Transcript Document
MENDELIAN GENETICS Laws of Heredity A. Origins of Genetics • Passing characteristics from parent to offspring is called heredity • Accurate study of heredity began with Austrian monk Gregor Mendel at his monastery gardens • Mendel used different varieties of garden pea plant – Could predict patterns of heredity which form modern-day genetics principles – Garden peas have eight observable characteristics with two distinct traits that Mendel counted and analyzed with each cross or breeding Pea Characteristics & Traits Characteristic Trait Flower color Purple v. white Seed color Yellow v. green Seed shape Round v. wrinkled Seed coat color Gray v. white Pod shape Round v. constricted Pod color Green v. yellow Flower position Axial v. top Plant height Tall v. dwarf Pictures • In nature, pea plants self pollinate since both reproductive organs (male stamen [pollen] & female pistil) are internal – Mendel physically removed stamens & dusted pistils with pollen from chosen plants to observe results by crosspollination • Parent plants are P generation • All offspring are F generation (from Latin filialis for son/daughter) – F1 generation = first offspring (children) – F2 generation = second offspring (grandchildren) P F1 F2 P F1 F1 F2 F2 F2 • Cross-pollinated two pure-bred plants with one very different trait (purple vs. white flowers) in P generation – Examined each F1 plant’s trait & counted them Monohybrid Cross P F1 F1 • Allowed F1 generation to self-pollinate to produce F2 generation – Examined each F2 plant’s trait & counted them F2 F2 • Mendel collected tons of data – his results are reproducible – Monohybrid cross of white flowers and purple flowers in P generation produced 100% purple flowers in F1 generation – Self-pollination of F1 produced 705 purple flowers & 224 white flowers in F2 generation P F1 F2 • Mendel’s ratios still hold true today – Crossing pure bred traits in monohybrid cross will ALWAYS express only one trait in F1 – Self-pollinating F1 will ALWAYS result in 3:1 ratio • 705:224 = 3:1 B. Heredity Theories & Laws • Mendel knew all ideas about “blending” characteristics was bogus – Developed four hypotheses: • 1. individual has two copies of gene, one from each parent From Dad From Mom • 2. there exists alternative versions of genes called alleles represented by letters A b C A B C From Dad From Mom D e F d e F • 3. if two different alleles occur together, one may be expressed while other is not – dominant and recessive – UPPERCASE alleles are dominant alleles » Trait gets expressed ALWAYS – lowercase alleles are recessive alleles » Trait only gets expressed if dominant is not A A present b C B trait will be expressed B C From Dad e trait will be expressed From Mom D e F d e F – When both alleles are identical, individual is considered homozygous for that trait • Homozygous dominant = both dominant (UPPERCASE) • Homozygous recessive = both recessive (lowercase) – When alleles are different, individual is considered heterozygous for that trait A A b B C C D d e e F F QUIZ YOURSELF TT = ? Homozygous dominant Tt = ? Heterozygous tt = ? Homozygous recessive XX = ? Homozygous dominant rr = ? Homozygous recessive • 4. when gametes (sperm/eggs or spores) are formed, alleles for each trait separate independently during meiosis – Occurs during anaphase bb pP HH Aa C. General Rules for Genes • Each gene is given allele letter – Letter is always first letter of dominant trait • Ex: yellow peas are dominant