Chapter 9/12 Fundamentals of Genetics & Inheritance Patterns

Transcription

1 Chapter 9/12 Fundamentals of Genetics & Inheritance Patterns

2 Chapter 9: HEREDITY The transmission of traits from parents to offspring.

3 Historically, The Austrian monk Gregor Mendel (born 1822), a gardener, scientist and mathematician who bred pea plants.

4 Developed a set of rules that accurately predicted patterns of heredity. The basis of genetics the study of heredity and the prediction of offspring. Used mathematics in making predictions quantitative (statistical) approach.

5 Mendel s Garden **Chose garden pea for many reasons: --small/inexpensive --short growing season --many offspring --traits easy to distinguish --male and female flower parts enclosed in the same flower

6 -Observed the inheritance of traits Trait: genetically determined variant of a characteristic -then used statistics to analyze his observations

7 Trait Flower color Seed color Seed shape Pod color Pod shape Flower position Plant height

8 This allowed for self-pollination. One parent plant allows for control of outside factors Makes it easy to identify parent s gene type To experiment, Mendel had to remove anthers to cross-pollinate

9 MENDEL S EXPERIMENTAL DESIGN In the beginning, he allowed plants to self-pollinate for several generations. 1)Produced true-breeding plants, all have the same trait. 2)P generation parental/starting generation (homozygous)

10 He then crossed 2 different P- generation varieties with contrasting traits. Example TT X tt P cross T T t t Tt Tt Tt Tt F 1 generation the first filial generation (or the results of the P generation cross)are all heterozygous.

11 Next, he allowed the F 1 generation to self-pollinate. F 1 generation T t T TT Tt t Tt tt F2 generation second filial generation (or the results of the F1 generation cross).

12 Observations: -One trait would be the only one to appear in the F 1 generation and it would appear in the F 2 generation in 3 of 4 plants.

13 Mendel s Results & Conclusions -Hypothesized that something within pea plants controlled their traits = factors - since each trait had 2 forms, traits must be controlled by 2 separate factors & occur in pairs in the offspring

14 Dominant trait the trait, if present, in the plant, will be expressed. Capital letters are used (to show dominance) Ex tall = T

15 One trait would not appear in the F1, but would appear in the F2 in 1 out of 4 plants. Recessive trait the trait that is often present but remains hidden. This trait is only expressed when it is with another recessive trait. Lower case letters short = t

16 Mendel s Proposals in Modern Terms **Each individual has 2 genes for every trait, one from mom and one from dad.

17 Phenotype or physical appearance = what it looks like (Certain environmental conditions may contribute to physical appearance of organisms along with their genetics.) Genotype or genetic makeup = what genes or alleles makeup the individual Ex TT, Tt, tt (for height) Allele: different or alternative forms of the same gene Ex height (T or t)

18 Homozygous or pure = both alleles for a gene are the same. They are either dominant (TT) or recessive (tt). Heterozygous or hybrid = one gene is dominant and one is recessive (Tt), an individual with 2 different alleles.

19 Mendel s Laws of Heredity The Law of Segregation members of each pair of genes/alleles will separate when gametes (sex cells) form Tt T t --when gametes come together at fertilization, embryo gets a pair of factors per characteristic

20 The Law of Independent Assortment The inheritance of one trait does not influence the inheritance of another trait. TtGg TG Tg tg tg --factors separate independently of one another during gamete formation

21 Support for Mendel s Conclusions Molecular genetics: study of the structure & function of chromosomes & genes -chromosomes occur in pairs; therefore genes occur in pairs also (genes are found on chromosomes) Allele: each of the 2 or more alternative forms of a gene (factors)

22 -During meiosis, gametes receive one chromosome from each homologous pair -- supports law of segregation -- during fertilization, offspring receive one copy of each gene (& chromosome) from each parent - however, law of independent assortment is only observed for genes located on separate chromosomes, or genes far apart on the same chromosome

23 Jumping Genes 1)TRANSPOSONS are genes that have the ability to move (jump) from one chromosomal location to another. 2)Once every few thousand cell divisions, will change its position on its chromosome. 3)This jumping gene can cause the gene to become inactive or result in a mutation.

24 In the 1940 s, Barbara McClintock discovered that certain genetic elements can move from one location in a chromosome to another, or even from one chromosome to another. If these jumping genes land in the middle of other genes, it may disrupt them.

