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Chapter 13: Patterns of Inheritance

Chapter Outline

Chapter 13: Patterns of Inheritance

 

13.0 Introduction
  1. Enigma of Heredity

    1. People in Different Parts of the World Vary in Appearance fig 13.1

    2. Members of a Family Tend to Look Alike

13.1 Mendel solved the mystery of heredity
  1. Early Ideas About Heredity: The Road to Mendel

    1. Variation in Appearance

      1. Similarities within families fig 13.2

      2. Certain characteristics are more common among families fig 13.3

    2. Classical Assumptions 1: Constancy of Species

      1. Heredity occurs within species

        1. Cannot create bizarre creatures by cross breeding

        2. Common animals are not combinations of breeding

      2. Variation occurs within boundaries of a species

    3. Classical Assumption 2: Direct Transmission of Traits

      1. Traits are transmitted directly

        1. Once thought body parts transmitted in sex cells

        2. Darwin proposed gemmules transmitted characteristics to offspring

      2. Thought male and female traits blended in offspring

    4. Koelreuter Demonstrates Hybridization Between Species

      1. Assumption 1 and assumption 2 together lead to paradox

        1. If no variation enters from outside species

        2. If variation blended with each generation

        3. In time, would result in little species variation

      2. Koelreuter hybridized tobacco plants

        1. Offspring appeared different from either parent

        2. Crosses of hybrids resulted in further variation

        3. Offspring resembled parents or grandparents

        4. Parental traits not blended

    5. The Classical Assumptions Fail

      1. Koelreuter's experiments signal beginning of modern genetics

      2. Traits masked for a generation, reappeared in next

      3. Alternative forms segregating among offspring

    6. Knight Studies Heredity in Peas

      1. Crossed true-breeding peas, purple and white flowers fig 13.4

        1. All offspring of first cross had purple flowers

        2. Offspring of next cross had both color flowers

        3. Purple flowers predominated over white flowers

      2. Failure to quantify results prolonged discovery of important concepts

  2. Mendel and the Garden Pea

    1. Carried Out First Quantitative Studies

      1. Austrian monk educated in a monastery fig 13.5

      2. Initiated experiments on plant hybridization fig 13.6

    2. Why Mendel Chose the Garden Pea

      1. Expected segregation among offspring, via early studies

      2. Many true-breeding traits, studied only seven

      3. Small plants, easy to grow, short generation time

      4. Male and female parts within flower fig 13.7

        1. Self-fertilized male and female from same flower

        2. Cross-pollinated female with other flower's pollen, cross-fertilization resulted

    3. Mendel's Experimental Design

      1. Reasons for Mendel's success

        1. Focused on only a few traits

        2. Selected comparable traits

      2. Conducted experiments in three stages

      3. Allowed several generations of self-fertilization

        1. Progeny produced only a single form of a trait

        2. Assured that forms of traits were transmitted regularly

      4. Conducted crosses between alternate forms of a trait fig 13.8

        1. Removed male parts from a flower with white flowers

        2. Fertilized with pollen from plant with purple flowers

        3. Performed reciprocal crosses white flower pollen on purple flower plant

      5. Allowed self-pollination of hybrids

        1. Allowed segregation of alternate forms of traits

        2. Counted number of offspring of each type per generation

        3. Quantification of results most important to studies

  3. What Mendel Found

    1. Mendel's Seven Traits Produced Recognizable and Scorable Results fig 13.9

    2. The F1 Generation

      1. First filial (F1) progeny resembled one of parents

      2. Trait expressed in F1 called dominant

      3. Trait masked in F1 called recessive

      4. All seven traits had dominant and recessive forms

    3. The F2 Generation

