Presentation is loading. Please wait.

Presentation is loading. Please wait.

"A set of genes represents the individual components of the biological system under scrutiny" Modifications of the "3:1 F2 monohybrid ratio" and gene interactions.

Similar presentations


Presentation on theme: ""A set of genes represents the individual components of the biological system under scrutiny" Modifications of the "3:1 F2 monohybrid ratio" and gene interactions."— Presentation transcript:

1 "A set of genes represents the individual components of the biological system under scrutiny" Modifications of the "3:1 F2 monohybrid ratio" and gene interactions are the rules rather than the exceptions" Genes to Phenotypes

2 One gene - one polypeptide??

3 1.Many alleles are possible in a population, but in a diploid individual, there are only two alleles 2.Mutation is the source of new alleles 3.There are many levels of allelic variation, e.g. a.DNA sequence changes with no change in phenotype b.Large differences in phenotype due to effects at the transcriptional, translational, and/or post-translational levels c.Transposable element activity Allelic Variation

4 Mutant Series at a Locus Vrs1 – see Komatsuda et al. (2007) PNAS 104:

5 Complete dominance: Deletion, altered transcription, alternative translation. The interesting case of aroma in rice: a loss of function makes rice smell great, and patent attorneys salivate.... Allelic Relationships at a locus

6 Incomplete (partial) dominance Example: Red x White gives a pink F1. The F2 phenotypes are 1 Red: 2 Pink: 1 White. Explanation: Red pigment is formed by a complex series of enzymatic reactions. Plants with the dominant allele at the I locus produce an enzyme critical for pigment formation. Individuals that are ii produce an inactive enzyme and thus no pigment. In this case, II individuals produce twice as much pigment as Ii individuals and ii individuals produce none. The amount of pigment produced determines the intensity of flower color. Allelic Relationships at a locus

7 Incomplete (partial) dominance Example: Red x White gives a pink F1. The F2 phenotypes are 1 Red: 2 Pink: 1 White. Perspectives: Enzymes are catalytic and heterozygotes usually produce enough enzyme to give normal phenotypes. This is the basis for complete dominance. However, upon closer examination, there are often measurable differences between homozygous dominant and heterozygous individuals. Thus, the level of dominance applies only to a specified phenotype. Allelic Relationships at a locus

8 Codominance Many molecular markers show codominant inheritance. Both of the alleles that are present in a heterozygote can be detected. A way of visualizing codominance is through electrophoresis. Allelic Relationships at a locus

9 Codominance An application of electrophoresis is to separate proteins or DNA extracted from tissues or whole organisms. An electric charge is run through the supporting media (gel) in which extracts, containing proteins or DNA for separation, are placed. Proteins or DNA fragments are allowed to migrate across the gel for a specified time and then stained with specific chemicals or visualized via isotope or fluorescent tags. Banding patterns are then interpreted with reference to appropriate standards. The mobility of the protein or DNA is a function of size, charge and shape. Allelic Relationships at a locus

10 Cross two lines together and the F1 deviates significantly from the mid-parent Seed segregates in F2 – Farm Saved Seed not possible Overdominance

11 Mid-Parent Hybrid Vigour (Heterosis) aaAAAa Additive effect -a Dominance effect d P1P2F1 Single Gene Model Yield Additive effect a m

12 Heterosis Significantly exceed mid-parent – F1 > (P1+P2)/2; AA>Aa>(0.5*(AA+aa))>aa – Of interest?? Significantly exceed best parent – F1 > P1; Aa>AA>aa – Most commercially useful

13 Cause of Heterosis Over-dominance theory – Heterozygous advantage, d > a – F1’s always better than inbreds Dispersed dominant genes theory – Character controlled by a number of genes – Favourable alleles dispersed amongst parents (d ≤ a) – Can develop inbreds as good as F1

14 Dispersed Dominance Completely dominant genes shared by parents Heterosis = Σ(d/a)f 2 (after Falconer) – d/a = Degree of dominance at each locus (= 1 for complete dominance) – f 2 = difference in gene frequency of parents – Maximum heterosis when parents are fixed for opposite alleles and dominance is complete P2 aabbCCDD F1 AaBbCcDd P1 AABBccdd x

15 Epistasis: Interaction between alleles at different loci Example: Duplicate recessive epistasis (Cyanide production in white clover). Identical phenotypes are produced when either locus is homozygous recessive (A-bb; aaB-), or when both loci are homozygous recessive (aabb). Non-Allelic Interactions

16 Parental, F1, and F2 phenotypes: Parent 1 x Parent 2 (low cyanide) F1 (high cyanide) F2 (9 high cyanide : 7 low cyanide) Duplicate Recessive Epistasis Cyanide Production in white cover

17 AAbbaaBBx AaBb ABAbaBab ABAABBAABbAaBBAaBb AbAABbAAbbAaBbAabb aBAaBBAaBbaaBBaaBb abAaBbAabbaaBbaabb Low Cyanide High Cyanide F1 F2 9 High : 7 Low Cyanide Duplicate Recessive Epistasis Doubled Haploid Ratio??

