Transcript Genetics
Mendel….Father of Genetics 1800s monk Studied plant breeding /agriculture & statistics http://www.mendelweb.org/Mendel.html Mendel’s paper in English 19th century theories of inheritance : blending Mendel’s studies showed Particulate inheritance.. Inheritance of discrete units (genes) Mendel's experiments Pea plants self- pollinate To make crosses Mendel cut off stamens and cross-pollinated artificially Each pea seed contains an embryo Vocab True breeding – all alleles same Hybrid – produced by crossing 2 true breeding parents (has one of each allele) P generation : first generation F1 generation: offspring of P F2 generation: offspring of 2 F1 individuals Mendel found: No blending, flowers one color or the other. All F1 looked same called that trait dominant Found that the other trait was hidden in some of the F1s but was passed on to F2 called that trait recessive Dominant Traits represented by CAPITAL letters recessive traits represented by lower case letters Descriptors Since genes(particles) can be hidden used Genotype to describe genes Phenotype to describe physical appearance Genotypes: Homozygous Dominant (AA) true breeding Homozygous recessive (aa) Heterozygous (Aa) hybrid Punnett squares Analytical tool for determining probabilities of inheritance ALL possible Gametes go on top and sides of square (1n) so only one letter Boxes inside square represent possible offspring So each must get 2 copies of gene (2n) 2 letters Phenotypes Ratio of possible offspring in box is … The probability that that type of offspring will be produced ratio 3:1 75% smooth : 25% wrinkled Genotypes Ratio of possible offspring in box is … The probability that that type of offspring will be produced ratio 1: 2 :1 1 SS : 2 Ss : 1 ss 1) Because statistics show that each allele is equally likely to be passed on….. Law of independent assortment 2) Bcs each parent only contributes one allele… Law of segregation : two alleles of parent separate & offspring only gets one of them Test Crosses Done to determine genotype Cross unknown individual with homo recessive if any recessive phenotype offspring then .. Unknown is heterozygous Types of Crosses Monohybrid only looks at one gene Dihybrid looks at probability of inheriting certain alleles of 2 different genes Each different gene MUST have a different letter of alphabet H = height H = tall h = short C= color C = purple c= white individuals needs 2 of each letter so… HHCC HHCc HHcc HhCC HhCc Hhcc hhCc hhcc Dihybrid Punnett squares All possible gametes on top Heterozygous for both traits makes .. 4 different gametes RrYy parent makes: RY or Ry….rY or ry Each box is a zygote so needs 2 letter r’s & 2 letter y’s 2 copies of each gene Any other parent will not make all 4 gametes so Punnett squares can be smaller Multiplication Rule (p213) To determine the chances of 2 different events happening together… Multiply the probabilities of each event P to get A & B = Probability A x Probability B Chance to get TTBB ¼ x ¼ = 1/16 Addition Rule If more than one way to get a desired outcome…. Add the probabilities of each way together. Chances to get 2 homozygous dominant genotype RRYYSs= ¼ x ¼ x ½ = 1/32 = 2/64 RRYYss= ¼ x ¼ x ¼ = 1/64 RrYYSS = ½ x ¼ x ¼ = 1/32 = 2/64 rrYYSS= 1/4 x ¼ x ¼ = 1/64 RRYySS = ¼ x ½ x ¼ = 1/32 RRyySS = ¼ x ¼ x ¼ = 1/64 ∑ = 9/64 Non-Mendelian Genetics Any genes that are not straight forward dominant or recessive allele Incomplete dominance & Co-dominance Multiple alleles (More than 2 alleles for a gene) Polygenic traits (traits controlled by more than one gene) Linked genes – genes that do not assort independently because they are on the same chromosome Sex linked genes – genes located on X or y Incomplete Dominance Neither allele completely dominates the other NO recessive (hidden) allele Both traits are partially expressed (blending) Letter symbols… must be same letter…same gene must both be capital letters ……….not recessive use superscripts to differentiate alleles gene is color so use C Cw = white Cy = yellow Do Punnett squares as normal but heterozygotes show a blended or in-between phenotype Draw Punnett square for the cross CwCw x CyCy F1 will be … Draw punnett square for F2 Co-Dominance 2 alleles both fully expressed (NO blending) Gene for feather color in chickens C co-dominant alleles for white (Cw) and black (Cb) Heterozygotes (Cw Cb )have some black feathers and some white feathers (NOT grey) Multiple alleles and Co-Dominance Human ABO blood types 3 alleles ….. 