      1. Planted F1 seeds to produce F2 (second filial) generation examining flower color

      2. Counted F2 generation, determined proportion of dominant to recessive fig 13.9

        1. Three fourths of plants exhibited dominant form

        2. One fourth of plants exhibited masked, recessive form

        3. Dominant:recessive ratio was close to 3:1 for all seven traits

      3. Obtained same results comparing round and wrinkled seed shape fig 13.10

    4. A Disguised 1:2:1 Generation

      1. Recessive individuals always bred true

      2. One third of dominant individuals bred true

      3. Two thirds of dominant individuals produced 3:1 progeny

      4. 3:1 ratio really 1:2:1 ratio fig 13.11

        1. 1/4 dominant, true-breeding

        2. 1/2 dominant, not true-breeding:recessive

        3. 1/4 true breeding, recessive

  4. Mendel's Model of Heredity

    1. Mendel Determined Four Things About the Nature of Heredity

      1. Plants did not produce blended, intermediate characteristics

      2. For each trait with two alternates, one was not expressed in F1 generation

      3. Expression of traits segregated among progeny of a cross

      4. Expression of F2 generation produced 3:1 Mendelian ratio

    2. Mendel's Model Has Five Elements

      1. Parents transmit factors that provide information about traits

      2. Each individual contains two factors for each trait

        1. May code for same form or alternative forms

        2. Diploid set of chromosomes in individuals, thus two forms

        3. Haploid chromosomes randomly distributed in gametes

      3. Not all copies of a factor are identical

        1. Alternate forms of factor called alleles

        2. Individual is homozygous when both alleles are the same

        3. Individual is heterozygous when alleles are different

        4. Position of gene on DNA is called its locus

      4. Alleles from each parent do not influence one another

        1. They remain discrete and "uncontaminated "

        2. They do not blend with one another

        3. They further segregate randomly when forming progeny

      5. Presence of a factor does not insure its expression

        1. Heterozygote dominant expressed, recessive unexpressed

        2. Genotype is the totality of the genes (blueprint)

        3. Phenotype is the expression of the genes (visible outcome)

    3. Complete Dominance of One Allele Over Another fig 13.12

      1. Exhibited in all seven traits studied by Mendel

      2. Exhibited by many human traits tbl 13.1

  5. How Mendel Interpreted His Results

    1. Mendel Tested His Model, Predicted Results

      1. Expressed model in terms of simple symbols

      2. Example: Purple-white flower cross (letter choice is most common trait)

        1. Dominant purple flowers designated P

        2. Recessive white flowers designated p

        3. True-breeding recessive thus designated pp

        4. True-breeding dominant designated PP

        5. Heterozygote designated Pp (dominant trait first)

        6. Mendel's original cross designated pp PP fig 13.13

    2. The F1 Generation

      1. Each parent can produce gametes of only its kind

        1. Purple gametes contain only P allele

        2. White gametes contain only p allele

      2. Resulting progeny all Pp, since P is dominant, all purple heterozygotes fig 13.13

      3. Recessive p allele present but not expressed

    3. The F2 Generation

      1. All are heterozygous, purple, cross = Pp Pp

      2. Alleles segregate randomly in gametes, either P or p

      3. Construct Punnett square to determine progeny of cross fig 13.14

      4. Square predicts 3:1 phenotypic ratio

    4. The Laws of Probability Can Predict Mendel's Results

      1. Expression of dominant trait is 3/4 (three chances in four)

      2. Expression of recessive trait 1/4 (one chance in four)

      3. Make simple predictions about outcomes of crosses

        1. If F1 parents are both Pp, determine probability of offspring being pp

        2. 1/2 chance that male parent donates p, 1/2 chance female donates p

        3. 1/2 1/2 = 1/4 chance of progeny being pp

        4. Same result as determined by Punnett square

    5. Further Generations

      1. Three kinds of F2 individuals