18 Precursor  Enzyme 1 (AA; Aa)  Glucoside  Enzyme 2 (BB; Bb)  Cyanide If Enzyme 1 = aa; end pathway and accumulate Precursor; if Enzyme 2 = bb; end pathway and accumulate Glucoside Duplicate Recessive Epistasis

19 Complete dominance at both loci, but homozygous recessive condition at one of the two loci is epistatic to the other. Flower Colour in blue-eyed Mary (Collinsia parviflora) Recessive Epistasis wwMMWWmmx WwMm WMWmwMwm WMWWMMWWMmWwMMWwMm WmWWMmWWmmWwMmWwmm wMWwMMWwMmwwMMwwMm wmWwMmWwmmwwMmwwmm F1 F2 WhiteMagenta Blue 9 Blue : 3 Magenta : 4 White Also Agouti : Black : Albino coat colour in mice

20 Complete dominance at both loci, but homozygous recessive condition at one of the two loci is epistatic to the other. Recessive Epistasis Enzyme 1 (w) is epistatic to Enzyme 2 (m)

21 Selfing the F1 gives a ratio of 12 white, 3 yellow and 1 green fruited plants Dominant Epistasis Example: Fruit colour in summer squash (Cucurbita pepo) Plant 1 has white fruit and Plant 2 has yellow fruit; the F1 of a cross between them has yellow fruit x

22 Example: Fruit colour in summer squash (Cucurbita pepo) Dominant Epistasis WWyywwYYx AaBb WYWywYwy WYWWYYWWYyWwYYWwYy WyWWYyWwyyWwYyWwyy wYWwYYWwYywwYYwwYy wyWwYyWwyywwYywwyy White FruitYellow Fruit White Fruit F1 F2 A Dominant allele at the W locus suppresses the expression of any allele at the Y locus W is epistatic to Y or y to give a 12:3:1 ratio

23 Example: fruit shape in summer squash. Plant 1 and Plant 2 both have Round fruit; the F1 of the cross produces a new phenotype – Disc-shaped fruit Selfing the F1 give an F2 ratio of 9 disc : 6 Round : 1 Long fruit Duplicate Interaction x

24 Example: fruit shape in summer squash. Duplicate Interaction AAbbaaBBx AaBb ABAbaBab ABAABBAABbAaBBAaBb AbAABbAAbbAaBbAabb aBAaBBAaBbaaBBaaBb abAaBbAabbaaBbaabb Round Fruit Disc Fruit F1 F2 Complete dominance at both loci but A-B- gives a new phenotype as does aabb Also called a Cumulative Duplicate effect to give a 9 : 6 : 1 ratio

25 Example: Seed capsule shape in Shepherd's purse (Capsella bursa-pastoris) Plant 1 has a heart–shaped seed capsule and Plant 2 has a narrow capsule Crossing the two produces an F1 with a heart-shaped capsule Duplicate Dominant Epistasis x Single Dominant Gene? Selfing the F1 produces the following F2 ratio: 15 heart to 1 narrow fruit

26 Example: Seed capsule shape in Shepherd's purse (Capsella bursa-pastoris) Duplicate Dominant Epistasis AABBaabbx AaBb ABAbaBab ABAABBAABbAaBBAaBb AbAABbAAbbAaBbAabb aBAaBBAaBbaaBBaaBb abAaBbAabbaaBbaabb Heart ShapeNarrow Shape Heart Shape F1 F2 A Dominant allele at either of the two loci produces a heart-shaped fruit A is epistatic to B or b and B is epistatic to A or a to give a 15:1 ratio

27 Example: Malvidin production in Primula (anthocyanin giving blue flower) Plants 1 and 2 lack blue pigment in their flowers. When crossed together, the F1 also lacks pigment Selfing the F1 produces blue flowered segregants Phenotypic ratio: 13:3; Not 3:1 Dominant Suppression Epistasis

28 Example: Malvidin production in Primula (anthocyanin giving blue flower) Dominant Suppression Epistasis KKDDkkddx KkDd KDKdkDkd KDKKDDKKDdKkDDKkDd KdKKDdKKddKkDdKkdd kDKkDDKkDdkkDDkkDd kdKkDdKkddkkDdkkdd Non blue Non Blue F1 F2 Malvidin production controlled by dominant allele at K locus but the pathway is blocked by a dominant allele at the suppressor locus D Produces a 13 : 3 ration, like feather colour in chickens

29 prepared by Dr. Paul Kusolwa Bell Pepper Colour Genetics

30 Example: Fruit colour in Bell Pepper Two genes without Epistasis - Additive CCrrccRRx CcRr CRCrcRcr CRCCRRCCRrCcRRCcRr CrCCRrCCrrCcRrCcrr cRCcRRCcRrccRRccRr crCcRrCcrrccRrccrr YellowBrown Red F1 F2 A Dominant allele at either of the two loci produces red fruit Dominant alleles at the C locus and no dominant alleles at R give yellow fruit Dominant alleles at the R locus and no dominant alleles at C give brown fruit Double recessive gives green fruit 9:3:3:1

