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Campbell and Reece Chapter 14.  Character: observable, heritable feature that may vary among individuals in a population  Trait: 1 of 2 or more detectable.

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Presentation on theme: "Campbell and Reece Chapter 14.  Character: observable, heritable feature that may vary among individuals in a population  Trait: 1 of 2 or more detectable."— Presentation transcript:

1 Campbell and Reece Chapter 14

2  Character: observable, heritable feature that may vary among individuals in a population  Trait: 1 of 2 or more detectable variants in a genetic character  True-breeding: refers to organisms that produce offspring of the same variety over many generations of self-pollination  Hybridization: cross-breeding 2 true- breeding individuals

3  Advantages of using peas: 1. several characters with “either-or” traits 2. short generation time 3. large #s of offspring 4. each flower contained both male & female organs


5  started with true breeders  cross-pollinated 2 contrasting, true- breeding pea varieties (hybridization)  true-breeding parents = P generation  their hybrid offspring = F 1 generation  F 1 self-pollinated = F 2 generation


7  did quantitative analysis of thousands of genetic crosses  deduced 2 principles of heredity: 1. Law of Segregation (monohybrids) 2. Law of Independent Assortment (dihybrids)



10  alternative versions of a gene

11 1. Alternative versions of genes (alleles) account for variations in inherited characters. 2. For each character, an organism inherits 2 copies of a gene, one from each parent. 3. If the 2 alleles @ a locus differ, then the dominant allele determines the organism’s appearance & the recessive allele has no noticeable effect on the organism’s appearance

12 4. Law of Segregation: the 2 alleles for a heritable character separate during gamete formation & end up in different gametes. (correlates to 2 homologous chromosomes separating in Meiosis I)

13  diagramatical device for predicting the allele composition of offspring from a cros between individuals froma known genetic makeup.  P signifies dominant  p signifies recessive

14  Homozygous: having 2 identical alleles for a given gene  Heterozygous: having 2 different alleles for a given gene  Phenotype: the observable physical & physiological traits of an organism, determined by its genetic makeup  Genotype: the genetic makeup or set of alleles of an organism  Testcross: breeding an organism of unknown genotype with a homozygous recessive to determine the unknown genotype



17  states basically that in a dihybrid cross each allele for the 2 characters being crossed has equal opportunity

18  Probabilities of all possible outcomes for an event = 1  outcome of any particular toss of a coin is unaffected by the results of any previous tosses

19  used to determine the probability that 2 or more independent events will occur together in some specific combination  multiply the probability of 1 event by the probability of the 2 nd event


21  the probability that any 1 of 2 or more mutually exclusive events will occur is calculated by adding their individual probabilities (which we calculate using the multiplication rule)



24  Alleles can show different degrees of dominance or recessiveness in relation to each other  Mendel’s peas characters were examples of complete dominance (all or none)  Incomplete Dominance: neither allele is completely dominant or recessive

25  Snapdragons

26  2 alleles affect the phenotype in separate, distinguishable ways


28  When a dominant allele coexists with a recessive allele in a heterozygote, they do not actually interact.  It’s in the pathway from genotype to phenotype that dominance & recessiveness come into play

29  Mendel’s peas Round/wrinkled  R allele codes for an enzyme that helps convert an unbranched form of starch  branched form in the seed  r codes for a defective form of same enzyme leading to an accumulation of unbranched starch which leads to excess water entering seed by osmosis

30  later, when seed dries it wrinkles  If R present, it makes enough enzyme to make enough branched starch to prevent wrinkling

31  sometimes depends on how closely we look  example: Tay Sachs disease  homozygous recessive  Those with it cannot metabolize certain lipids in neurons  lipids accumulate  child suffers neurological events (seizures, blindness, degeneration of motor & mental performance)

32  when study heterozygotes vs. homozygous dominant individuals: heterozygotes have an intermediate level of the activity of enzyme that metabolizes this lipid than do homozygous dominant individuals  on biochemical level acts like incomplete dominance since ½ the normal enzyme activity is sufficient to prevent lipid accumulation, heterozygotes have normal phenotype  on molecular level it is really an example of codominance

33  dominant allele not always more frequent allele in a population  example: polydactyly  extra fingers or toes  1/400 babies born in USA  some caused by presence of a dominant allele

34  most genes exist in >2 allelic forms  example: ABO blood groups


36  most genes have multiple phenotypic effects

37  Greek: standing apart  phenotypic expression of a gene at one locus alters that of a gene at 2 nd locus  example: color of labs

38  Quantitative Characters: phenotypes vary in gradation along continuum in a population (height, skin color)  Polygenic Inheritance: an additive effect of 2 or more genes on a single phenotypic character, several genes  single phenotype (converse of pleiotrophy: 1 gene  several characters)



