Genetics Notes Week 2
Genetics Notes Week 2 Bios 206
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This 4 page Class Notes was uploaded by Becca Sehnert on Saturday January 23, 2016. The Class Notes belongs to Bios 206 at University of Nebraska Lincoln taught by Dr. Christensen in Fall 2016. Since its upload, it has received 33 views. For similar materials see Genetics in Biological Sciences at University of Nebraska Lincoln.
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Date Created: 01/23/16
LECTURE 4 Correlation of Mendel’s postulates with the behavior of chromosomes In a dihybrid cross (Aa Bb x Aa Bb) what is the probability of having progeny that are heterozygous for both genes? a. 1/16 b. 3/16 c. 4/16 d. 8/16 e. 9/16 1/4 AA 1/4 BB AaBb 1/2 Aa 1/2 Bb 1/4 aa 1/4 bb Chi-square analysis evaluates the influence of chance on genetic data • Chance deviation from expected outcome is diminished by larger sample size • Null hypothesis: there is no difference between observed results and expected results • Difference is due to chance variation/random fluctuations • Chi-square (x ) analysis used to test how well data fit null hypothesis If a coin is flipped , with following results, when do you question the null hypothesis (non-counting coil)? a. 6 heads, 4 tails b. 30 heads, 20 tails c. 60 heads, 40 tails d. 120 heads, 80 tails e. 300 heads, 200 tails For dihybrid cross, null hypothesis is that 2 genes assort independently Expectation based on null hypothesis O - E = Phenotype Observed Expected Deviation d^2 d^2/e A_B_ 587 567 20 400 0.71 A_bb 197 189 8 64 0.34 aaB_ 168 189 -21 441 2.33 aabb 56 63 -7 49 0.78 total 1008 1008 0 4.16 Null hypothesis that 2 genes sort independently Expect 9:3:3:1 ratio Squared makes deviations hurt more and all same size D^2/e all added up is the chi squared value Degrees of freedom • 3 degrees of freedom for this chart th • Can mess with 3 boxes but 4 fixed cuz have to get correct total • Degrees of freedom = n – 1 • = df X^2 value can be interrupted in terms of corresponding probability value (p) For chart, P > 0.20 -- Accept hypothesis Pedigrees reveal patterns of inheritance in humans • Pedigree shows family tree with respect to given trait • Collect data you have. Can’t make it, only use it • Analysis reveals patterns of inheritance • Standard pedigree conventions in Fig 3-12 • Representative pedigrees shown in Fig 3-13 • • Parents connected by single horizontal line, vertical lines lead to offspring • If parents related (consanguineous), such as first cousins, connected by double line What is genotype of individual II-2? a. BB b. Bb c. Bb d. BB or Bb e. Impossible to determine What are genotypes of individuals I-3 and I-4? a. Both BB b. Both Bb c. One Bb, other BB d. One Bb, other BB or Bb e. One BB, other bb Alleles alter phenotype in different ways • Alt forms of gene are called alleles • Mutation is source of alleles • “Wild type” allele is one that occurs most frequently in nature In trihybrid cross such as mendel’s Gg Ww Dd X Gg Ww Dd how many possible phenotypes can result? a. 8 Types of mutations • Mutation may cause reduction or loss of wild-type function • Loss of function mutation • Loss if complete = “null allele” • Also gain-of-function alleles • Phenotypic traits may be influenced by 1+ genes Genetic Symbols • Gene has 1 name, 1 symbol • Diff alleles distinguished either by capitalization of superscripts, or both • Dominant alleles often indicated by uppercase (G) • The + superscript used to indicate wild type allele A • Other dominant alleles use different superscripts (I ) • Recessive allele often indicated by lowercase letter (d) or by superscript (v ) • Symbols don’t always distinguish dominant and recessive alleles CHAPTER 4 Incomplete dominance • Snap dragons • Get same genotypic and phenotypic ratio • Andalusian chickens –splash and black o Cross to make andalusian blue LECTURE 5 In peppers, incompletely dominant gene determines spiciness:HH is hot, Hsh is medium hh is mild. Long shape (ll) is recessive to round (L_), orange color (rr) is recessive to red (R_). In a cross of HhLlRr to HhLlRr, what proportion of progeny will be meuim, long, and red? a. 1/64 b. 3/64 c. 6/64 d. 9/64 e. 12/64 In Codominance, influence of both alleles in a heterozygote is clearly evident • Codominance • FINISH SLIDE Mult alleles of gene may exist • Multiple alleles (>2) can be studied only in populations, because any individual will have at most 2 alleles of any given gene. ABO blood groups • Each individual is A, B, AB, or O • 1 gene, 4 phenos • 3 alleles: I I and I (I ) • I and I are dominant to i • I and I show codomiance o I adds enzyme to terminal to H substance o I allele responsible for modified enzyme that adds terminal to galactose o O pheno has absence of either terminal o I is a null allele (complete loss of function) Lethal alleles represent essential genes • Loss of function mutation can be tolerated in heterozygote • May be lethal in homozygous state • Would conclude this is essential gene • Recessive lethal allele may produce phenol when heterozygous o Agouti mouse vs yellow (lethal) mouse o Homo recessive is lethal, die before birth o Thus, smaller litters o Both recessive and dominant phenotypes In mice, yellow allele of agouti (AY) has dominant yellow fur and recessive lethal phenotype. Tailless allele (T +) has dominant taillessness and recessive lethal phenol. Two tailless yellow mice are crossed. Agouti tails: agouti tailless: yellow tails: yellow tailless At: AT: A t: A T ¼ of the offspring of each allele die en-eutero 1/3 TT 1/9th 1/3 AA 2/3 Tt 2/9th AAY 2/3 AAY 1/3 TT 2/9th 2/3 Tt 4/9th a. 9:3:3:1 b. 3:1:3:1 c. 1:2:2:4 d. 1:3:3:9 e. 1:2:1:2 Dominant lethals • Mutation is dominant lethal allele, heterozygote will die • Huntington disease is one example • For dominant lethal to persist, affected individuals must reproduce before dying o OI is weak bones, high likelihood she will break a bone o Half as many working collagen fibers o UNBREAKABLE –movie. Looks pretty good •
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