over green peas Y = yellow, y = green YY yy Yy • Ex: purple flowers are dominant over white flowers P = purple, p = white PP Pp pp • Ex: In roses, pink petals are dominant over white petals, and tall stems are dominant over short stems. P = pink, p = white T = tall, t = short Cross a male heterozygous pink tall with a female homozygous white short Male = PpTt Female = pptt • Some traits are dominant – only one dominant allele needed in genome to show phenotype • Some traits are recessive – both recessive alleles needed to express phenotype Dominant Trait Polydactyl (P) No Hitchhiker’s Thumb (T) Tongue rolling (R) Free-hanging ear lobe (F) Widow’s Peak (W) Brown eyes (B) Left thumb on top (L) Recessive Trait Non-polydactyl (p) Hitchhiker’s thumb (t) No tongue rolling (r) Attached ear lobe (f) No widow’s peak (w) Blue or green eyes (b) Right thumb on top (l) Plus four others! Polydactyl (PP or Pp) No Hitchhiker’s Thumb (T) TT or Tt tt Tongue Rolling (R) RR or Rr rr Free-Hanging Earlobes (F) FF or Ff ff Widow’s Peak (W) WW or Ww ww Brown Eyes (B) BB or Bb bb Left Thumb on Top (L) LL or Ll ll Mid-Digit Hair (H) HH or Hh hh Cleft Chin (C) CC or Cc cc Dimples (D) DD or Dd dd Freckles (F) FF or Ff ff D. Laws of Heredity • During meiosis (forming haploid gametes from diploid cells), chromatids separate during anaphase II – Law of segregation: two alleles for character separate when gametes are formed Male Parent (Tt) Alleles segregate (separate) into gametes Female Parent (Tt) T t Alleles segregate (separate) into gametes Alleles pair up in all combos Alleles segregate (separate) into gametes Alleles pair up in all combos • Mendel studied whether different characteristics were inherited together or separately – Conducted dihybrid crosses where two traits are studied – Concluded that traits NOT inherited together & developed law • Law of independent assortment: alleles of different genes separate independently during gamete formation in meiosis Law of Independent Assortment Male Parent (TtBb) Traits separate independently Female Parent (TtBb) Tb TB tB tb TTBB TTBb TtBb 16 total! E. Punnett Square • Easiest way to represent Laws of Segregation and Independent Assortment is through Punnett Square – Cross a homozygous dominant yellow pea with a green (homozygous recessive) – Genotypes: YY & yy (1 trait, 4 alleles = 4 combos) Step 1: separate alleles from genotypes & place on top & down side Step 2: determine possible combinations by crossing alleles y y Y Yy Y Yy Yy Yy y y Y Yy Y Yy Yy Yy • Have to analyze findings from crossings – Genotypic ratio: 4 Yy – Phenotypic ratio: 4 yellow peas Curi Family Eye Color • My dad has green eyes • My mom has brown eyes – Knowing that I have brown eyes, what is my GENOTYPE? • Brown is dominant (B) • Green is recessive (b) –Dad must be bb b B Bb b Bb B Bb Bb • According to Punnett Square, all my parents’ children should have BROWN eyes –In reality, my brother has green eyes. What does this mean? • Mom’s genotype must be Bb b B Bb b Bb b bb bb This means that there is a 50% (2/4 or ½) chance that each child my parents had could have green eyes. I lucked out. Monohybrid Cross Examples • 1. Aliens with two eyes are dominant over aliens with one eye. Cross a heterozygous twoeyed male with a homozygous one-eyed female. t t T Tt Tt t tt tt – Genotypic ratio: 2:2 – Phenotypic ratio: 2:2 (2 Tt: 2 tt) (2 two eyes: 2 one eye) • 2. Orange carrots are dominant over purple carrots. Cross a male purple carrot with