25 4)Ex-indian corn w/ diff. colored kernels

26 Predicting the Results of Monohybrid Crosses -use Punnett squares: diagram that distributes the probable inherited traits to offspring Monohybrid cross: cross in which only 1 characteristic is tracked -offspring called monohybrids

27 Examples: In peas, purple flower color is dominant to white. Let s cross a plant that is homozygous dominant by a plant that is homozygous recessive. What are the genotypes of the parents? PP & pp

28 - Remember that during gamete formation, the alleles for each trait separate p P Pp P Pp p Pp Pp

29 What are the genotypes of the offspring? All Pp = heterozygous What are the phenotypes of the offspring? All purple! 100%

30 Now cross a homozygous purple plant by a heterozygous purple plant. What are the genotypes of the parents? PP & Pp What are genotypes of the offspring? What are the phenotypes of the offspring? What are the ratios the phenotypes occur in?

31 Genotypes: 50% PP (homo. dom.) P P PP P PP & 50% Pp (hetero.) p Pp Pp Phenotypes: All purple! 100%

32 Now you do it! Cross 2 heterozygous purple pea plants. What are the genotypic ratios & phenotypic ratios of the offspring? 25% PP, 50% Pp, 25% pp 75% will be purple 25% will be white

33 What could you say is true of the parent purple flower? Homozygous dominant

34 Predict offspring using a Punnett Square Show genotype and phenotype. 1)heterozygous tall X heterozygous tall 2)homozygous tall X short 3)heterozygous tall X short 4)heterozygous tall X homozygous tall

35 1) 2) T t T T T TT Tt t Tt Tt t Tt tt t Tt Tt 3) T t 4) T t t Tt tt T TT Tt t Tt tt T TT Tt

36 Testcross -An individual w/ unknown genotype is crossed with a homozygous recessive individual -Can determine the genotype of any individual whose phenotype is dominant -- if any of the offspring produced express the recessive trait, you know the genotype of the unknown organism is heterozygous

37 ie: person w/ wavy hair is intermediate between straight & curly haired parents Incomplete dominance Complete dominance: one allele is completely dominant over another -like the examples we have shown Incomplete dominance: occurs when the phenotype of a heterozygote is an intermediate between the phenotypes that are dominant & recessive

38 -Both alleles are equally strong Ex: Red plant X white plant = pink plant Genotypes? Red plant = RR White plant = WW Pink plant = RW

39 In snapdragon flowers, where pink is the intermediate. P 1 = Pink X Red R W Phenotype: R RR RW Red 2 R RR RW Pink 2 Genotype: RR 2 RW - 2

40 Codominance -Occur when both alleles are expressed in the heterozygous offspring -Neither allele is dominant or recessive, nor do the alleles blend (like in incomplete dominance) Ex: roan cattle/horses & brindle colored dogs & flowers w/ 2 different colored petals & blood type

41 Roan horses have both red and white hair, brindle dogs have both brown and black hair

42 Multiple Alleles -Genes w/ 3 or more alleles (instead of just 2) Ex: human blood type (3 forms--presence of A, presence of B, lack of both = o) is codominant: both A and B alleles are expressed in a heterozygote; blood type is a combination of any 2 of the 3 possible forms

43 blood type is the presence or absence of up to 2 proteins Phenotype (blood type) A B AB O Genotype AA or AO BB or BO AB OO

44 Dihybrid Crosses - 2 characteristics are tracked (2 sets of different alleles). -Predict offspring using Punnett Square. Show genotype and phenotype. Cross a heterozygous tall, hetero-green plant by a heterozygous tall, hetero-green

45 USE FOIL F = FIRST TG O = OUTSIDE Tg I = INSIDE tg T t G g L = LAST tg

46 TG Tg tg tg TG Tg tg TTGG TTGg TtGG TtGg TTGg TTgg TtGg Ttgg TtGG TtGg ttgg ttgg tg TtGg Ttgg ttgg ttgg Phenotype: tall green 9 tall yellow 3 short green 3 short yellow 1 Genotype: TTGG - 1 TtGG - 2 ttgg - 1 TTGg - 2 TtGg - 4 ttgg - 2 TTgg - 1 Ttgg - 2 ttgg - 1

