Mendel and the Gene Idea

Transcription

1 Chapter 4 Mendel and the Gene Idea PowerPoint Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

2 Overview: Drawing from the Deck of Genes What genetic principles account for the passing of traits from parents to offspring? The blending hypothesis the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) In a freely mating population, a uniform population of individuals would eventually arise

3 Fig. 4-

4 Overview: Drawing from the Deck of Genes What genetic principles account for the passing of traits from parents to offspring? The blending hypothesis the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) In a freely mating population, a uniform population of individuals would eventually arise The particulate hypothesis the idea that parents pass on discrete heritable units (genes) Genes retain their separate identities in offspring Mendel documented a particulate mechanism through his experiments with garden peas

5 Fig. 4-

6 Concepts in this Chapter. Mendel used the scientific approach to identify two laws of inheritance 2. The laws of probability govern Mendelian inheritance 3. Inheritance patterns are often more complex than predicated by simple Mendelian genetics 4. Many human traits follow Mendelian patterns of inheritance

7 CONCEPT 4.: MENDEL USED THE SCIENTIFIC APPROACH TO IDENTIFY TWO LAWS OF INHERITANCE

8 Concept 4.: Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments Mendel grew up on a small farm and received agricultural training in school Joined a monastery in 843, failed teaching exam Went to University of Vienna (85-853) Influenced by a physicist to do experimental study and a botanist to study variations in plants Returned to the monastery, taught school, started breeding peas for study in 857

9 Mendel s Experimental, Quantitative Approach Advantages of pea plants for genetic study: There are many varieties with distinct heritable features, or characters (such as flower color) character variants (such as purple or white flowers) are called traits Have a short generation time and produce many offspring per mating Mating of plants can be controlled Each pea plant has male (stamens) and female (carpels) sexual organs Self-fertilize in nature Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

10 Mendel s Experimental, Quantitative Approach Mendel chose to track only those characters that varied in an either-or manner Eg purple vs white flowers Avoided traits that varied on a continuum He also used varieties that were true-breeding Plants that produce offspring of the same variety when they self-pollinate

11 Fig. 4-2a TECHNIQUE 2 Parental generation (P) Stamens Carpel 3 4

12 Fig. 4-2b RESULTS First filial generation offspring (F ) 5

13 Mendel s Experimental, Quantitative Approach In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation The hybrid offspring of the P generation are called the F generation When F individuals self-pollinate, the F 2 generation is produced

14 Fig. 4-2a TECHNIQUE 2 Parental generation (P) Stamens Carpel 3 4

15 Fig. 4-2b RESULTS First filial generation offspring (F ) 5

16 The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F hybrids were purple If the blending model were correct, he d have seen pale purple flowers When Mendel crossed the F hybrids, many of the F 2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F 2 generation

17 Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers

18 Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F Generation (hybrids) All plants had purple flowers

19 Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants

20 Mendel s Experimental, Quantitative Approach Mendel reasoned that only the purple flower factor was affecting flower color in the F hybrids The white flower factor was not lost, just masked Mendel called the purple flower color a dominant trait and the white flower color a recessive trait Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits What Mendel called a heritable factor is what we now call a gene

21 Table 4-

22 Mendel s Model Mendel developed a hypothesis to explain the 3: inheritance pattern he observed in F 2 offspring Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes

23 Mendel s Model The first concept is that alternative versions of genes account for variations in inherited characters For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome

24 Fig. 4-4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

25 Mendel s Model The second concept is that for each character an organism inherits two alleles, one from each parent Each diploid organism has a pair of homologous chromosomes and therefore two copies of each gene Mendel made this deduction without knowing about the role of chromosomes The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel s P generation Alternatively, the two alleles at a locus may differ, as in the F hybrids

26 Mendel s Model The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism s appearance, and the other (the recessive allele) has no noticeable effect on appearance In the flower-color example, the F plants had purple flowers because the allele for that trait is dominant

27 Mendel s Model The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis If different alleles are present 50% of the gametes will receive one allele and 50% the other.

28 Mendel s Model Mendel s segregation model accounts for the 3: ratio he observed in the F 2 generation of his numerous crosses The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

29 Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p

30 Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F Generation Appearance: Genetic makeup: Gametes: Purple flowers Pp / 2 P / 2 p

31 Fig P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F Generation Appearance: Genetic makeup: Gametes: Purple flowers Pp / 2 P / 2 p F 2 Generation P Sperm p Eggs P p PP Pp Pp pp 3

32 Useful Genetic Vocabulary An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Unlike homozygotes, heterozygotes are not truebreeding

33 Useful Genetic Vocabulary Because of the different effects of dominant and recessive alleles, an organism s traits do not always reveal its genetic composition Therefore, we distinguish between an organism s Phenotype- physical appearance Genotype- genetic makeup In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes

34 Fig. 4-6 Phenotype Genotype Purple PP (homozygous) 3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) White pp (homozygous) Ratio 3: Ratio :2:

35 The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous The answer is to carry out a testcross breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous

36 Fig. 4-7a TECHNIQUE Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If PP If Pp or Sperm Sperm p p p p Eggs P Pp Pp Eggs P Pp Pp P Pp Pp p pp pp

37 Fig. 4-7b RESULTS All offspring purple or / 2 offspring purple and / 2 offspring white

38 Fig. 4-7 TECHNIQUE Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If PP If Pp or Sperm Sperm p p p p P Eggs Pp Pp P Eggs Pp Pp P Pp Pp p pp pp RESULTS All offspring purple or / 2 offspring purple and / 2 offspring white

39 The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F offspring produced in this cross were monohybrids individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross

40 The Law of Independent Assortment Mendel identified his second law of inheritance by following two characters at the same time Crossing two true-breeding parents differing in two characters produces dihybrids in the F generation, heterozygous for both characters A dihybrid cross, a cross between F dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

41 Fig. 4-8a EXPERIMENT P Generation YYRR yyrr Gametes YR yr F Generation Predictions Hypothesis of dependent assortment YyRr Hypothesis of independent assortment Predicted offspring of F 2 generation Eggs / 2 / 2 YR yr YR Sperm / 2 / 2 yr YYRR YyRr YyRr yyrr or Eggs / 4 / 4 / 4 YR Yr yr YR Yr Sperm yr / 4 / 4 / 4 / 4 yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr 3 / 4 / 4 Phenotypic ratio 3: / 4 yr YyRr Yyrr yyrr yyrr 9 / 6 3 / 6 3 / 6 / 6 Phenotypic ratio 9:3:3:

42 Fig. 4-8b RESULTS Phenotypic ratio approximately 9:3:3:

43 Fig. 4-8 EXPERIMENT P Generation YYRR yyrr Gametes YR yr F Generation Predictions Hypothesis of dependent assortment YyRr Hypothesis of independent assortment Predicted offspring of F 2 generation Eggs / 2 / 2 YR yr Sperm / 2 YR / 2 yr YYRR YyRr YyRr yyrr or Eggs / 4 / 4 / 4 YR Yr yr Sperm / 4 YR / 4 Yr / 4 yr / 4 yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr 3 / 4 / 4 Phenotypic ratio 3: / 4 yr YyRr Yyrr yyrr yyrr 9 / 6 3 / 6 3 / 6 / 6 RESULTS Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3:

44 The Law of Independent Assortment Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together

45 Summary

46 CONCEPT 4.2: THE LAWS OF PROBABILITY GOVERN MENDELIAN INHERITANCE

47 Concept 4.2: The laws of probability govern Mendelian inheritance Mendel s laws of segregation and independent assortment reflect the rules of probability When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene s alleles

48 The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities Probability in an F monohybrid cross can be determined using the multiplication rule Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 2 chance of carrying the dominant allele and a chance of carrying the recessive allele 2

49 Fig. 4-9 Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Sperm / 2 R / 2 r / 2 R R R R r Eggs / 4 / 4 r r / 2 r R r / 4 / 4

50 The Multiplication and Addition Rules Applied to Monohybrid Crosses The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities The rule of addition can be used to figure out the probability that an F 2 plant from a monohybrid cross will be heterozygous rather than homozygous

51 Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes each character is considered separately then the individual probabilities are multiplied together

52 Fig. 4-UN

53 CONCEPT 4.3: INHERITANCE PATTERNS ARE OFTEN MORE COMPLEX THAN PREDICTED BY SIMPLE MENDELIAN GENETICS

54 Concept 4.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

55 Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes

56 Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Like the purple flower In incomplete dominance, the phenotype of F hybrids is somewhere between the phenotypes of the two parental varieties Not blending because the traits are separable with further crosses Offspring show :2: phenotypic and genotypic ratios

57 Fig P Generation Red C R C R White C W C W Gametes C R C W

58 Fig P Generation Red C R C R White C W C W Gametes C R C W F Generation Pink C R C W C R Gametes / 2 / 2 C W

59 Fig P Generation Red C R C R White C W C W Gametes C R C W F Generation Pink C R C W C R Gametes / 2 / 2 C W F 2 Generation Eggs / 2 C R C R Sperm / 2 / 2 C W C R C R C R C W / C W 2 C R C W C W C W

60 Degrees of Dominance In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways M and N blood groups of humans due to the presence of specific molecules on the surface of red blood cells People of the M group (MM) have one kind of molecule on their red blood cells People of the N group (NN) have a different kind People of the MN group (MN) have both the M and N phenotype

