figure Queen Victoria Albert Alice Louis Alexandra Alexis Czar Nicholas II of Russia Female Male Hemophilia Carrier Normal

151 Figure 9.22-

152 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.

153 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,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 6 million men living today

154 Figure 9.23

155 You should now be able to. Describe the theory of pangenes and the blending hypothesis. Explain why both ideas are now rejected. 2. Define and distinguish between true-breeding organisms, hybrids, the P generation, the F generation, and the F 2 generation.

156 You should now be able to 3. Define and distinguish between the following pairs of terms: homozygous and heterozygous; dominant allele and recessive allele; genotype and phenotype. Also, define a monohybrid cross and a Punnett square.

157 You should now be able to 4. Explain how Mendel s law of segregation describes the inheritance of a single character. 5. Describe the genetic relationships between homologous chromosomes. 6. Explain how Mendel s law of independent assortment applies to a dihybrid cross. 7. Explain how and when the rule of multiplication and the rule of addition can be used to determine the probability of an event.

158 You should now be able to 8. Explain how family pedigrees can help determine the inheritance of many human traits. 9. Explain how recessive and dominant disorders are inherited. Provide examples of each. 0. Compare the health risks, advantages, and disadvantages of the following forms of fetal testing: amniocentesis, chorionic villus sampling, and ultrasound imaging.

159 You should now be able to. Describe the inheritance patterns of incomplete dominance, multiple alleles, codominance, pleiotropy, and polygenic inheritance. 2. Explain how the sickle-cell allele can be adaptive. 3. Explain why human skin coloration is not sufficiently explained by polygenic inheritance. 4. Define the chromosome theory of inheritance. Explain the chromosomal basis of the laws of segregation and independent assortment.

160 You should now be able to 5. Explain how linked genes are inherited differently from nonlinked genes. 6. Describe T. H. Morgan s studies of crossing over in fruit flies. Explain how Sturtevant created linkage maps. 7. Explain how sex is genetically determined in humans and the significance of the SRY gene. 8. Describe patterns of sex-linked inheritance and examples of sex-linked disorders. 9. Explain how the Y chromosome can be used to trace human ancestry.

161 Figure 9.UN0 Homologous chromosomes Alleles, residing at the same locus Fertilization Paired alleles, different forms of a gene Meiosis Gamete from the other parent Haploid gametes (allele pairs separated) Diploid zygote (containing paired alleles)

162 Figure 9.UN02 Incomplete dominance Red RR White rr Pink Rr

163 Figure 9.UN03 Single gene Pleiotropy Multiple characters

164 Figure 9.UN04 Multiple genes Polygenic inheritance Single characters (such as skin color)

165 Figure 9.UN05 chromosomes located on Genes alternative versions called (a) at specific locations called if both are the same, the genotype is called if different, the genotype is called (b) (c) heterozygous the expressed allele is called the unexpressed allele is called (d) (e) inheritance when the phenotype is in between is called (f)