47 Studying Two Traits at once: (Dihybrid Cross) AA or Aa = purple; aa = white BB or Bb = tall; bb = short

48 Probability the likelihood that a trait will occur. The larger the sample, the closer you will come to the predicted ratios. -- a decimal, percentage, or fraction Probability = # of times an event is likely to occur Total # of possible occurrences

49 Ex: probability of flipping heads? ½, 50%, or 0.5 What s this got to do with genetics? - Helps us determine the probability of offspring having a certain genotype or phenotype

50 Ch 12: Inheritance Patterns & Human Genetics Early 1900 s: Thomas Hunt Morgan experimented w/ fruit flies (Drosophila melanogaster) - Have 4 pairs of chromosomes: 3 autosomes, 1 pair of sex chromosomes

51 Sex chromosomes: contain genes that determine the gender of an individual Autosomes: chromosomes that do not directly determine sex Males = XY Females = XX - Most plants & some fish lack sex chromosomes entirely

52 Sex Determination -Sex chromosomes separate during meiosis -Egg cells will only receive X chromosomes, sperm could receive X or Y

53 Sex-determining Region Y: gene on Y chromosome that codes for protein that causes the gonads of embryo to develop into testes; otherwise, gonads become ovaries Therefore, sperm determines sex of offspring!

54 Effects of Gene Location -When working w/ fruit flies, Morgan noticed that the gene for eye color was related to the sex of the organism - only males could be white-eyed Sex-linked traits: traits that are coded for by alleles on a sex chromosome - X chromosome is larger = more X-linked traits than Y-linked traits

55 Linked Genes -Pairs of genes that tend to be inherited together Set of linked genes = linkage group -Found on same chromosomes -Can become rearranged during crossingover of meiosis 1

56 -Most X-linked alleles do not have homologous counterparts on the Y chromosome -Since males have only 1 X, if that X carries recessive alleles, the male will exhibit that sex-linked trait -Males exhibit more sex-linked disorders than women (eg = color blindness)

57 X-Linked Traits -More common in males than in females -Males inherit their X-chromosomes from their mothers (& none from father) - therefore, males have only allele to rely on, whereas females have 2 ie: colorblindness (more common in males than females)

58 Sex-Influenced Traits -Males & females can show different phenotypes even w/ the same genotype -Usually autosomal ie: baldness

59 Hemophilia a condition that impairs the blood s ability to clot *A dozen genes code for proteins involved in blood clotting and any can mutate. *Two of these are found on the X chromosome and make them sex-linked.

60 Since females are XX, they have three different possibilities when it comes to sex-linkage. In hemophilia, Genotype Phenotype X N X N Normal female (2?) X N X n X n X n Hemophiliac female

61 Since males are XY, they only have two possibilities. In hemophilia, Genotype X N Y X n Y Phenotype Normal male Hemophiliac male

62 Cross a man who has hemophilia with a woman who is a carrier. X n Y Genotype X N X n - 1 X N X N X n X N Y X n X n - 1 X n X n X n X n Y X N Y - 1 X n Y - 1 Phenotype Normal female 1 Normal male 1 Hemophiliac female - 1 Hemo male - 1

63 Dominant Allele Disorders - more than 200 different human traits controlled by single dominant alleles 1.Huntington Disease: autosomal dominant condition where brain tissue gradually deteriorates - unfortunately, symptoms don t appear until 30 s-40 s (after children)

64 2.Alzheimer s Disease also brain deterioration but in different ways

65 Recessive Allele Disorders 1.Sickle-cell Anemia a defective form of hemoglobin causes RBC s to bend in a sickle shape --Problems include: a.can t carry much O2 b.cells rupture easily c.cells get stuck in blood vessels, very painful. --Advantage: a heterozygous individual is protected from malaria

66 2. Cystic Fibrosis- mucus build-up in respiratory system -Breathing treatments needed -Medication to decrease mucus -Affected die young -Western Europe caucasian

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68 Non-disjunction Disorders -Chromosomes fail to separate in meiosis -Results in monosomy or trisomy 1. Down Syndrome - trisomy on chromosome #21

69 - A gamete receives an extra copy of a chromosome & 1 gamete receives none

70 2. Edwards Syndrome - trisomy on chromosome #18 *affects almost every organ system

71 3. Klinefelter Syndrome - trisomy in male *occurs in 1 of every 2000 births *has male sex organs but are sterile (XXY)

