Presentation on theme: "Haixu Tang School of Informatics"— Presentation transcript:
1 Haixu Tang School of Informatics Basic geneticsHaixu TangSchool of Informatics
2 Mendel’s two innovations Developed pure linesa population that breeds true for a particular traitCounted his results and kept statistical notes
3 Phenotypes of Mendel's pea plants round or wrinkled seed phenotypeyellow or green seed phenotypered or white flower phenotypetall or dwarf plant phenotype
4 Results from Mendel's Experiments Parental CrossF1 PhenotypeF2 Phenotypic RatioF2 RatioRound x Wrinkled SeedRound5474 Round:1850 Wrinkled2.96:1Yellow x Green SeedsYellow6022 Yellow:2001 Green3.01:1Red x White FlowersRed705 Red:224 White3.15:1Tall x Dwarf PlantsTalll787 Tall:227 Dwarf2.84:1
5 PhenotypesDominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F1 generation from the cross of two pure linesRecessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation
6 ConclusionThe hereditary determinants are of a particulate nature. These determinants are called genes.Each parent has a gene pair in each cell for each trait studied. The F1 from a cross of two pure lines contains one allele for the dominant phenotype and one for the recessive phenotype. These two alleles comprise the gene pair.One member of the gene pair segregates into a gamete, thus each gamete only carries one member of the gene pair.Gametes unite at random and irrespective of the other gene pairs involved.
7 Some termsAllele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individualAllelic pair - the combination of two alleles which comprise the gene pairHomozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interestHeterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygoteGenotype - the specific allelic combination for a certain gene or set of genes
12 Genotype of the F2 individuals PhenotypesGenotypesGenetic DescriptionF2 Tall Plants1/3 DD 2/3 DdPure line homozygote dominant HeterozygotesF2 Dwarf Plantsall ddPure line homozygote recessiveThus the F2 is genotypically 1/4 Dd : 1/2 Dd : 1/4 dd
13 Backcross: Dd x ddThe cross of an F1 hybrid to one of the homozygous parents; for pea plant height the cross would be Dd x DD or Dd x dd; most often, though a backcross is a cross to a fully recessive parentBackcross One or (BC1) Phenotypes: 1 Tall : 1 DwarfBC1 Genotypes: 1 Dd : 1 dd
14 MonohybridMonohybrid cross - a cross between parents that differ at a single gene pair (usually AA x aa)Monohybrid - the offspring of two parents that are homozygous for alternate alleles of a gene pairRemember --- a monohybrid cross is not the cross of two monohybrids.
15 Variations to Mendel's First Law of Genetics Codominance - a relationship among alleles where both alleles contribute to the phenotype of the heterozygoteIncomplete dominance - the F1 produces a phenotype quantitatively intermediate between the two homozygous parents;
21 Mendel's Law of Independent Assortment We have followed the expression of only one gene. Mendel also performed crosses in which he followed the segregation of two genes. These experiments formed the basis of his discovery of his second law, the law of independent assortment.
22 Dihybrid crossDihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)Dihybrid- an individual heterozygous for two pairs of alleles (AaBb)
27 Mendel's Second Law - the law of independent assortment During gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair.
28 backcross - F1 dihybrid x Female GametesGWGwgWgwMale GametesGgWw (Yellow,round)Ggww (Yellow,wrinkled)ggWw (Green,ggww (Green,The phenotypic ratio of the test cross is:1 Yellow, Round Seed1 Yellow, Wrinkled Seed1 Green, Round Seed1 Green, Wrinkled Seed
29 The Chi-Square TestAn important question to answer in any genetic experiment is how can we decide if our data fits any of the Mendelian ratios we have discussed. A statistical test that can test out ratios is the Chi-Square or Goodness of Fit test.Degrees of freedom (df) = n-1, where n is the number of classes
30 An exampleLet's test the following data to determine if it fits a 9:3:3:1 ratio.Observed ValuesExpected Values315 Round, Yellow Seed(9/16)(556) = Round, Yellow Seed108 Round, Green Seed(3/16)(556) = Round, Green Seed101 Wrinkled, Yellow Seed(3/16)(556) = Wrinkled, Yellow 32 Wrinkled, Green(1/16)(556) = Wrinkled, Green556 Total Seeds Total SeedsNumber of classes (n) = 4df = n = 3Chi-square value = 0.47
31 A Chi-Square Table Probability Degrees of Freedom 0.9 0.5 0.1 0.05 0.0110.020.462.713.846.6420.211.394.615.999.2130.582.376.257.8211.3541.063.367.789.4913.2851.614.359.2411.0715.09
32 Pleiotropic Effects and Lethal Genes In 1904, a cross was made between a yellow-coated mouse and a mouse with a gray coat. The gray- coated mouse was extensively inbred and therefore was considered to be pure bred.
33 Next a cross was made between two yellow mice Next a cross was made between two yellow mice. What genetic ratio would we expect to see? Yy x Yy should give a ratio of 3 yellow:1 gray. The result, though, was a ratio of 2 yellow to 1 gray mice. How can this result be explained? Let's first set up a Punnett Square.
34 TestcrossAll testcross data with the yellow mice give a 1:1 ratio. This ratio is typical of what is seen with heterozygous individuals.All of the yellow mice from the cross of two heterozygous yellow mice are genotypically Yy. Somehow the YY genotype is lethal. The 2:1 ratio is the typical ratio for a lethal gene.
35 Lethal geneLethal Gene - a gene that leads to the death of an individual; these can be either dominant or recessive in nature.Pleiotropic gene - a gene that affects more than one phenotype
38 EpistasisThe interaction between two or more genes to control a single phenotype
39 Modifier GenesInstead of masking the effects of another gene, a gene can modify the expression of a second gene. In mice, coat color is controlled by the B gene. The B allele conditions black coat color and is dominant to the b allele that produces a brown coat. The intensity of the color, either black or brown is controlled by another gene, the D gene. At this gene, the dominant D allele controls full color whereas the recessive d allele conditions a dilute or faded expression of the color expression at the B gene. Therefore, if a cross is made among mice that are BdDd, the following phenotypic distribution will be seen:9 B_D_ (black)3 B_dd (dilute black)3 bbD_ (brown)1 bbdd (dilute brown)The D gene does not mask the effect of the B gene, rather it modifies its expression.
40 Gene linkageOne experiment was performed by Bateson and Punnett with sweet peas. They performed a typical dihybrid cross between one pure line with purple flowers and long pollen grains and a second pure line with red flowers and round pollen grains. Because they knew that purple flowers and long pollen grains were both dominant, they expected a typical 9:3:3:1 ratio when the F1 plants were crossed.
41 ObservedExpectedPurple, long (P_L_)284215Purple, round (P_ll)2171Red, long (ppL_)Red, round (ppll)5524Total381
42 Linked Genes On The Same Chromosome F1 GameteTestcross DistributionGamete Typepr+ vg+1339Parentalpr+ vg151Recombinantpr vg+154pr vg1195
43 Coupling and repulsion F1 GameteTestcross DistributionGamete Typepr+ vg+1339Parentalpr+ vg151Recombinantpr vg+154pr vg1195