        1. Pure-breeding white flowers (pp)

        2. Heterozygous purple flowers (Pp)

        3. Pure-breeding purple flowers (PP)

      2. Closer examination of 3:1 ratio indicates 1:2:1 genotypic ratio

    6. Mendel's First Law of Heredity: Segregation

      1. Alternative forms encoded by discrete alleles

      2. Alternative alleles separate at random along metaphase plate

      3. Explained segregation without cellular knowledge, chromosomes or meiosis

    7. The Testcross

      1. Experimental cross with homozygous recessive called a testcross

      2. Used to determine genotype of dominant phenotype

      3. Observing phenotype insufficient, PP and Pp appear same

      4. Cross unknown to organism of known lineage fig 13.15

        1. Homozygous dominant (PP) produces dominant phenotype (Pp or PP)

        2. Heterozygous (Pp) produces all possible genotypes of offspring (PP, Pp, pp)

        3. Homozygous recessive as known parent (pp) produces two totally different results

          1. All Pp offspring indicates PP unknown

          2. Half Pp, half pp offspring indicates Pp unknown

      5. Mendel's testcross helped determine identity of 3:1 F1 generation fig 13.15

        1. Predicted resulting 1:1 ratio

        2. Observed 1:1 ratio as predicted

      6. Testcross used to determine genotype when two genes involved

        1. F2 individual with both dominant traits (A_B_)

        2. Could be any one of four genotypes: AABB, AaBB, AABb or AaBb

        3. Performed test cross A_B_ aabb

        4. Determined F2 genotype based on whether further generations bred true

          1. Was AABB if both bred true

          2. Was AaBB if only B bred true

          3. Was AABb if only A bred true

          4. Was AaBb if neither bred true

    8. Mendel's Second Law of Heredity: Independent Assortment

      1. Mendel questioned effect of traits upon one another

      2. Genes located on different chromosomes assort independent of one another

      3. Step 1: Establish pure-breeding lines differing in two traits

      4. Step 2: Cross contrasting pairs of traits

        1. Results in F2 generation of identical dihybrids

        2. Dihybrids are individuals heterozygous for two genes

      5. Step 3: Allow dihybrids to self-fertilize

        1. 1/4 chance for a single trait to occur

        2. 1/4 x 1/4 = 1/16 for any pair to occur

        3. Predicts 9:3:3:1 ratio

      6. Example: Shape=Round (R) or wrinkled (r) and color= Yellow (Y) or green (y)

        1. Four types of gametes possible: RY, Ry, rY, ry

        2. Sixteen types of offspring all equally possible fig 13.16

        3. Nine individuals possess R_Y_ = round, yellow

        4. Three individuals possess R_yy = round, green

        5. Three individuals possess rrY_ = wrinkled, yellow

        6. One individual possesses rryy = wrinkled, green

        7. Observation fulfills prediction

      7. Genes assort independently when traits on different chromosomes

      8. Chromosomes assort independently during meiosis in gamete formation

  6. Mendelian Inheritance Is Not Always Easy to Analyze

    7.
    1. Mendel's Work Rediscovered After His Death

    2. Modified Mendelian Ratios

      1. Difficult to obtain simple ratios in some dihybrid crosses

      2. Difficult to clearly identify four phenotypic classes

    3. Epistasis

      4.
      1. Observed in varieties of corn producing purple anthocyanin pigment

        1. Emerson cross two true-breeding varieties without any purple pigment

        2. All F1 plants produced purple seeds

        3. F2 cross produced 56% plants with pigment, 44% without

        4. Deduced two genes responsible for seed coat color

          1. modified ratio of 9:7

      2. Why was Emerson's ratio modified?

        3.
        1. Genes may act sequentially in biochemical pathway

          1. Defect early in pathway blocks rest of pathway

          2. Cannot determine if rest of pathway is working properly

        2. Gene interaction where one gene modifies expression of other gene

        3. Pigment producing pathway explained

          4.
          1. Molecule (colorless) ® intermediate (colorless) ® anthocyanin (purple)

