How is each group the same? GENETICS AND MENDEL How is each group - PDF document

2/20/2013 How is each group the same? GENETICS AND MENDEL How is each group different? HISTORY OF DISCOVERERY OF HEREDITY • Up to 1800’s: theory of blending inheritance Heredity • 1851: Gregor Mendel, father of heredity transmission of traits from parents to offspring – studied pea plants – prevented self pollination Genetics – used cross pollination study of heredity Mendel’s Experiment Genetic Terms Alleles : different forms of the same gene ex: tall / short green seed / yellow seed smooth / wrinkled curly hair/ straight hair Dominant : stronger trait which is expressed, written as a capital letter ex: T Recessive: weaker trait which is not expressed when paired with a dominant trait, written as a lower case letter ex: t Homozygous/Pure : appearance of 2 identical alleles of a gene on a chromosome ex: TT, tt Heterozygous/Hybrid : appearance of 2 different alleles of a gene on a chromosome ex: Tt 1

2/20/2013 Mendel: Experiment 1 Genetic Terms 1. he crossbred purebred plants of opposite traits • self pollination: ( parental/P generation ) fertilization of plant’s 2. resulting offspring were first egg by pollen of same filial (F1 generation) plant 3. he self pollinated F1 generation • cross pollination: 4. resulting offspring were fertilization of plant’s second filial (F2 generation) egg by pollen of 5. he performed 100’s of crosses another plant and documented results Mendel’s Three Laws of Heredity Results: Experiment 1 I. Law of Dominance and Recessiveness P 100% homozygous dominant: SS One factor (gene) in a pair may mask the other factor F1 (gene) preventing it from having an effect. 100% heterozygous dominant: Ss dominant : allele codes for a protein that works F2 3:1 ratio dominant to recessive traits recessive : allele codes for a protein that smooth : wrinked doesn’t work **genes always occur in pairs** THIS WORK FORMED BASICS OF GENETICS AND HOLDS TRUE TODAY ! ex: TT, Tt : tall tt: short Mendel’s Three Laws of Heredity Mendel’s Three Laws of Heredity Ill. Law of Independent Assortment II. Law of Segregation Factors (genes) for different traits are distributed to reproductive cells (gametes) independently of each other. The two factors for a trait segregate (separate) during the formation of egg and sperm and each reproductive - Mendel also crossed plants that differed in cell (gamete) receives only one factor for each trait two characteristics ex: height, coat color ex: male would give one trait : T or t female would give one trait: T or t - He found that traits from dominant factors did not appear together offspring could have these combinations: - Factors for each trait were not connected TT, Tt, tt 2

2/20/2013 MENDEL’S 4TH LAW Mendel was very innovative because he applied math (probability) to Biology. PRINCIPLE OF ZERO POPULATION GROWTH Probability “If your parents had no offspring, Possibility that an event will occur chances are 3:1 you won’t either” Probability = # one kind of event # of all events Genetic Crosses Punnett Square Chart used to predict probabilities of genetic crosses (RC Punnett) Phenotype : external appearance of an organism Genotype : actual genetic makeup of an Monohybrid Cross organism Punnett Square steps: Crossing of one set of traits. 1. determine which trait is dominant or recessive 2. determine genotype (remember – genes come in pairs) 3. write down letters to represent the gene pairs 4. write down the cross 5. make a square with 4 sections 6. put one pair across top (male), one pair down the side (female) 7. fill in boxes with the gene pairs Dihybrid Cross Crossing of two sets of traits. Traits Test Cross Y yellow R round Procedure where an individual of unknown y green r wrinkled genotype is crossed with a homozygous recessive individual. P generation homozygous round, yellow X homozygous wrinkled, green RRYY x rryy 3

