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GREGOR MENDEL • • • • Austrian monk Studied at the University of Vienna Discovered the basic principles of heredity Worked with breeding garden peas – Self pollinating – Perfect flowers – Artificially cross pollinated Mendel’s Pea Plants • True breeding – all offspring same variety • Hybridization – crossing 2 contrasting true breeding varieties • P generation – parental generation (true breeding) • F1 – first filial (hybrids) • F2 – second filial (from self pollinating F1s) Mendel tracked heritable characters for 3 generations MENDEL’S LAWS 1. Alternative versions of genes (alleles) account for variations in inherited characters. 2. For each character, an organism inherits two alleles, one from each parent. 3. If the 2 alleles differ, then one, the dominant allele is fully expressed in the organism’s appearance; the recessive allele has no noticeable effect on the organism’s appearance 4. The 2 alleles for each character segregate during gamete production. Law of Segregation • Letters represent alleles (upper case for dominant alleles & lower case for recessive alleles) • P generation – true breeding plants, matching alleles (PP or pp) • Gametes contain only one allele • Fusion of gametes → hybrid F1s • When hybrids produce gametes, the 2 alleles segregate • Half the gametes receive the P allele and half receive the p allele • Punnett squares show possible combinations of alleles in gametes • Each square is a possible offspring VOCABULARY TEST CROSS PROBABILITY LAWS 1. CHANCE HAS NO MEMORY 2. RULE OF MULTIPLICATION: the chance of 2 independent events occurring together is the product of their individual probabilities. 3. RULE OF ADDITION: the probability of an event that can occur in 2 or more different ways is the sum of the separate probabilities. PROBABILITY PRACTICE 1) If a coin is tossed 7 times and lands heads all seven times, what is the chance of getting heads again? 2) A couple has 3 girls; what is the chance that their fourth child will be another girl? 3) In a deck of 52 cards, what is the chance of drawing: a) Any red card? b) Any ace? c) Any heart? PROBABILITY PRACTICE 1. From a normal deck of cards, what is the chance of drawing the Jack of Hearts? 2. When tossing 2 dice, what is the probability of getting a “6” up on both? 3. What is the probability of 2 parents, with genotypes AabbCc x AaBBCc, having a child with the genotype AaBbCc? AaBbRr x Aabbrr • What fraction of the offspring will have the following genotypes? – aabbrr - AaBbRr - aaBbrr • What fraction of the offspring will have at least two recessive phenotypes? a) list all possible genotypes b) calculate probabilities (rule of multiplication) c) pool probabilities (rule of addition) AaBbRr x Aabbrr Predict the gametes • • • • • • • Formula 2n (n = # of heterozygous pairs) AA → 20 →1 gamete type (A) Aa → 21 → 2 gametes (A or a) AABb → 21 →2 gametes (AB or Ab) AaBb → 22 → 4 gametes (AB, Ab, aB, ab) AaBbDd → 23 → 8 gametes AaBbDdFf → 24 →16 gametes AaBbDd AaBbDdFf Incomplete Dominance • Alleles for red and white, neither is dominant. • Hybrids are a blend of the two alleles and are phenotypically pink • There is NO allele for pink, therefore NO true breeding pink flowering plants. • Codominance: both alleles equally expressed. Human blood type, cow coloring MULTIPLE ALLELES EPISTASIS • Gene at one locus alters the phenotypic expression of a gene at a second locus • Gene for fur color: (B) black (b) brown • 2nd gene deposition of pigment: (C) color (c) white Polygenic Traits • Skin color and height in humans • Additive effect of 2 or more genes on 1 phenotype • Quantitative characters – variation along a continuum • Dots represent “units” of darkness NATURE and NURTURE The product of a genotype is a range of phenotypic possibilities over which there may be variation due to environmental influence. Norm of reaction - Hydrangea flower color and pH (blue-pink) - Human blood typing: little range (genotype mandates phenotype. - Human blood counts: vary with altitude, fitness, infectious agents Norms of reaction broadest for polygenic traits like skin color. Multifactorial characters: both genetic & environmental influences. Mendelian Inheritance in Humans • Recessive Disorders – Cystic fibrosis – Tay-Sachs disease – Sickle-cell disease - PKU • Dominant Disorders – Achondroplasia – Huntington’s disease • Multifactorial Disorders -Polydactyly SICKLE CELL DISEASE • Pleiotrophy – one disorder, multiple effects • Incomplete dominance • Heterozygotes – sickle cell trait, advantage (resistant to malaria) PEDIGREES INHERITANCE PATTERN? FETAL TESTING THE CHROMOSOMAL BASIS OF INHERITANCE The Chromosomal Basis of Mendel’s Laws Segregation R & r alleles Segregate Only one long chromosome In each gamete Fertilization recombines the R & r alleles Independent Assortment Long and short chromosomes; Arranged in 2 equally likely ways They assort independently Fertilization 9:3:3:1 ratio THOMAS HUNT MORGAN • Worked with Drosophila • Wild type – phenotype most common in the wild. Red eyes, gray, normal wings • Mutant phenotypes – white eyes, ebony, vestigial wings • Discovered sex linkage • Sex linked genes – on sex chromosomes Sex Linked Inheritance • Sex linked genes: on the X or Y chromosome • X & Y NOT homologous • X-linked genes: males being XY have only one copy/allele, females XX have two copies/alleles • X-Linked recessive – more common in males • X-Linked dominant – more common in females SEX DETERMINATION • Humans – sex determined by presence or absence of Y chromosome; XX-female; XY-male • Fruit flies – # of X chromosomes; XXfemale; XY-male; XXY-female • Birds – females are heterogametic Transmission of Sex-Linked Recessive Traits Father w/ trait transmits to all daughters Carrier passes trait to ½ sons ½ daughters Sons afflicted Carrier w/ afflicted male; 50% of children afflicted LINKAGE • Law of Linear Order: genes on the same chromosome are linked and are inherited in a block. • Drosophila – 2 linked genes; inherited together – body color (G gray, g ebony) – wing size (L long, l vestigial) • Test cross by Morgan produced unexpected results CROSSING OVER MAPPING • Linkage map – genetic map based on recombination frequencies • Map units – one map unit = 1% recombination frequency • Cytological maps – locate genes with respect to chromosomal features like stained bands Recombination frequencies: used to construct genetic map The probability of a crossover between 2 loci is proportional to the distance separating the loci Sex linked Disorders in Humans • Duchenne Muscular Dystrophy • Hemophilia • Red/Green Color blindness X Inactivation NONDISJUNCTION leads to aneuploidy (trisomy, monosomy, polyploidy) CHROMOSOMAL MUTATIONS Human Chromosomal Disorders • • • • • • Down Syndrome (Trisomy 21) Turner Syndrome (XO, monosomy X) Klinefelter syndrome (XXY) XYY, XXX Cri du chat (cry of the cat) deletion in #5 CML (chronic myeloid leukemia) translocation of # 22 • Down Syndrome due to translocation of #21 Genomic Imprinting • Prader-Willi Syndrome: – Mental retardation, obesity, short stature, small hands & feet (father) • Angelman Syndrome: – Spontaneous laughter, jerky movements, other motor and mental symptoms (mother) • Same cause – partial deletion of chromosome #15 • Genomic Imprinting – gene on one chromosome silenced