genetics & inheritance genetics & inheritance the chromosome theory of inheritance

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  • Genetics & InheritanceThe Chromosome Theory of Inheritance

  • Chromosome Theory of Inheritance:Genes that code for various traits are found on chromosomes which are made of DNA and found in the nucleus of each cell

  • Thomas Hunt Morgan (1910): Studies Drosophilia melanogaster ;common fruit fly

    Discovered the gene for eye color and wing size were both located on the same chromosome (X) = LinkageLinkage = genes located on the same chromosomeThe closer the genes are on the chromosome, the greater the likelihood of crossing overThe Law of Independent Assortment does not apply to linked genes!

  • Gene Linkage

  • Barbara McClintock & Harriet Creighton (1950): Studied Zea mays ; corn

    Looked at chromosome #9They noticed that 2 copies of it were different sizesConclusion = Crossing Over had occurred, an abnormal event caused a piece of another chromosome to attach itself to one of the copies of chromosome #9Crossing Over disrupts normal Linkage Groups!

  • Human Chromosomes

    Chromosomes: Humans have 46 individual chromosomes in every cell of the body except the sex cells

    22 pairs = Autosomes (regular, information carrying chromosomes)

    1 pair = Sex Chromosomes, determines gender (XX = female, XY = male)

  • Which Parent determines the sex of the child?

  • MODES OF INHERITANCEDifferent ways of inheriting genetic traits1. Complete Dominance: dominant allele completely masks out the recessive trait (AA, Aa)Autosomal Dominant = trait carried on an autosome by a dominant gene (A)Autosomal recessive = trait carried by a recessive allele (aa)Example: flower color in pea plantsP = Purple, p = white


    2. Incomplete Dominance: One allele of a gene pair is not fully dominant over the other; being heterozygous produces an intermediate form, no masking occurs (AA, Aa, aa)

    Example: Flower color in snapdragons

    RR = red, rr = white, Rr = pink

  • MODES OF INHERITANCE3. Codominance: both alleles express themselves when heterozygous (Blood types A, AB, B, O). There is no true recessive trait.AB Blood type is an example of Codominance = Both alleles in the heterozygous form (IAIB ) end up expressing themselves equally. Both traits show up in the phenotype.Example: coat color in horsesRR = red , WW = White , RW = roan(RR = red , RR = White , R R = roan)RR x WWRW x RW


    4. Polygenic Traits: trait is controlled by more than one pair of genes (Eye color, Skin color, Human Height). This usually results in continuous variation. Polygenic inheritance is considered the opposite of Pleiotropy.

    5. Pleiotropy = when a single gene has more than one phenotypic expression. Example: Sickle Cell Anemia = misshapen red blood cells ultimately causes other problems such as anemia, pneumonia, heart & kidney failure, bone abnormalities, and impaired mental functioning.

  • MODES OF INHERITANCE6. Epistasis = one gene affects the phenotypic expression of a second gene. (Skin pigmentation) One gene codes for color, the other codes for the amount of pigment.

  • MODES OF INHERITANCE7. Multiple Alleles: a gene having more than two alleles (Blood types)PHENOTYPES% of PopulationGENOTYPESA 39% IAIA , IAIiB 12%IBIB , IBIi*AB 4% IAIBO 45%IiIi

  • Blood TypesDiscovered in 1900 by Dr. Karl LandsteinerBased on the presence or absence of specific agglutinogens (clotting factors) on the surface of red blood cells (RBCs = Erythrocytes)For example: a person with blood type A posseses A-antigens and Anti-B antibodiesThe blood will clot if a foreign Antigen is present

  • Donor vs. RecipientUniversal Donor = Blood type O ; contains no A or B antigens No Clotting reactionUniversal Recipient = Blood type AB ; contains both A & B antigens Will recognize antigens from any blood type

  • Rh FactorRh Factor = (Rhesus monkey) You either have it (+) or you dont (-) Sensitization can occur by:.)Rh+ blood transfused into Rh- person.)Rh- mother carries a fetus who is Rh+


  • MODES OF INHERITANCE8. Sex-Linked Inheritance: trait carried on the sex chromosomes; usually the X (XX = female; XY = male)X-Linked Recessive = Xa (Colorblindness, Hemophilia)X-Linked Dominant = XA; Y-Linked (Rare)Carrier = person not affected by the trait but can pass it on to offspring = XA Xa Only females can be carriers for sex-linked traits because if a male has the gene, he will also exhibit the trait!

  • MODES OF INHERITANCE9. X inactivation: one of the two X chromosomes in a female does not uncoil during embryonic development. The chromosome that remains coiled is called a Barr Body and contains genes that will not get expressed. This could cause a sex-linked trait to affect a female that would normally be only a carrier.

  • MODES OF INHERITANCE10. Sex-Influenced Traits: expressed in both sexes, but they are expressed differently (Pattern Baldness)B= Normal; b= Baldfemale (bb) = bald; male (bb or Bb) =bald

    BB x BbBb x Bb

  • MODES OF INHERITANCE11. Sex-Limited Traits: autosomal traits expressed in only one sex (Lions mane)

  • MODES OF INHERITANCE12. Dihybrid Crosses: follow 2 traits at a time (AaBb)Example: A = purple flowers, a = white B = Tall, b = shortIf you cross two parents, where the father is AABB and the mother is aabb: the possible gametes are AB x ab. This can be determined using the F.O.I.L method.

  • Dihybrid CrossesF = firstO = outsideI = insideL = lastAABB x aabb --> RrYy x RrYyResults = 100% AaBb

  • GENETIC DISORDERSDisorders or diseases related to a persons genes or chromosomes; inherited in the same ways as other traits.2 Mechanisms exist:

  • Genetic Disorders1. Inherited on Genes: inherited as a trait (Autosomal, sex-linked, sex influenced, etc...)colorblindnesshemophilia bleeders diseasemuscular dystrophyalbinismProgeria

  • Genetic Disorders2. Chromosome Abnormalities: not caused by a geneA.) Extra or Missing Chromosomes. Aneuploidy = abnormal chromosome numberNon-Disjunction = failure of chromosome pairs to separate properly during meiosis, end up with daughter cells having either too many or not enough chromosomes in them.Ex. Down Syndrome Trisomy 21

  • Genetic DisordersB.) Mutated Chromosome = damaged DNA, genes located in that section are damagedDeletion: missing gene or piece of chromosomeDuplication: extra piece, genes duplicatedTranslocation: gene switches chromosomesInversion: fragment of gene gets turned around

  • SCREENING FOR DISORDERS1. Karyotyping = genetic map of all the chromosomes that an organism possesses2. Amniocentesis = test done before birth, take sample of amniotic fluid (C.V.S.) 3. Genetic Counseling = determine family medical history

  • THE END!!


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