          2. Plant needs one functional copy of gene for both enzymes fig 13.17

          3. Of 16 genotypes, 9 code for at least one dominant gene for both enzymes

          4. Remaining 7 (3+3+1) lack dominant alleles at one or both loci

            5.
    4. Continuous Variation

      1. Most traits reflect action of polygenes, genes act sequentially or jointly

      2. Trait shows a range of small differences like height and weight

        1. Genes all segregate independently, see gradation in degree of difference fig 13.18

        2. Gradation called continuous variation

      3. Analyze traits by grouping individuals into categories, not dealing with raw data

        1. Example: Height

        2. Measure height, round fractions to nearest higher value

        3. Each value,phenotypic category, plotted as histogram fig 13.18b

        4. Graph approximated bell-shaped curve

    5. Pleiotropic Effects

      1. Pleiotropic gene has more than one effect on phenotype

      2. Example: Cuenot's experiments on yellow fur in mice

        1. Dominant trait, unable to obtain true-breeding strain

        2. Homozygous yellow offspring died

        3. Allele for yellow fur not only produced color, but was lethal

      3. One gene affects many traits (in polygeny many genes affect one trait)

      4. Difficult to predict since genes perform other unknown functions

      5. Characteristic of many inherited disorders

        1. Cystic fibrosis symptoms affect many organs

        2. Are effects of one mutated gene encoding chloride ion transmembrane channel

      6. Sickle-cell anemia

        1. Defect in oxygen-carrying hemoglobin molecule

        2. Produces numerous systems and organ damage

    6. Lack of Complete Dominance

      1. Alternative alleles not dominant or recessive fig 13.19

      2. Some heterozygous are intermediate between condition of each parent

      3. Example: Japanese four-o'clock flower color, parents red or white

        1. F1 offspring all pink in color

        2. F2 offspring ratio 1:2:1 (red:pink:white

        3. Thus all heterozygotes pink, intermediate in color

    7. Environmental Effects

      8.
      1. Expression of gene modified by environment

      2. Traits usually sensitive to temperature or light

      3. Example: Arctic fox fur pigment made only in warm weather fig 13.20

      4. Example: ch allele in Himalayan rabbits and Siamese cats

        5.
        1. Allele encodes heat-sensitive version of tyrosinase

        2. Enzyme mediates production of dark pigment melanin

          3.
        3. ch version of allele inactivates above 33° C

        4. Where body temperature is lower (ears, snout, feet, tip of tail) melanin is produced

        5. Where body temperature is higher (body) melanin not produced (white)

          6.

13.2 Genes are on chromosomes
  1. Chromosomes: The Vehicles of Mendelian Inheritance

    1. Many Organelles Segregate in Meiosis, Chromosomes Not Obvious Carriers of Heredity

    2. The Chromosomal Theory of Inheritance

      1. Support for Sutton's theory formulated in 1902

        1. Reproduction involves union of egg and sperm

        2. Mendel's theory expected hereditary contributions of two individuals

        3. Since sperm contains virtually only a nucleus, hereditary material resides there

      2. Diploid adults have two copies of each chromosome, gametes have one copy

      3. Chromosomes segregate in meiosis, characteristic consistent with Mendel's theory

    3. Problems with the Chromosomal Theory

      1. If Mendelian traits determined by genes on chromosomes

      2. And if Mendelian independent assortment reflects meiotic assortment

      3. Why is number of assorting traits far greater than number of chromosome pairs?

    4. Morgan's White-Eyed Fly

      1. Helped support chromosomal theory of inheritance

      2. Discovered mutant, white-eyed male fruit fly fig 13.21

        1. Crossed with wild type red-eyed female

        2. All progeny had red eyes, concluded red eye color dominant

      3. Crossed progeny of F1 generation

        1. 82%: 18% approximated 3:1 ratio red to white eyes

        2. All recessive white eye flies were male

      4. Testcross F1 females to white-eyed male

        1. 1:1:1:1 ratio

        2. Eye color and sex equally represented, female and male could have white eyes

    5. Sex Linkage

      1. Sex determination in fruit flies

        1. Two kinds of sex chromosomes, X and Y

        2. XX = female, XY = male

      2. Morgan's explanation: Eye color gene associated with sex chromosome fig 13.22

        1. Eye color gene located on the X chromosome

        2. Since Y chromosome doesn't have eye color gene, eye color is white

      3. Sex linked trait

      4. White-eye trait is recessive to red-eye trait

    6. Morgan's Experiment Presented First Clear Data that Genes Resided on Chromosomes

      1. Segregation of white-eye gene corresponded to segregation of sex chromosomes

      2. Traits segregate because chromosomes do during meiosis

  2. Genetic Recombination

    1. More Independently Assorting Factors than Chromosomes

      1. de Vries suggested that homologous chromosomes exchange elements in meiosis

      2. Janssens discovered chiasmata in chromosomes during meiosis

        1. Of the four chromatids , two crossed, two did not

        2. Proposed exchange of chromosomal arms

        3. Not accepted due to questioning of required precision of exchange

    2. Crossing Over

      1. Stern's experiments on fruit flies fig 13.23

        1. Examined two sex-liked traits on chromosomes with abnormal ends

        2. Observed corresponding changes in genetic traits

      2. Crossing over can occur anywhere along length of homologues

        1. Crossing over more likely if genes far apart

        2. Independent assortment still occurs if genes far apart fig 13.24

    3. Using Recombination to Make Genetic Maps

      1. Crossing over more likely if genes are far apart

      2. Frequency of crossing over used to determine relative positions of genes

        1. Distance between genes = frequency of crossing over

        2. One map unit, centimorgan = 1% recombination

      3. Modern technology creates more precise gene maps

        1. Map position of restriction sequences

        2. Sequences recognized by DNA-cleaving restriction endonucleases

      4. Recombination maps still valuable for widely separated genes

      5. Three-point cross

        1. Monitor recombination among three or more genes

          1. Syntenic alleles located on same chromosome

          2. Linked genes do not assort independently

        2. Three-point cross involves three linked genes

        3. First genetic map constructed by Sturtevant on fruit flies fig 13.25

          1. Wild type (+) is most frequent allele of a locus

          2. Other alleles given specific symbols

      6. Analyzing a three-point cross

        1. Traits examined were all located on the X chromosome

          1. y = yellow body color (normal is grey)

          2. w = white eye color (normal is red)

          3. min = miniature wing length (normal is 50% longer)