61 The Relationship between Dominance and Phenotype A dominant allele does not subdue a recessive allele; alleles don t interact Alleles are simply variations in a gene s nucleotide sequence

62 One dominant allele results in enough of the enzyme to synthesize adequate amounts of branched starch The Relationship between Dominance and Phenotype Round vs wrinkled pea shape The dominant allele (round) codes for an enzyme that helps convert an unbranched form of starch to a branched form in the seed The recessive allele (wrinkled) codes for a defective form of this enzyme Leads to accumulation of unbranched starch Excess water to enter seed by osmosis When seed dries it wrinkles If a dominant allele is present no excess water enters the seed and it does not wrinkle on drying

63 The Relationship between Dominance and Phenotype For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype

64 The Relationship between Dominance and Phenotype Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain Only children with two copies of the Tay-Sachs allele have the disease At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant The activity level of the lipid-metabolizing enzyme is reduced in heterozygotes At the molecular level, the alleles are codominant Heterozygotes produce equal numbers of normal and dysfunctional enzyme molecules

65 The Relationship between Dominance and Phenotype Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes

66 The Relationship between Dominance and Phenotype The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage In this example, the recessive allele is far more prevalent than the population s dominant allele

67 Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I A, I B, and i. The enzyme encoded by the I A allele adds the A carbohydrate, whereas the enzyme encoded by the I B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

68 Fig. 4- Allele I A I B i Carbohydrate A B none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype Red blood cell appearance Phenotype (blood group) I A I A or I A i A I B I B or I B i B I A I B AB ii O (b) Blood group genotypes and phenotypes

69 Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

70 Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes

71 Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in mice and many other mammals, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

72 Fig. 4-2 BbCc BbCc Eggs Sperm / 4 / 4 / 4 / 4 BC bc Bc bc / 4 BC BBCC BbCC BBCc BbCc / 4 bc BbCC bbcc BbCc bbcc / 4 Bc BBCc BbCc BBcc Bbcc / 4 bc BbCc bbcc Bbcc bbcc 9 : 3 : 4

73 Polygenic Inheritance Some characters cannot be defined as eitheror as Mendel s genes were Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Skin color in humans is an example of polygenic inheritance

74 Fig. 4-3 AaBbCc AaBbCc Sperm / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 Eggs / 8 / 8 / 8 / 8 / 8 Phenotypes: Number of / 64 6 / 64 5 / / 64 5 / 64 6 / 64 / 64 dark-skin alleles:

75 Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

76 Fig. 4-4

77 Nature and Nurture: The Environmental Impact on Phenotype Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype

78 Integrating a Mendelian View of Heredity and Variation An organism s phenotype includes its physical appearance, internal anatomy, physiology, and behavior An organism s phenotype reflects its overall genotype and unique environmental history

79 Fig. 4-UN2 Degree of dominance Description Example Heterozygous phenotype same as that of homozygous dominant PP Pp Incomplete dominance of either allele C R C R C R C W C W C W Heterozygotes: Both phenotypes expressed I A I B Multiple alleles In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters Sickle-cell disease

80 Fig. 4-UN2 Degree of dominance Description Example Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes C R C R C R C W C W C W Codominance Heterozygotes: Both phenotypes expressed I A I B Multiple alleles Pleiotropy In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters ABO blood group alleles I A, I B, i Sickle-cell disease

81 CONCEPT 4.4: MANY HUMAN TRAITS FOLLOW MENDELIAN PATTERNS OF INHERITANCE

82 Concept 4.4: Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for genetic research Generation time is too long Parents produce relatively few offspring Breeding experiments are unacceptable However, basic Mendelian genetics endures as the foundation of human genetics

83 Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Rather that manipulate breeding, study results of matings that have already occurred Inheritance patterns of particular traits can be traced and described using pedigrees

84 Fig. 4-5a Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left)

85 Fig. 4-5b st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW or Ww ww Widow s peak (a) Is a widow s peak a dominant or recessive trait? No widow s peak

86 Fig. 4-5c st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff or FF Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?

87 Fig. 4-5 Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left) st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW or Ww ww Widow s peak No widow s peak (a) Is a widow s peak a dominant or recessive trait? st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?