a heterozygous orange carrot. O o Oo o Oo – Genotypic ratio: 2:2 – Phenotypic ratio: 2:2 o oo oo (2 Oo: 2 oo) (2 orange: 2 purple) • Dihybrid (two traits) cross can be trickier – Cross heterozygous purple flowers, heterozygous yellow pea with another of the same. – Genotypes: PpYy & PpYy (2 traits, 8 alleles = 16 combos!) Step 1: find possible gametes PY Py pY py (two traits each!) for each PPYy PpYY PpYy PPYY PY parent by doing FOIL method (first, outside, inside, last) & Py PPYy PPyy PpYy Ppyy place on top & down side Step 2: determine possible pY PpYY PpYy ppYY ppYy combinations by crossing alleles, py PpYy Ppyy ppYy ppyy making sure same alleles are together PY Py pY py PY PPYY PPYy PpYY PpYy Py PPYy PPyy PpYy Ppyy pY PpYY PpYy ppYY ppYy py PpYy Ppyy ppYy ppyy • Analyze results – Genotypic ratio: 1:2:2:4:1:2:1:2:1 (1 PPYY: 2 PPYy: 2 PpYY: 4 PpYy: 1 PPyy: 2 Ppyy: 1 ppYY: 2 ppYy: 1 ppyy) – Phenotypic ratio: 9:3:3:1 (9 purple/yellow: 3 purple /green: 3 white/yellow: 1 white: green) Simple dihybrid rules … • Always will be maximum of 4 phenotypes – Trait A vs. Trait B, Trait C vs. Trait D = 4 phenotypes • AC, AD, BC, BD • Heterozygous AaBb vs. Heterozygous AaBb will always have same phenotypic ratio – 9 AB, 3aaB, 3Abb, 1aabb = 9:3:3:1 • Heterozygous AaBb vs. Homozygous aabb will always have same phenotypic ratio – 4 AB, 4 aaB, 4 Abb, 4 aabb = 4:4:4:4 Dihybrid Cross Examples • 1. Red ants are dominant over black ants, and long antennae are dominant over short antennae. Cross a black short antennae male with a heterozygous red long female. – Genotypes: rrll & RrLl – Gametes: rl, rl, rl, rl & RL, Rl, rL, rl RL rl RrLl Rl Rrll rL rrLl rl rrll rl RrLl Rrll rrLl rrll rl RrLl Rrll rrLl rrll rl RrLl Rrll rrLl rrll • Genotypic ratio: 4:4:4:4 (4 RrLl: 4 Rrll: 4 rrLl: 4 rrll) • Phenotypic ratio: 4:4:4:4 (4 red/long: 4 red/short: 4 black/long: 4 black/short • 2. Green frogs are dominant over brown frogs, and spots are dominant over no spots. Cross a heterozygous green spotted female with the same type of male. – Genotypes: GgSs & GgSs – Gametes: GS, Gs, gS, gs & GS, Gs, gS, gs gS gs GS Gs GS GGSS GGSs GgSS GgSs Gs GGSs GGss GgSs Ggss gS GgSS GgSs ggSS ggSs gs GgSs Ggss ggSs ggss • Genotypic ratio: 1:2:2:4:1:2:1:2:1 • Phenotypic ratio: 9:3:3:1 Curi Family Eye Color & Ear Shape • My father has green eyes and free-hanging ear lobes (homo or hetero?) while my mother has brown eyes (heterozygous) and free-hanging ear lobes (homo or hetero?). What are the possible outcomes for the children? – Know that my mother is heterozygous for brown eyes since my brother has green eyes – What about free hanging ear lobes? • I have free hanging, but my brother and sister are attached! What does that mean about my parents? –Both parents MUST be heterozygous for freehanging ears! • Genotypes: bbFf & BbFf • Gametes: bF, bf, bF, bf & BF, Bf, bF, bf BF Bf ? ? bf BbFf ? Bbff bF BbFF ? ? BbFf bf BbFf Bbff ? bF bf bF BbFF BbFf bbFF bbFf bbFf bbff BRO SIS bbFF bbFf bbFf bbff me F. Probability • Getting ratios of genotypes & phenotypes is actually calculating probability – Probability: likelihood that particular event (genotype or phenotype) will occur – Calculated by dividing number of predicted outcomes by number of total outcomes – Ex: 3 peas are yellow, 1 is green • Words: 3 out of 4 are yellow • Ratios: 3:1 yellow • Decimals: 0.75 yellow, 0.25 green (add up to 1.0) • Percentages: 75% yellow, 25% green (add to 100%) • Fractions: ¾ yellow, ¼ green (add to 4/4)