        2. Crossed female with all three mutations to male without any

          1. All progeny were heterozygotes

          2. Any resultant crossing-over produced recombinant chromosomes

          3. Detected changes in progeny

        3. Conducted testcross of female heterozygotes to triply recessive males

          1. Males contribute Y with no genes or X with all three recessives

          2. Tabulation of results of cross tbl 13.2

          3. Consider traits in pairs to determine if a crossover event was involved

          4. Similarly analyze other pairs to determine position of other genes

          5. Greatest distance 33.8 separates outside genes y and min

          6. gene w is between them, much closer (1.3 vs. 32.6) to y

    4. The Human Genetic Map

      1. Human genetic maps used to determine genetic disorders fig 13.26

      2. Genetic engineering permits isolation and sequencing of specific genes

        1. Differences in nucleotide sequence may suggest therapies

        2. May allow replacement of non-functional gene with functional one

      3. Human genome project

        1. Collective attempts to map entire set of human chromosomes

        2. Map initially consists of library of thousands of fragments

        3. Screen library to determine which fragment contains gene of interest

        4. Entire genomes of smaller genomes (some yeast, bacteria) already determined

        5. Completion of project within the decade

13.3 Human genetics follows Mendelian principles
  1. Multiple Alleles: The ABO Blood Groups

    1. Most Genes Possess More than Two Possible Alleles

      1. Often neither allele is completely dominant over any other

      2. Alleles are then considered to be co-dominant

    2. ABO Blood Groups Contain Co-Dominant Genes

      1. Three alleles encode cell surface antigens

        1. Antigens are sugars that are attached to lipids on blood cell surface

        2. Gene encoding the enzyme that adds the sugar is designated I

        3. Allele B(IB) codes for galactose

        4. Allele A (IA) codes for galactosamine

        5. Allele O (i) codes for neither sugar

      2. IA and IB are codominant and can be expressed together

      3. IA and IB are both dominant over i

      4. Four phenotypes produced from three alleles

        1. Type A: Genotype IAIA or Iai adds only galactosamine

        2. Type B: Genotype IBIB or Ibi adds only galactose

        3. Type AB: Genotype IAIB adds both sugars

        4. Type O: Genotype ii adds neither sugar

      5. Four phenotypes called ABO blood groups

      6. Blood may agglutinate due to presence of antigens fig 13.27

        1. Type A blood recognizes type B blood and reacts with B antigens

        2. Type A blood recognizes type AB blood reacts with B antigens

        3. Type A blood does not recognize type O blood since no antigens

        4. Type AB blood does not recognize either A or B as foreign

    3. The Rh Blood Group

      1. Associated with presence of Rh cell surface markers

        1. Rh-positive possess marker, most adult humans

        2. Rh-negative lacks marker, fewer in number

        3. Rh-negative is homozygous recessive condition

      2. Blood may agglutinate due to presence of antigens

        1. Rh-negative mother, Rh-positive child (Rh-positive father)

        2. Rh-positive blood crosses placenta into mother's blood

        3. Induces production of anti-Rh antibodies in mother's blood

        4. In later pregnancy, Rh antibodies can cross back

        5. Cause next Rh-positive baby's blood to clump: Erythroblastosis fetalis

  2. Human Chromosomes

    1. Morphology of Human Chromosomes

      1. 46 chromosomes in 23 pairs

      2. Divided into seven groups: A to G fig 13.28

    2. Sex Chromosomes

      1. 22 pairs of autosomes, 2 sex chromosomes

      2. XX is normal female

      3. XY is normal male

        1. Y has few active genes, counterparts to X alleles

        2. Genes for maleness present on Y

        3. Male possesses at least one Y

    3. Barr Bodies

      1. With XX, females do not produce twice as much protein as male

        1. One X inactivated in form of Barr body

        2. Other X active and expressed, activity of X is random in each cell

        3. Barr body stains deeply, attached to nuclear membrane fig 13.29

  3. Human Abnormalities Due to Alterations in Chromosome Number

    1. Primary Nondisjunction

      1. Caused by failure of chromosomes to separate in meiosis

      2. Can result in severe abnormalities

    2. Nondisjunction Involving Autosomes

      1. Monosomics possess one less copy of an autosome, do not survive

      2. Trisomics possess one extra copy of an autosome, most do not survive

        1. Trisomy in one of five smallest chromosomes may survive

        2. Extra 13, 15, 18 causes severe developmental defects

        3. Extra 21 or 22 may survive to adulthood, retarded skeletal and mental development

      3. Down Syndrome

        1. Results from extra chromosome 21 fig 13.30

          1. Usually entire chromosome is present

          2. In 3% only small portion is present, called translocation Down syndrome

        2. Gene causing syndrome may be similar to one that causes Alzheimer's disease

          1. Correlation with cancer-causing genes

          2. Cancer, leukemia more common in Down syndrome

        3. Arises from primary nondisjunction during meiosis

          1. More likely to occur in pregnancy of older women fig 13.31

          2. Woman produces all eggs by time she is born

          3. Men produce new sperm continually

    3. Nondisjunction Involving the Sex Chromosomes

