Chapter 12 Patterns of Heredity & Human Inheritance.

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  • Chapter 12Patterns of Heredity & Human Inheritance

  • 12.1: Mendelian Inheritance ofHuman TraitsPedigree- graphic representationof genetic inheritanceDifferent symbols are used torepresent:Males and femalesIndividuals affected or unaffected by a particular traitCarrier- a heterozygote for a recessive trait that does have exhibit the trait in their phenotype, but carries the allele for the trait in his or her genotype

  • Recessive DisordersDisorders that appear or are expressed when the genotype is homozygous recessive

  • Examples of recessive disorders:1. Cystic fibrosisResults in accumulation of thick mucus in lungs and digestive tract2. Tay-Sachs diseaseLipids are not broken down in brain cellsAs lipids accumulate, brain function declines and eventually leads to deathEspecially common on those of eastern-European Jewish descent

  • 3. Phenylkentonuria (PKU)Amino acid phenylalanine cannot be broken downAccumulation leads to mental retardationTreated by a diet low in phenylalanine

  • Dominant DisordersDisorders that appear or are expressed when the genotype is heterozygous or homozygous dominant

  • Examples (not necessarily disorders)1. Cleft chin, widows peak, hitchhikers thumb, mid-digit hair

    2. Huntingtons diseaseResults in breakdown of certain areas of the brainNo treatment

  • 12.2: When heredity follows different rulesMost inheritance does not follow the simple patterns established by Mendel

  • Incomplete DominanceHeterozygotes have an appearance that is in between the phenotypes of the two homozygotesCapital letters will be used for both traits because neither is dominant

  • Flower color

  • Example problem for incomplete dominance:

  • Multiple AllelesEvery trait we have discussed so far has two allelesSome traits have more than two possible allelesHowever, each individual only has two of them (one on each of two homologous chromosomes)

  • A, B, O blood groupsLetters refer to carbohydrates found on the surface of red blood cellsPossible alleles for blood type:IA- carbohydrate A is presentIB- carbohydrate B is presenti- no carbohydrate is present

    Both IA and IB are dominant over i

  • Possible Genotypes and PhenotypesIAIA- type AIBIB- type BIAIB- type ABIAi- type AIBi- type Bii- type O

  • Example problem for multiple alleles:

  • CodominanceNotice that when IA and IB are paired together, both alleles are expressed, blood type is ABCodominance- both alleles are expressed in heterozygotes, both alleles are dominant

  • Sickle Cell Disease- an example of codominanceSymptoms:Breakdown of red blood cellsClumping of cells & clogging of vesselsAccumulation of sickle cells in spleenHeart failure, pain, fever, brain damage, weakness, kidney failure

  • Genotypes & PhenotypesN- normalS- sickle cells

    SS- suffer from full-blown diseaseSN- usually healthy, but may have some symptoms at high altitudesNN- no symptoms

  • About 1 in 10 African Americans is a carrier for sickle cell disease, but the disease is very rare in Americans of other ancestryWhy is this gene so common if the effects are so serious?

  • Heterozygote advantage- heterozygotes for a particular trait have some advantage that homozygotes do notIn the case of sickle cell heterozygotes, they are resistant to malariaIn locations where malaria is prevalent, heterozygotes tend to live longer and produce more offspring

  • Malaria

  • Example problem for sickle cell disease:

  • Sex DeterminationThere are two possible sex chromosomes in humans, X or YFemales- XXMales- XY

    Sperm determines the sex of the offspring

  • Example problem for sex determination:

  • Sex-Linked TraitsGenes located on the sex chromosomesMost sex-linked genes are located on the X chromosome- phenotype can be seen in both males and femalesA few are located on the Y chromosome- phenotype can only be seen in males

  • Sex-linked disordersColor-deficiency (blindness)Malfunction of light-sensitive cells in eyesRed-green- see gray tones instead of red or greenInvolves several X-linked genesNormal color vision- see over 150 colorsColor-deficient- see fewer than 25 colors

  • Can females be colorblind?Possible alleles:XB- normalXb- color deficient

    Possible genotypes & phenotypes:XBXB- normal femaleXBY- normal maleXBXb- carrier femaleXbXb- color deficient femaleXbY- color deficient male

  • Example problem for sex-linked traits:

  • Can males be carriers of X-linked traits?Why not?

  • HemophiliaX-linked geneExcessive bleeding wheninjured- lack clotting factorMay bleed to death afterminor injuryExcess blood pools in joints

  • Duchenne muscular dystrophyProgressive weakening & loss of muscle tissueDeath usually occurs by age 20

  • Polygenic InheritancePolygenic inheritance- the additive effects of two or more genes appear in a single phenotypic characteristicThese characteristics tend to occur on a continuumWhat are some examples of human characteristics that occur on a continuum?

  • Skin tone

  • External Influences on PhenotypeExternal environment can affect the expression of certain genotypesExamples of external influences:TemperatureLightNutritionChemicals

  • Internal Influences on PhenotypeGene expression may be affected by internal environments:Presence or absence of hormonesStructural differencesAge

  • 12.3: Complex Inheritance of Human TraitsSee examples from section 12.2 notes

  • KaryotypeOrderly display of magnified images of an individual's chromosomesCan detect chromosomal abnormalities

  • Abnormalities in Chromosomal NumberUsually, a human embryo with an abnormal number of chromosomes is miscarried.Some abnormalities in chromosomal number upset genetic balance less drastically

  • Causes of chromosomal number abnormalitiesNondisjunction- when members of a chromosome pair fail to separateDuring Meiosis I:Homologous chromosomes do not separateAll gametes have abnormal numbersDuring Meiosis II:Sister chromatids do not separate2 gametes are normal, 2 are abnormal

  • If either of these events occurs, surviving offspring that results from these abnormal gametes will have abnormal karyotypes

  • Disorders Resulting from NondisjunctionTrisomy 21Result of nondisjunction in chromosome 21Affected individuals have 3 copies of chromosome 21Leads to Down Syndrome

  • Down Syndrome

  • Klinefelters SyndromeExtra X in males- XXYSterilityBreast enlargementNormal intelligenceAlso includes multiple disjunctions- XXYY, XXXY, XXXXY- more mental retardation

  • Klinefelters Syndrome

  • Jacobs SyndromeExtra Y in males- XYYNormal malesTaller than average

  • Superwoman Syndrome (metafemales)Extra X in females- XXXNormal femalesLimited fertility

  • Turner SyndromeFemales lacking an X- XOShort statureWebbing of neckSterile, poor development of secondary sex characteristicsNormal intelligence

  • Turner Syndrome


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