88 Pedigree Analysis Pedigrees can also be used to make predictions about future offspring We can use the multiplication and addition rules to predict the probability of specific phenotypes For example, what is the probability that a child from heterozygous parents (WwFf) will have a widow s peak and attached earlobes? (widow s peak (Ww) and free earlobes (Ff))

89 Pedigree Analysis Probability of having a Widow s peak with two Ww parents is ¾ Probability of having attached earlobes with two Ff parents is ¼ Therefore the probability of having a widow s peak and attached earlobes: ¾ * ¼ = 3/6

90 Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner They range from relatively mild )albinism) to life-threatening (cystic fibrosis)

91 The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented) Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

92 Fig. 4-6 Parents Normal Normal Aa Aa Eggs A a Sperm A AA Normal Aa Normal (carrier) a Aa Normal (carrier) aa Albino

93 The Behavior of Recessive Alleles If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele Most societies and cultures have laws or taboos against marriages between close relatives

94 Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine, chronic bronchitis and bacterial infections The extracellular chloride also disables a natural antibiotic made by some body cells Without treatment affected children die before age 5 With treatment they can live past their late 20s or even 30s

95 Sickle-Cell Disease Sickle-cell disease affects one out of 400 African- Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells When oxygen levels are low sickle cell hemoglobin aggregates into long rods that deform red blood cells into a sickle shape Symptoms include physical weakness, pain, organ damage, and even paralysis Regular blood tranfusion can prevent brain damage and drugs can treat other problems

96 Sickle-Cell Disease The two alleles are codominant, both are sythesized Carriers have sickle cell trait Healthy but may suffer sickle cell symptoms under blood-oxygen stress in 0 African Americans has sickle-cell trait Individuals with sickle cell disease have increased resistance to the malaria parasite, which infects red blood cells

97 Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Achondroplasia is a form of dwarfism caused by a rare dominant allele

98 Fig. 4-7 Parents Dwarf Normal Dd dd Sperm Eggs d D Dd Dwarf d dd Normal d Dd Dwarf dd Normal

99 Huntington s Disease A lethal dominant allele can escape elimination if it causes death at a relatively advanced age after the individual has past the trait to his or her children Huntington s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Any child born to a parent with Huntington s disease has a 50% chance of having the disease and disorder Afflicts in 0,000 in the USA The gene has now been sequenced and a test developed to detect the Huntington s allele

100 Multifactorial Disorders Many diseases, such as heart disease and cancer, have both genetic and environmental components Little is understood about the genetic contribution to most multifactorial diseases

101 Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

102 Counseling Based on Mendelian Genetics and Probability Rules Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders For example, both John and Carol had brothers who died of the same recessive disease John, Carol and their parents do not have the disease What are the chances that the their children will have the disease?

103 Deductions Let s use A to symbolize the normal allele, a will be the disease allele Both John s parents and Carol s parents must have been carriers of the disease (Aa) Neither John nor Carol is homozygous recessive (aa) because neither has the disease, so they must be each either AA or Aa

104 Calculations John and Carol each has a 2/3 chance of being a carrier (Aa) And a /3 chance of being homozygous dominant (AA) The probability that a child will have the disease if both are carriers is 2/3 (chance of John being a carrier) * 2/3 (Chance of Carol being a carrier) * ¼ (chance of the offspring of two carriers being homozygous recessive) = /9 If their first child is born with the disease then we know that John and Carol are both carriers In that case the chance that their next child will have the disease is also ¼.

105 Tests for Identifying Carriers For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately

106 Fetal Testing In amniocentesis, the liquid that bathes the fetus is removed and tested 4 th to 6 th week of pregnancy Fetal cells cultured and karyotypes

107 Fig. 4-8a Amniotic fluid withdrawn Fetus Centrifugation Placenta Uterus Cervix Fluid Fetal cells Several hours Biochemical tests Several weeks (a) Amniocentesis Several weeks Karyotyping

108 Fetal Testing In chorionic villus sampling (CVS), a sample of the placenta is removed and tested 8 th to 0 th week of pregnancy Faster karyotyping

109 Fig. 4-8b Placenta Chorionic villi Fetus Suction tube inserted through cervix Biochemical tests Several hours Fetal cells Karyotyping Several hours (b) Chorionic villus sampling (CVS)

110 Fig. 4-8 Amniotic fluid withdrawn Fetus Centrifugation Fetus Placenta Uterus Cervix Placenta Chorionic villi Suction tube inserted through cervix Fluid Fetal cells Several hours Several weeks Biochemical tests Several hours Fetal cells (a) Amniocentesis Several weeks Karyotyping Several hours (b) Chorionic villus sampling (CVS)

111 Fetal Testing Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero Video: Ultrasound of Human Fetus I

112 Newborn Screening Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States PKU Occurs in in births Individuals accumulate the amino acid phenylalanine and its derivative phenylpyruvate in blood to toxic levels Leads to mental retardation If disorder is detected early a special low diet in phenylalanine usually promotes normal development

113 Summary

114 You should now be able to:. Define the following terms: true breeding, hybridization, monohybrid cross, P generation, F generation, F 2 generation 2. Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype 3. Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F 2 generation

115 You should now be able to: 4. Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance 5. Define and give examples of pleiotropy and epistasis 6. Explain why lethal dominant genes are much rarer than lethal recessive genes 7. Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling