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Unit 3: Genetic Continuity Chapters 16, 17, & 18 1 Pages 522 - 639 (MHR) Mendelian Genetics $ Traits - distinguishing characteristics or easily observed features of an individual $ Heredity - is the passing of genetic traits (e.g. hair or eye colour) from one generation to the next. It results in similarities between family members. $ Genetics - is the branch of biology that studies how traits are passed from generation to generation. Genetics in history $ believed sperm contained fully formed embryo that grew inside womb of female (fig 16.2, pg 526) $ others believed theory of blended inheritance. $ traits from each parent blended together in offspring $ E.g. tall + short = medium $ couldn’t explain appearance or disappearance of distinct traits from one generation to another $ The Austrian monk Gregor Mendel had other ideas... Gregor Mendel (1822-1884) pg 527 $ first scientist to correctly interpret patterns of inheritance $ experimented with pea plants over 8 years (1853-1861) to study inheritance $ Pea plants were a good choice because they: 1. Have seven distinct traits, each of which has only 2 possible forms e.g. tall vs short, smooth vs wrinkled $ 2. Can self-pollinate or cross-pollinate $ self-pollination - eggs from one plant are fertilized by pollen of the same plant $ cross-pollination - eggs from one plant are fertilized by pollen from another plant 3. Grow and reproduce quickly (unlike lab rats) To prevent self-pollination Mendel eliminated either the male or female reproductive Biol 3201 2 structures of the flower $ to cross pollinate he brushed pollen from one plant onto the female part of another plant. $ cross-pollination results in offspring from 2 different plants (offspring from selfpollination is from one plant only) $ cross-pollination allowed Mendel to mate plants with different traits (characteristics) $ to ensure validity of his experiments, Mendel first bred purebred plants $ $ took 2 tall plants and bred them together. seeds produced were planted and grew. $ some were tall, others short. $ tall plants were selected and 2 bred together again $ repeated ‘til only tall plants produced $ then called purebred i.e. they descended from ancestors of a distinct type (like our own Newfoundland Dog or Labrador Retriever who are purebreds!!) $ created purebreds for all seven traits $ called the purebred plants P generation or parent generation $ then the parent plants with different characteristics of the same trait were crossed (Fig 16.5, pg 529) $ e.g. tall with short (height) $ green with yellow (colour) $ “characteristics (trait) $ $ offspring of this cross were the first filial generation or F1 generation also called hybrid because they contained both characteristics for a particular trait (even if only one characteristic was apparent) $ (here we are only crossing one trait, height. We can therefore say that this was a monohybrid cross) Biol 3201 3 $ 100% of the F1 group were tall (i.e. no blending!!) the short characteristic seemed to disappear! $ Mendel concluded that the trait for being tall must be dominant, and short must be recessive $ dominant - the characteristic is always expressed/apparent $ recessive - the characteristic is present but not expressed unless there are only recessive characteristics present $ repeated these experiments many times and always same results. Sooo... he developed his principle of dominance i.e. when contrasting traits are crossed, the offspring express only the dominant trait $ the next part of his experiment, the F1 generation self-pollinated, which produced the second filial generation (F2 generation) $ repeated this test many times and every trial gave the same results: 3 /4 of the F2 offspring were tall and 1/4 were short (fig 16.5, pg 529) $ from the F2 offspring he developed his law of segregation or 1st law of heredity which stated that “traits are determined by pairs of factors from each parent. These factors separate during gamete formation, giving each offspring only one factor from each parent” (Fig 16.7, pg 530) $ we now call his “factors” genes (part of the chromosome that controls expression of traits) $ genes can occur in different forms i.e. alleles (dominant or recessive) $ Mendel’s factors are sometimes called unit characters because they are inherited independent of other traits $ his theory is called unit theory from his discovery that genes are inherited as independent units. $ if we use letters to represent the different alleles, a purebred tall plant would be TT (2 uppercase T’s). It is homozygous (2 alleles are the same) Biol 3201 4 $ a purebred short would be tt, also homozygous (both alleles are the same) $ the F1 generation were all heterozygous, i.e. the tall parent donated a T and the short parent donated a t, so the result is Tt (2 alleles are different) (fig 16.6, pg 530) $ to represent Mendel’s experiments symbolically: Purebred Purebred Tall Short TT tt P generation Tt F1 generation (100% tall) $ (come back to this business of crosses after probability) PROBABILITY (pg 531) $ “the chance or likelihood of a particular outcome” $ usually expressed as a ratio $ important to genetics because enables us to predict expected outcomes of genetic crosses $ can use product rule or Punnett squares Product rule $ the chances of independent events occurring at the same time is equal to the product of the chances of these events occurring separately $ e.g. What is the likelihood of getting 2 heads if we flip 2coins? -The chance of getting one head is ½ -The chance of getting another head is ½ -The chance of getting both heads at same time is ½X ½= 1/4 $ Rule of independent events: chance has no memory i.e. if you toss 2 heads in a row, the Biol 3201 5 $ $ likelihood of tossing another head is still ½. however the chances of flipping 3 heads in a row is ½ x ½ x ½= 1/8 genetic e.g. - 2 heterozygous tall plants (Tt) are crossed ½ the gametes are T ½ the gametes are t T might combine with another T or a t. Punnett Squares (pg 532) $ visual representation of possible offspring allele combinations $ $ used to calculate chance (probability) of inheriting a particular trait shows all possible gamete combinations from set of parents (egg/sperm) $ $ $ dominant allele - capital letter!! (e.g. T) recessive allele - lowercase letter!! (e.g. t) Example: P generation TT x tt Tall plant T T t Tt Tt t Tt Tt Short Plant TT tt F1 generation 100% of offspring is tall $ Punnett sq’s give predictions as to possible outcomes from a given set of alleles $ can use to determine genotype and/or phenotype - genotype - genetic makeup of an organism - phenotype - appearance of trait in organism Biol 3201 6 $ E.g. F1 phenotype is F1 Tt T t T TT Tt t Tt tt generation has genotype Tt but tall cross (self pollination): Tt } F2 generation 3 tall:1 short $ Complete dominance - the type of inheritance where both heterozygotes and dominant homozygotes have the same phenotype. e.g. Both Tt and TT are tall. The “tall allele” is dominant Practice problems 1,2, & 3 page 533 (MHR text) Test Cross (pg 533-534 MHR) $ How can we tell if a plant showing a dominant phenotype is heterozygous or homozygous? $ $ $ e.g. how can we tell if a tall plant is TT or Tt? Or if a yellow pea plant is YY or Yy? Perform a test cross!! (the crossing of the unknown individual with a homozygous recessive individual) $ e.g. we cross the unknown tall plant (either TT or Tt) with a known short plant (know tt). $ If unknown plant is TT ... all offspring will have genotype Tt and will be tall Biol 3201 7 (phenotype). $ If unknown plant is Tt ... ½ offspring will be tall and ½ will be short i.e. ½ will be Tt and ½ tt. $ Do the Punnett Square to see why (see fig 16.10, pg 534 for another example) Dihybrid Cross (pg 536 MHR) $ Pea plants have many traits (colour, height, shape etc.) $ Are they inherited together or separately? Does pea shape influence pea colour? $ Mendel bred 2 more types of purebred plants $ homozygous dominant for both shape and colour i.e. round (RR) and yellow (YY) seeds $ homozygous recessive for both shape and colour i.e. wrinkled (rr) and green (yy) seeds $ then he performed a dihybrid cross which is a cross of two plants that differ in two traits i.e. one is round & yellow and the other is wrinkled and green. (Fig 16.12 pg 536) $ F1 generation - all round & yellow (all heterozygous: RrYy) $ F2 generation -9 round :3 round : 3 wrinkled : 1 wrinkled yellow green yellow green $ 9:3:3:1 made sense if alleles for different traits were inherited separately i.e. alleles for colour not connected to alleles for shape $ Mendel’s 2nd law of inheritance or the law of independent assortment states that inheritance of alleles for one trait does not affect the inheritance of alleles for another trait. $ offspring may have characteristics that were not present in the parents $ Punnett square: all possible gamete combinations for one parent go across top and all possible gamete combinations for the other parent go down the side. Biol 3201 8 e.g. F1 RrYy x Possible are RY, Ry, RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy Rryy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy cross RrYy. combos rY, ry RrYy RrYy R - round; r - wrinkled Y - yellow; y - green Biol 3201 9 Beyond Mendel’s Laws (pg 541-543) $ Mendel’s pea plants demonstrated complete dominance. $ Other organisms show different patterns of inheritance Incomplete dominance (fig 16.15 pg 541) $ Occurs when neither of the alleles present is dominant $ A blending of the traits occurs $ Many examples in plants including snapdragons, and Japanese four-o’-clock flowers $ e.g. red or white snapdragon flowers are homozygous whereas pink ones are heterozygous $ Here RR = red, R’R’ = white, RR’ = pink $ Two red alleles are necessary to produce a red flower i.e. RR $ If only 1 R, not enough pigment to be red. Therefore blends with white and appears pink. $ $ Still follows Mendel’s laws of segregation and independent assortment. Offspring don’t show Mendelian ratio because neither allele is dominant $ Fig 16.15 pg 541 shows P, F1 and F2 generations. $ $ $ P generation F1 generation F2 generation RR x R’R’ All pink (all RR’) 1 red RR: 2 pink RR’: 1 white R’R’ RR’ x RR’ R R’ R RR RR’ R’ RR’ R’R’ Biol 3201 10 Co-dominance (fig 16.16 pg541) $ both alleles for a trait are dominant i.e. both alleles of a gene are expressed at the same time in a heterozygous individual $ e.g. feather colour in chickens black - BB, white - WW $ cross black rooster with white hen $ blend? Get grey feathers? $ If only black dominant, would get all black offspring. $ No. Get 1 chicken with some white and some black feathers. Results in a checkered pattern. See fig 16.16 pg541 $ also seen in horses. Cross a red horse with a white horse and get a roan horse (red and white hair) Multiple Alleles (pg 542-543) $ when a gene has more than 2 alleles for a given trait $ e.g. human blood types: 3 alleles - A, B, and O. $ table 16.1 pg 542 blood types & genotypes $ people have 2 of the 3 alleles: IA, IB, or i $ $ IA and IB are dominant over i IA and IB are co-dominant when both present $ $ $ $ IAi or IA IA result in blood type A (hetero- or homozygous) IBi or IB IB result in blood type B (hetero- or homozygous) IAIB results in blood type AB (heterozygous) ii results in blood type O (homozygous) $ skin colour, eye colour is controlled by even an greater number of alleles $ continuous variation that can’t be split into easily defined categories $ e.g. cross a heterozygous person with type A blood with an individual heterozygous for Biol 3201 11 type B blood. What are the possible offspring phenotypes? IA i IB I AI B IBi i I Ai ii $ ii is the genotype for type O $ IAIB is the genotype for type AB $ IBi is the genotype for type B $ IAi is the genotype for type A Modern Ideas $ Mendel said each trait has 2 ‘factors’ which separate during gamete formation $ went unnoticed until 1900's when better technology showed cellular processes & chromosomes The Chromosome Theory of Inheritance (pg 545-546) $ Walter Sutton and Theodor Boveri (scientists) studied meiosis Biol 3201 12 $ $ $ saw chromosome behaviour during meiosis was like that of Mendel’s factors (fig 16.19 pg546) they made 3 key observations - chromosomes occur in pairs which segregate during meiosis - align independently of one another - inheritance of one doesn’t affect other chromosome being found in the gamete formed basis for chromosome theory of inheritance - genes are on chromosomes - it is the segregation and independent assortment of chromosomes that accounts for patterns of inheritance Morgan’s discoveries $ Thomas Morgan - 1910 studied eye colour in fruit flies $ crossed 2 red-eyed parents - got white-eyed male $ principle of dominance?? $ crossed a red-eyed female offspring of the white-eyed male with normal red-eyed male - all female offspring had red eyes - ½ male offspring had red eyes - ½ male offspring had white eyes $ eye colour linked to sex $ concluded gene for eye colour located on X chromosome $ 1st time gene linked to specific chromosome $ found that some genes didn’t follow law of independent assortment, tended to be inherited together. $ genes located on same chromosome usually don’t segregate. $ this rule doesn’t always apply $ linked genes do sometimes segregate Biol 3201 13 $ $ $ $ this is due to gene-chromosome theory - genes exist at specific sites in a linear fashion along chromosomes. Pairs of homologous chromosomes separate during gamete formation, and form two gametes. Each gamete contains a separate allele for each trait. During fertilization, chromosomes from one gamete will combine with another gamete. $ crossing over can occur during cell division (fig 16.20 pg 546) see “crossing over” overhead $ genes very close on chromosome - inherited together $ genes separated on chromosome - likely to be separated by crossing over $ more distance between genes - more likely cross over will occur $ crossing over breaks gene linkages and creates variation $ restate Mendel’s law of independent assortment: if crossing over doesn’t occur, genes on different chromosomes will assort independently while genes on the same chromosome will be inherited together. Sex Determination and Chromosomes $ chromosomes in cells from males and females are identical except one pair: the sex chromosomes (pair 23) $ other 22 pairs of chromosomes called autosomes $ sex chromosomes determine sex of organism $ $ females - two X-chromosomes as 23rd pair males one X- and one Y- chromosome as 23rd pair $ female gametes only have X-chromosomes when separated in meiosis $ $ male gametes produce both X and Y chromosomes cross male and female Biol 3201 14 $ $ X Y X XX XY X XX XY half offspring - female (XX); half - male (XY) could say male responsible for determining sex of offspring Sex-Linked Inheritance (pg 546-547) $ Some inherited traits depend on the sex of the parent carrying the trait $ Why?? $ The trait is carried on the sex chromosome $ Sex linked inheritance - the transfer of genes on the X or Y chromosome from one generation to the next $ a gene only on the X chromosome is called X-linked $ a gene only on the Y chromosome is called Y-linked $ most of the known sex-linked genes are X-linked, very few are Y-linked. Maybe because Y-chromosome is much smaller. $ Morgan hypothesized that the gene for eye colour in fruit flies is on the X chromosome $ said white eyed trait was recessive $ to get white eyed female both parents would need to donate an allele for white eyes $ fig 16.22 pg 547. Punnett square of XrY x XRXR - XrY - white eyed male, and XRXR - red eyed female Xr Y Biol 3201 15 XR XRXr XRY XR XRXr XRY $ male donates Y chromosome to all male offspring therefore doesn’t donate allele for eye colour $ female must donate the X chromosome to all male offspring $ female offspring get one X from father, and one of the mother’s X chromosomes $ all female offspring are heterozygous and red-eyed XRXr and all male offspring red eyed XRY red eyed male XRY with a heterozygous red eyed female XRXr XR Y XR XRXR XRY Xr XRXr Xr Y Sex linked traits and Punnett squares $ Same as autosomal trait squares except: $ $ alleles on X if X-linked and none on Y chromosome e.g. Xr Y and XRXR when writing the genotypes and phenotype ratios we ALWAYS include the sex in the ratio. $ Remember autosomal inheritance usually involves pairs of genes, gender is irrelevant $ sex-linked inheritance involves pairs of genes on the X chromosomes in the female and a single gene in the male $ sex linked defects/disorders are more common in males than females $ males only need one recessive allele for trait to be seen $ females need two recessive alleles to display trait, or could be a carrier if heterozygous. Biol 3201 16 $ red-green colour blindness, hemophilia, muscular dystrophy are all connected to sex $ more common in males X-Linked Recessive Inheritance (pg 558-559) 1. Colour blindness fig 16.36, pg 559 $ 8% men, 0.4% women red-green colour blind $ both alleles for “seeing” red & green on X chromosome $ if one allele absent, can’t distinguish between colours $ why more in men?? $ $ defective allele is recessive condition so woman must have 2 recessive alleles (XbXb) one from a colour blind father, and one from her mother man only needs to inherit one recessive allele from mother XbY woman can have normal vision but be a carrier XBXb $ $ XBXB Normal vision females males XBY Normal Vision or or XBXb or carrier (Normal vision) XbXb Colour blind XbY Colour Blind $ for a female to be colour blind BOTH chromosomes must have the gene for colour blindness 2. $ $ $ $ Hemophilia recessive, sex (X-linked) disorder blood doesn’t clot properly XHXH or XHXh or XHY all normal phenotype Xh Xh or Xh Y hemophilia 3. Muscular dystrophy $ progressive wasting of skeletal muscle Biol 3201 17 $ carried on X chromosome Karyotypes (pg 559) $ useful in diagnosing genetic disorders $ $ illustration of chromosomes arranged in homologous pairs chromosomes stained, pictures enlarged, arranged in pairs according to size, banding, etc. $ humans have 46 chromosomes in somatic cells: 44 are autosomes and 2 are sex chromosomes Do Core Lab #6: Karyotype Lab Polygenic Traits $ also ‘multiple gene inheritance’ $ traits determined by a number of different contributing genes present at different locations $ expression depends on sum of influences of all the genes $ skin & eye colour, susceptibility to cardiovascular disease, athletic ability, ear length in corn. Biol 3201 18