41  for humans: very old ?  generally, genotype is NOT associated with a rigidly defined phenotype  see range of phenotypic possibilities due to environmental influences  phenotypic range is called: norm of reaction for a genotype  generally, broadest for polygenic characters

42  The environment contributes to the quantitative nature of polygenic characters which are referred to as multifactorial  influenced by genetics & environment (nutritional status, exposure to infectious disease, general well-being)

43  in place of looking at organisms as single gene  single phenotype  view organism as whole: emergent properties of all genes  all aspects of its phenotype  In most cases, a gene’s impact on phenotype is affected by genes & by the environment

44  In light of all the possibilities of gene interaction it was extremely lucky that Mendel chose to study inheritance in the garden pea he chose.


46  a diagram of a family tree with conventional symbols, showing the occurrence of heritable characters in parents & offspring over multiple generations




50  generally, the recessive homozygous either has a malfunctioning protein or no protein at all  heterozygous individuals produce enough of the normal protein to have “normal” phenotype & are called carriers

51  generally, genetic disorders are NOT evenly distributed among all groups of people  uneven distribution results from different genetic histories of world’s people when populations were more geographically isolated

52  when a disease-causing recessive allele is rare it is relatively unlikely that 2 carriers will meet & mate  if the 2 carriers are closely related (1 st cousins) the probablity of passing on recessive traits increases (consanguinous matings)


54  little or no pigment in skin, hair, eyes  affects: vision, skin

55  most common lethal genetic disease in USA  1/2500 people of European descent have CF  4% are carriers  normal allele codes for membrane protein that functions in transport of Cl-  affected individuals have defective or no Cl- membrane channel

56  result of abnl Cl- channel:  abnl high Cl- in extracellular fluid  mucus that coats certain cells to become thicker, stickier than normal  mucus more tenacious, builds up in pancreas, lungs, digestive tract, testes  has pleiotropic effects: poor digestion & absorption of nutrients (fats), chronic bronchitis, frequent bacterial infections, infertility (males), diabetes

57  autosomal recessive  growths grow on nerves  skin changes (3-5% growths cancerous)  hearing loss  bone damage

58  most common inherited disorder among people of African descent  1/400 African-Americans  single a.a. substituted in hgb  homozygous recessive individuals: all RBCs sickle shaped


60  when O 2 content of affected individual is low (hi altitudes, physical exertion) the sickle cell hgb molecules aggregate into rods  sickle shape to RBC  sickled RBCs will clog small vessels  weakness, pain, organ damage, paralysis  transfusion help prevent brain damage  no cure

61  1/10 African-Americans  unusually high frequency of heterozygosity considering homozygous recessive phenotype has such detrimental effects  Malaria parasite spends part of its life cycle in RBCs & even with only some sickeled cells present it lowers the density of the parasite  reduced malarial symptoms

62  those that are lethal less common than recessive disorders  most cause death of afflicted individual  all lethal alleles arise by mutations in gametes

63  form of dwarfism found in heterozygotes  1/2500 people have achondroplasia (0.01% of US)  If you do not have this form of dwarfism you are homozygous recessive for it

64  example of a lethal dominant allele that is passed on to offspring (50%) because it does not cause death until individual in mid-forties (phenotype normal til then)  degenerative, irreversible, untreatable disease of nervous system  can test DNA (tip of chromosome 4)


66  genetic (usually polygenic) + environmental components  examples:  heart disease  alcoholism  schizophrenia  bipolar disease

67  use multiplication rule to determine if potential parents are carriers  each child represents an individual event  it is incorrect to think: “If we have 1 child with a recessive disease then our next 3 children will have the normal phenotype”.

68  available for several of the recessive disorders  law passed in 2008 forbids discrimination by insurance carriers (or employers) from dropping coverage for known carriers

69  Amniocentesis:  amniotic fluid sample taken in 2 nd trimester  karyotype done on fetal cells  biochemical marker assayed  Chorionic Villus Sampling:  1 st trimester test by sampling placental tissue (1 layer formed by fetus)

70  newest technology: test mom’s blood  find fetal cells  culture & test them  Ultrasound (US) can identify many anatomical abnl in fetus  Fetoscopy: scope in amniotic cavity for diagnosis, possible treatment  Intra-amniotic surgery: repair neurotube defects, heart defects…..


72  most hospitals screen using heel prick  PKU (phenylketonuria)  recessive  1/10,000 to 1/15,000  cannot metabolize phenylalanine  causes severe drop in mental capacity  TX: diet free of phenylalanine


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