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Mendel & Genetics Chapter 11
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Gregor Mendel Austrian Monk Researched inheritance using pea plants
Teacher
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Why pea plants? Variation Characters – heritable feature
Ex: flower color Trait – variable Ex: purple or white Controlled plant “mating” by cutting stamens
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Mendel's Experiments Established true breeding lines – offspring the same as the parent P generation - parent Hybridized true breeding lines P X P = F1 generation – first filial Hybridized F1 generation F1 x F1 = F2 generation – second filial generation P Generation (true-breeding parents) Purple flowers White F1 Generation (hybrids) All plants had purple flowers F2 Generation
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Alleles Allele – alternate version of a gene
Accounts for variations in inherited characters Organisms inherit 2 alleles for each character One from each parent Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers
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Dominant vs. Recessive Dominant allele – determines the organisms appearance if present Represented as a capital P = purple Recessive allele – masked if the dominant allele is present Appears if both alleles are recessive Represented as lower case p = white
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1st law = law of segregation
Two alleles for a heritable characteristic separate (segregate) during gamete formation and end up in different gametes Separate during anaphase I Egg or sperm only gets one of the alleles (haploid)
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Punnett Square Predicts the possibilities or possible combinations of offspring from known parental genes.
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Homozygous vs. Heterozygous
Homozygous – same Homozygous dominant PP – genotype Purple - phenotype Homozygous recessive pp – genotype White - phenotype Heterozygous – different Pp – genotype
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Testcross Possible genotypes for a purple flower?
Pp or PP Impossible to know just by looking at the flower Cross with homozygous recessive pp Analyze results 100% purple – PP 50% purple 50% white - Pp
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2nd law – law of independent assortment
Each pair of alleles segregates independently of other allele pairs during gamete formation
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Monohybrid vs. Dihybrid
One Trait Cross – one pair of contrasting characteristics Flower color – purple or white PP, Pp, or pp Two Trait Cross – two pairs of contrasting traits Demonstrates law of independent assortment Flower color and height – purple or white and tall or dwarf PPTT, PpTT, PPTt, PpTt, PPtt, Pptt, ppTT, ppTt
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Sample Two Trait Cross PPTt x PpTt Possible Gametes
PPTt – PT or Pt PpTt – PT, Pt, pT, pt PT Pt pT pt The size of the punnet square is determined by the number of traits being crossed. 3 traits: 2x = 23 or 8 gametes possible an 8x8 square 4 traits: 2x = 24 or 16 gametes possible a 16x16 square
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Heterozygous dihybrid cross
GgWw x GgWw G = green, g = yellow W = wrinkled, w = round Gametes for both the same GW, Gw, gW, gw Always ends up in a 9:3:3:1 phenotypic ratio 9 – dominant – dominant Green and wrinkled 3 – dominant – recessive Green and round 3 – recessive – dominant Yellow and wrinkled 1 – recessive – recessive Yellow and round
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Probability # of times an event occurs # of opportunities it can occur
Rr Segregation of alleles into eggs alleles into sperm R r 1⁄2 1⁄4 Sperm Eggs Figure 14.9 Multiply individual allele probabilities to calculate overall probability (AND) Add probability of 2 OR more mutually exclusive events (the heterozygotes can be produced in 2 different ways) PpTt x PpTt ¼ PP, ½ Pp, ¼ pp ¼ TT, ½ Tt, ¼ tt Probability of PPTt ? ¼ PP x ½ Tt = 1/8 chance of PPTt Probability of PpTt? ½ Pp x ½ Tt = ¼ chance of PpTt
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Practice probability PpTtGg x PPTtgg
What are the chances of producing recessive genotypes for 2 characteristics? PP = ½, Pp = ½, pp = 0 TT = ¼, Tt = ½, tt = ¼ GG = 0, Gg = ½, gg = ½ PPttgg = ½ x ¼ x ½ = 1/16 Ppttgg = ½ x ¼ x ½ = 1/16 ppTTgg = 0 x ¼ x ½ = 0 ppTtgg = 0 x ½ x ½ = 0 ppttGg = 0 x ¼ x ½ = 0 ppttGG = 0 x ¼ x 0 = 0 Total probability = 2/16 (remember the additional rule!)
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Incomplete Dominance Incomplete – traits are expressed together
Ex: red (CRCR) + white (CWCW) snapdragon flowers = pink (CRCW) The heterozygote produces less red pigment so the flower appears pink. NOT BLENDING! F2 Phenotypic and genotypic ratios are the same.
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Codominance Tay-Sachs Disease: codominant at the molecular level, and equal # of functional and dysfunctional enzymes are produced in the heterozygote, there is enough functional enzyme to break down the lipids in the brain, and the person is phenotypically normal. Codominance – both phenotypes show up – no recessive trait Blood Groups: M and N or A and B are Codominant, O is recessive over both MM: MN: NN A: Iai or IAIA B: IBi or IBIB O: ii AB: IAIB
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Multiple Alleles More than two allele forms Ex: blood type A, B, O
AB – codominant O - recessive Table 14.2
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Pleiotropy One gene has more than one phenotypic effect
Ex: Sickle cell anemia & cystic fibrosis
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Epistasis One gene alters another gene at a different location
BC bC Bc bc 1⁄4 BBCc BbCc BBcc Bbcc bbcc bbCc BbCC bbCC BBCC 9⁄16 3⁄16 4⁄16 Sperm Eggs One gene alters another gene at a different location B = black b = brown C = color c = albino
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Polygenic Inheritance
AaBbCc aabbcc Aabbcc AaBbcc AABbCc AABBCc AABBCC 20⁄64 15⁄64 6⁄64 1⁄64 Fraction of progeny Quantitative characteristics Varied degrees of the characterstic Multiple genes affect one phenotype Ex: Skin Color, hair color, eye color
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Multifactoral Impact Environment plays a role in gene expression – phenotype Ex: flower color Shades may vary based on acidity of the soil Ex: Height Actual height may vary based on childhood nutrition and sleep patterns. Figure 14.13
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Pedigrees Track traits through generations
Appearance of trait is shaded Absence of trait is unshaded
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Recessively inherited disorders
Must have both recessive alleles to have the disease Heterozygotes are carriers Increased probability of passing on a recessive disease when close relative mate Albinism Cystic Fibrosis Sickle Cell
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Dominantly Inherited Dissorders
Figure 14.15 Only need one dominant gene to have the disorder Achondroplasia Dwarfism – heterozygous Deadly when homozygous dominant Cartilage doesn’t form into bone during development0 Mild response to HGH Huntington’s Disease
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Multifactoral Diseases
Genetic component plus environmental factors Heart Disease Cancer Diabetes Alcoholism Mental Disorders
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Genetic Testing and Counseling
Fetal Testing Amniocentisis Chronic Villus Sampling – CVS
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