72 4. Metafemale - trisomy in female (XXX) *occurs in 1 of every 1000 births *limited fertility but otherwise appear normal

73 . Turner Syndrome - monosomy in female (XO) *occurs in 1 of every 5000 births *no mature sex organs, sterile

74 Chromosome Mapping -The farther apart 2 genes are located on a chromosome, the more likely a cross-over will occur Chromosome map: diagram that shows the linear order of genes on a chromosome -% of crossing-over for 2 traits is proportional to the distance between them on a chromosome

75 Mutations -A change in the nucleotide-base sequence of a gene or DNA molecule Germ-cell mutations: occur in an organism s gametes -don t affect org itself but can be passed on to offspring

76 Somatic-cell mutations: take place in org s body cells & can affect organism -Cannot be inherited lethal mutations: cause death, often before birth - There can be beneficial mutations; these will help organism to survive & reproduce = gives evolutionary advantage

77 Chromosome Mutations -Involve changes in the structure of a chromosome or the loss or gain of a chromosome 5 Types: (book only gives 4) Deletion, Duplication, Inversion, Translocation, Nondisjunction

78 1-DELETION = a fragment of the chromosome breaks off and is lost (only dealing with one homologue) For example, in this picture gene 3 has broken off and been lost. becomes Williams Syndrome deletion of about 15 genes on 1 of the homologous chromosomes in chromosome #7 (Where did gene 3 run off to?)

79 2-DUPLICATION = chromosome fragment attaches to a homologue now one homologue has 2 sets of (same) info. and the other is missing info. (Old Homologue 2) (Homologue 1 is left without genes 1 & 2. Homologue 2 ends up with both copies of genes #1 & 2.) (New Homologue 2) OLD {12} NEW {12}345678

80 3-INVERSION = chromosome breaks off and reattaches in reverse order (only dealing with one homologue) {234} Becomes {432}

81 4-TRANSLOCATION = a fragment breaks off and attaches to a non- homologue (Example chromosome 1 has a piece break off and attach to chromosome number 2 which is a non-homologue) (What will the new chromosome #1 look like?) Chromo.#1 Chromo. #2 New #2

82 1. Substitution: one nucleotide replaces another; could result in wrong aa being used in protein Gene Mutations - Change that occurs within a single gene or other segment of DNA on a chromosome Point mutation: substitution, addition, or removal of a single nucleotide 2 Types:

83 2. Frameshift mutation: results from insertion or deletion of a nucleotide that effects how mrna sequence is read to produce proteins

84 Inheritance of Traits -Use pedigrees to trace traits thru generations Pedigree: diagram that shows how a trait is inherited over several generations Squares = males Filled in = had trait circles = females empty = no trait Lines show unions/matings & offspring

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86 -Pedigrees help determine patterns of inheritance -If trait is sex-linked, mostly males will exhibit it; most sex-linked traits are recessive -If autosomal dominant, every individual w/ trait will also have parent w/ trait - 2 heterozygous parents can have children w/ recessive trait

87 Carriers: individuals w/ 1 copy of the recessive allele (for the trait), but do not exhibit the disease - Are shown on a pedigree by using a half-filled in circle or square

88 Genetic Traits Polygenic inheritance: most human characteristics are controlled by several genes Ex: eye color, height, hair color, skin color

89 Detecting Genetic Disease Genetic screening: examination of a person s genetic makeup -karyotypes, blood tests for proteins, or tests of DNA Amniocentesis: amniotic fluid is removed from amnion (sac of water surrounding fetus)

90 Chorionic villi sampling: sample of cells from chorionic villi (grow between uterus & placenta) -Both analyze fetal cells, chromosomes, & proteins to detect genetic disorders Genetic counseling: process of informing person or couple about their genetic makeup; informs them of problems that might affect their offspring -can advise them how to lower risk factors

91 Treating Genetic Disease 1.Treat the symptoms thru therapy or medicine. 2.Gene therapy: technique that places a healthy copy of a gene into the cells of a person whose copy of the gene is defective - 2 types: somatic cell gene therapy & germ cell gene therapy

92 Somatic cell gene therapy: only body cells are altered; extension of normal medicine to improve symptoms -Is temporary & must be repeated Germ cell gene therapy: attempts to alter sex cells - Poses more risks & ethical issues because future generations could be affected