2/20/2013 Cross F1 generation: RrYy P Generation RY Ry rY ry rryy RY RRYY RRYy RrYY RrYy ry ry ry ry RrYy Ry RRYy RRyy RrYy Rryy RY RrYy RrYy RrYy RrYy rY RrYY RrYy rrYY rrYY rrYy rrYy RY RrYy RrYy RrYy RrYy RRYY ry RrYy Rryy rrYy rrYy rryy RY RrYy RrYy RrYy RrYy F2 generation : phenotype ratio RY RrYy RrYy RrYy RrYy 9 : 3 : 3 : 1 yellow yellow green green round wrinkled round wrinkled F1 generation : 100% RrYy heterozygous round yellow (only genotype possible) When crossing two hybrids- phenotype ratio will always be 9:3:3:1 Theories of Heredity 1902: Walter Sutton , Columbia University - Observed that genes are located on chromosomes. Product Rule - Realized chromosomes behaved exactly same as carriers of genetic information would do. Chance of 2 or more independent events 1903: Chromosome Theory of Heredity occurring together equals product of 1. Genes are located on chromosomes and each chances of each of the separate gene occupies a specific place (locus) on a chromosome. occurrences. 2. Genes can exist in several forms. (alleles) 3. Each chromosome contains only one of the alleles for each of its genes. Sutton – believed that genes move in sets on a chromosome. Theories of Heredity, cont. Exchange of chromatids pieces of a homologous pair during synapsis at a chiasma... 1902/03: Thomas Hunt Morgan , Columbia Univ. - proved gene linkage, won Nobel prize in 1933 Gene linkage : attachment of certain genes to each other on a chromosome is GREATER the FARTHER (by chemical bonds that keep them together) apart 2 genes are Linkage groups : group or packages of genes located on one chromosome which are usually inherited together is proportional to relative (they do not undergo independent assortment) distance between 2 linked genes - groups can be independently assorted, but always go together - linkage groups are actually chromosomes 4

2/20/2013 Morgan worked with Drosophila Mendelian genetics would produce : 1 : 1 : 1 : 1 (new generation every 4 weeks) grey grey black black - demonstrated gene linkage: norm. small norm. small Actual observation G grey body (dom) W normal wings (dom) - 42% grey normal (84% chromosomes like parents) - 42% black small g black body (rec) w small wings (rec) -------------------------------------- - 8% black normal (16% new combinations) - 8% grey small P GGWW x ggww Recombinants F1 100% GgWw (grey normal wings) Individuals with new genetic combinations ***this indicated gene for body color and wing size were LINKED ***** ------------------------------------------------------- GgWw x ggww (test cross) Sturtevant (worked with Morgan) Sex Determination 1905: Nettie Stevens – - responsible for discovering crossing over: • studied mealworms process where portions of genes overlap - female cells had 20 large pairs of chromosomes each other and cause new allele combinations - male cells had 19 lg. chrom, 1 small chromosome during synapsis in prophase I. • she then studied drosophila - crossing over occurs at random along - female cells had 4 pairs linkage groups and close together alleles - male had 3 alike pairs, 1 mismatched (1 looked like a hook) rarely cross, farther distance alleles cross over more often ** autosomes – non sex chrom, matched ** sex chromosomes – mismatched pair - used crossing over to make gene maps - female chromosomes - X - male chromosomes - Y - gene map diagram of allele - during meiosis, 4 resulting gametes have either only an X or Y positions on a particular chromosome - 1:1 ratio of male to female offspring (50% probability in mating) Which sex determines sex of Sex Linkage offspring? - 1909: discovered by Morgan (worked with drosophila) Experiment: - homogametic sex (XX) - in a large batch of red eyed flies, they found 1 white eyed fly (actually a mutation) 2 same sex chromosomes R red, dominant r white, recessive -------------------------------- P red eye female x white eye male F1 100% red eyed hybrids (Rr) - heterogametic sex (XY) Mated F1 2 different sex F2 3 : 1 chromosomes ¾ red ¼ white **this confirmed Mendels work **BIG DISCOVERY - ALL WHITE EYES WERE MALES MORGAN DISCOVERED A SEX LINKED TRAIT** 5

2/20/2013 Sex Linked Genes (X linked) How is the gene for white eyes - genes carried by either sex chromosome related to sex??? (generally carried on X chrom, missing on Y chromosome) R r R Lets revisit the experiment by Morgan Cross F1 : X X x X Y F2 3 : 1 R X R x X r Y P X red white phenotype red female white male r R R X R Y (all red eyes) F1 X or X X Y 1 homo red female red red R R R R X 1 hetero red female (carrier) genotypes X X X Y hetero hemizygous 1 hemi red male 1 hemi white male female male r R r r X X X X Y This is known as “criss cross inheritance ” Hemizygous : dominant gene present and expressed, recessive gene missing P  F1  F2 male female male (express) (carrier) (1/2 sons express) What would happen in F3 if hybrid red female was • Why is this important? crossed with expressing male? - sex linked traits not limited to drosophila R X r x X r Y X - occur in all species including humans r Y X 1 carrier female R R X r X R Y F3 X X 1 normal male r r X r X r Y X 1 male expresses X 1 female expresses 6