      1. The X chromosome fig 13.32

        1. Produces XX gamete and O gamete

        2. XX plus normal X results in XXX individual

          1. Two Barr bodies, one active X

          2. Sterile, but otherwise normal female

        3. XX plus normal Y results in XXY individual

          1. Kleinfelter syndrome

          2. Sterile male with female characteristics

        4. O plus normal Y results in nonviable YO individual

        5. O plus normal X results in XO individual

          1. Turner syndrome

          2. Sterile female with specific appearance, low-normal mental capabilities

      2. The Y Chromosome

        1. Produces YY gametes

        2. YY plus normal X results in XYY individual

          1. Fertile males with normal appearance

          2. Greater numbers of individuals in penal institutions

          3. Controversial theory about antisocial behavior in XYY males

          4. Most XYY males do not develop such behaviors

  4. Human Genetic Disorders tbl 13.3

    1. Variant Alleles Exist in Populations

      1. Mutation involves random changes in genes

      2. Variant alleles are rarely produced by mutations

      3. Variant detrimental alleles exist in populations

        1. Are usually recessive to other alleles

        2. Are maintained in populations in heterozygous carriers

      4. Genetic disorder: Detrimental gene at high frequency in population

    2. Most Gene Defects Are Rare: Tay-Sachs Disease fig 13.33

      1. Causes fatal brain deterioration in children

      2. Highest occurrence in Jewish populations

        1. in 300,000 of overall population exhibit disease

        2. in 28 in specific population carry defective gene

        3. 1 in 3500 of same population exhibit disease

      3. Carriers do not exhibit disease

      4. Allele codes for nonfunctional form of hexosaminidase A

        1. Cannot degrade gangliosides in brain cell lysosomes

        2. Lysosomes fill with gangliosides, swell and burst, killing brain cells

    3. Gene Defects Are Inherited in Families: Hemophilia

      1. Loss of activity in blood clotting factors

      2. Disorder due to recessive condition

        1. Most clotting proteins located on autosomes

        2. Two (VII and IX) located on X chromosome, are sex-linked

          1. More prominent in males since they possess only one X

          2. If X defective, no proteins made

          3. Y lacks comparable allele

      3. Most common form has defective IX

        1. Called Royal hemophilia, prominent in family of Queen Victoria fig 13.34

        2. Carried into royal families of Europe fig 13.35

    4. Gene Defects Often Affect Specific Proteins: Sickle-Cell Anemia

      1. Improper transport of oxygen due to defective hemoglobin

        1. Red blood cells become stiff and sickle-shaped fig 13.36

        2. Blood cells clog blood vessels, are unable to enter small vessels

        3. Affected individuals usually have intermittent illness, shortened life span

      2. Results from alteration in single amino acid

        1. Valine replaces glutamic acid at a single location on protein's outer edge

        2. Causes "sticky patch" that causes hemoglobin molecules to adhere to each other

        3. Forms long chains of hemoglobin molecules

      3. Disorder of homozygotes but heterozygotes slightly affected

      4. Most common disorder among those of African descent

      5. Heterozygosity for sickle-cell confers resistance to malaria fig 13.37

    5. Not All Genes Are Recessive: Huntington's Disease

      1. Hereditary condition caused by dominant allele fig 13.38

      2. Causes progressive deterioration of brain cells

      3. Maintained in population, 1 in 24,000 affected

        1. Symptoms develop after reproductive activity

        2. Allele often transmitted prior to its expression

    6. Some Genetic Defects May Soon Be Curable: Cystic Fibrosis fig 13.39

      1. Most common genetic disorder in Caucasians

        1. in 20 carry single copy of defective gene

        2. 1 in 1800 are homozygous recessive, exhibit disease

      2. Affected individuals secrete clogging mucus

      3. Defect in transport of chloride ions across membranes

        1. Homozygous individual cannot regulate chloride-transport channel

        2. Heterozygote not affected due to presence of one normal copy of gene

        3. Responsible gene is cystic fibrosis transmembrane regulator (CFTR)

      4. Gene transfer therapy

        1. Working copies of CFTR inserted into affected individuals

        2. Success in mice

        3. Attempts still being made on humans

  5. Genetic Counseling

    1. In Absence of Cures Seek to Not Produce Children with Disorders

      1. Genetic counseling

        1. Identify parents at risk

        2. In high-risk pregnancy, likelihood that child will inherit disorder

      2. High risk of Down syndrome in older women fig 13.31

    2. Prenatal Diagnosis of Disorders

      1. Amniocentesis fig 13.40

        1. Sample amniotic fluid during fourth month

        2. Observe fetus and position via ultrasound fig 13.41

        3. Fetal cells grown in culture

        4. Cells examined for major chromosomal damage

      2. Chorionic villi sampling

        1. Sample placental tissue

        2. Can be performed earlier than amniocentesis at eight weeks

      3. Tests for genetic disorders

        1. Enzyme activity tests: PKU, Tay-Sachs

        2. Association with genetic markers

          1. Cut DNA with restriction enzymes

          2. Observe restriction fragment-length polymorphisms, RFLPs fig 13.42

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