31 Gene Interaction Control Pattern A-B-A-bbaaB-aabbRatio Additive No interaction between loci 93319:3:3:1 Duplicate Recessive Dominant allele from each locus required 93319:7 Duplicate Dominant allele from each locus needed 93319:6:1 Recessive Homozygous recessive at one locus masks second 93319:3:4 Dominant Dominant allele at one locus masks other :3:1 Dominant Suppression Homozygous recessive allele at dominant suppressor locus needed :3 Duplicate Dominant Dominant allele at either of two loci needed :1 Dihybrid Ratio & some modifications

32 Testing the Difference How easy it to differentiate between a 1:1 ratio and a 9:7 – Consequence? How easy is it to separate a 13:3 ratio from a 3:1 ratio – Consequence Are doubled haploids of F2s better at teasing apart interactions at two or more loci?

33 Gene Interaction Control Pattern AABBAAbbaaBBaabbRatio Additive No interaction between loci 11111:1:1:1 Duplicate Recessive Dominant allele from each locus required 11111:3 Duplicate Dominant allele from each locus needed 11111:2:1 Recessive Homozygous recessive at one locus masks second 11111:1:2 Dominant Dominant allele at one locus masks other 11112:1:1 Dominant Suppression Homozygous recessive allele at dominant suppressor locus needed 11113:1 Duplicate Dominant Dominant allele at either of two loci needed 11113:1 Dihybrid Ratio & some modifications

34 Fruit shapes in squash Di locus Dominant to spherical or pyriform Duplicate genes with cumulative effects When Di is present together with Spherical S locus Di is dominant = Disc fruits When Di present with recessive s = spherical fruits When didi/ss = long or pyriform fruits Modified Ratio = 9: 6: 1 6 Spherical = Di/s & di/S_ 1 Pyriform di/s 9 Discs Di_S_

35 Duplicate Recessive - Squash Pathway involving two genes Wt = warty fruits Dominant to non warty wt Hr = hard rind hr = intermediate texture 9:7 B_ Hr_ Wt_ Y_

36 Bicolor fruits = locus B pleiotropic for fruits and leaves For yellow and green color patches BB or Bb Extent of yellow or green: Model with 2 incompletely dominant additive loci, Ep1 and Ep 2, proposed for enlarging the yellow patches Genotypes Bb with a dosage of 0 to 1 Ep alleles = bicolor green and yellow fruits Dosage of 2-4 dominant Ep alleles extends the yellow coloration Genotypes 9 B_Ep_ Extended yellow 3 B_epep Yellow narrow 3 bbEp_ Green extended 1bbepep green More Epistasis in Squash

37 Three or more genes interactions in ornamental gourds Gb = green bands; gb for no bands Gr/G = green rind (gr/g buff skin) L-2 = color intensity (yellow /orange) Gr Gb L-2 Ep-1/2 Tri-Genic Interactions….

38 A genetically stimulating tour of vernalization sensitivity in barley : Something as simple as growth habit can involve all types of allelic variants and interactions Flowering in Cereals; an Epistatic Model

39 VRN-H2 Repression of VRN-H1

40 Allelic Variation at VRN-H1

41 Winter or Facultative Crops??

42

43 Assembling the right alleles

44

45 Mendelian genetic analysis: the "classical" approach to understanding the genetic basis of a difference in phenotype is to use progeny to understand the parents. If you use progeny to understand parents, then you make crosses between parents to generate progeny populations of different filial (F) generations: e.g. F1, F2, F3; backcross; doubled haploid; recombinant inbred, etc. Parent / Progeny Relationships

46 Mendelian genetic analysis: the "classical" approach to understanding the genetic basis of a difference in phenotype is to use progeny to understand the parents. The genetic status (degree of homozygosity) of the parents will determine which generation is appropriate for genetic analysis and the interpretation of the data (e.g. comparison of observed vs. expected phenotypes or genotypes). o The degree of homozygosity of the parents will likely be a function of their mating biology, e.g. cross vs. self-pollinated. Necessary parameters for genetic analysis

47 Mendelian genetic analysis: the "classical" approach to understanding the genetic basis of a difference in phenotype is to use progeny to understand the parents. Mendelian analysis is straightforward when one or two genes determine the trait. Expected and observed ratios in cross progeny will be a function of o the degree of homozygosity of the parents o the generation studied o the degree of dominance o the degree of interaction between genes o the number of genes determining the trait Necessary parameters for genetic analysis


Download ppt ""A set of genes represents the individual components of the biological system under scrutiny" Modifications of the "3:1 F2 monohybrid ratio" and gene interactions."

Similar presentations


Ads by Google