2 dominant and one recessive Gene designated as letter I for inherited antigenic substance Sex linked genes Genes on X or y chromosome Most often on X y very small mostly codes for male trait Writing sex linked genotypes : XX and Xy Use superscripts to designate alleles XNXn x XNy Chromosome Theory of Inheritance Morgan – first solid evidence Used fruit flies…good choice because… produce hundreds of offspring mature quickly small /cheap only 4 chromosomes (3 autosomes, Xy) mutations created new phenotypes Morgan’s breakthrough Natural phenotypes are called wild type (+) Wild type ff have red eyes Found mutant male w/ white eyes Used letter w to represent gene for eye color And w+ for wild type just w for white Crossed mutant male with normal femal w+ x w all F1 had red eyes so … Trait is recessive F2 generation…. Some white but only males So differences between sex. Only difference XX or Xy chromosome Morgan tried a reciprocal cross Change which parent has the mutation All males get white eyes Supports w gene is on X chromosome Do Punnett square for X+X+ Do Punnett square for XwXw x Xwy x X+y Supports Chromosomal Theory of Inheritance: individual genes are carried by specific chromosomes New alleles created by mutation May be helpful, harmful, lethal, or silent May be dominant or recessive Some cause genetic disorders Examples to know: cystic fibrosis sickle-cell disease/trait achondroplasia Huntington’s disease Hemophilia Cystic Fibrosis Most common lethal disorder in U.S.A. 4% of European Americans carriers Recessive mutant gene for cell membrane protein Chloride channels fail = thick mucus Problems with lungs & pancreas Chronic illness shortened life span With treatment 50% survival to 30s Research into gene therapy promising Sickle Cell Anemia Most common disorder of African Americans 0.1 % African Americans have sickle cell trait Co-Dominant mutation to Hemoglobin gene One single nucleotide is substituted Changes one amino acid Mutated hemoglobin causes RBC to sickle and block capillaries if anaerobic Sickle Cell Genes: co-dominant Hn = normal hemoglobin Hs = mutated hemoglobin HnHn = normal HnHs = sickle cell trait HsHs = sickle cell disease….organ/brain damage Phenotype looks incomplete dominance….same symptoms just much less severe Heterozygote Advantage High % of sickle cell trait individuals from tropical areas Linked to improved malaria survival Malaria = Protist parasite that lives in RBC HnHs = reduced parasite densities in blood… better survival rates Achondroplasia : (w/o cartilage formation) Dominant mutation in gene that forms cartilage into bone Heterozygotes = short appendages Homozygous dominant = still born 80% new mutations, 20% inherited from parent Huntington’s disease Lethal dominant allele Late onset so gene is passed on before onset Mutated Huntington protein causes brain damage and is fatal Genetic test available Hemophilia X-linked recessive mutation Mutant clotting factor No treatment = 11 year life expectancy Now nearly normal with treatment Environmental & Genetic disorders Different alleles predispose to a disorder Environmental factors determine if disorder is expressed Multifactorial disorders Most are polygenic Cardiovascular health, cancer, alcoholism, bipolar disorder Genetic Counseling/ testing 1) Genetic counseling -Use pedigrees to determine genotypes 2) Pre-conception genetic testing: a) fertility drugs cause multiple eggs to complete meiosis I and emerge from ovary b) eggs collected by surgery c) polar body tested for known genetic defect d) if mom is heterozygous and defect is in polar body, then egg is healthy Genetic Testing Embryo Embryos created by invitro fertilization At 8 cell stage one cell is removed and tested (day 3) Only healthy embryos implanted into mom 4) Genetic Testing of Fetus A) Chorionic Villi – sampling removes fetal cells from placenta (10-12 weeks) B) Amniocentesis – samples fetal cells from amniotic fluid (15-18 weeks) 10 weeks 15 weeks 4 inches long 1.7 ounces