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Lab. 11 Deviation of Mendel’s second law “Dihybrid” Part 2 Main topics: What’s Epistasis? Epistasis ratio Deviation of Mendel’s 1. Complement “chickens comb” second law “Dihybrid” 2. Crossing over between two gene “Gene interaction” 3. Epistasis Mendel’s Law II “ratio= 9:3:3:1” 3. Epistasis definition: The genes responsible for expression of the trait in the phenotype. In the case of simple Mendelian inheritance each gene is responsible for the expression of only one phenotypic trait. But, in reality the situation is more complicated. For example, the same gene may act on the expression of multiple traits or the same trait can evolve under the action of multiple genes. Typically, the interaction of genes has a biochemical nature, i.e. it's based on the combined action of proteins, whose synthesis is controlled by certain genes. Interact with each other can both allelic and non-allelic genes. The interaction of several genes that are located in different loci called non-allelic. Non-allelic gene interactions can be divided into two groups: complementary gene interaction and epistatic gene interaction. What is the difference between them? Perhaps, it can be explained in simple words. Complementary genes - is non-allelic genes that act together on trait expression, i.e. the combination of these genes in the genotype give us a new phenotype. And epistasis it is the interaction of non-allelic genes, in which one of them suppress the action of another. A gene that suppresses the action of another non-allelic gene called Principles of genetics 1 suppressor or epistatic gene, and can be marked by the letters I or S. And the suppressed gene called hypostatic. Both dominant and recessive genes can interact each other. Therefore, there is a dominant and recessive epistasis. When a dominant allele of one gene suppresses the action of another gene, it is called dominant epistasis. And when the recessive allele of epistatic gene in the homozygous state suppresses the action of another gene, it is called recessive epistasis. Often, the interaction of non-allelic genes can be explained both in terms of complementary interaction of genes and in terms of epistatic interactions of genes. For the complementary interaction of genes the ratio of phenotypes depend on whether or not an independent phenotypic expression in these genes. What is epistasis? A gene interaction in which the effects of an allele at one gene hide the effects of alleles at another gene The genes may work against each other (antagonistically) resulting in one gene masking or suppressing the expression of another gene. Or they may work together in a complementary fashion. 9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1 Principles of genetics 2 Epistasis Gene interaction “color” 1. Recessive Epistasis (9:3:4) 2- Dominant epistasis (12:3:1) The presence of a recessive allele at one One copy of a dominant epistatic allele (B) locus prevents the expression of another of one gene masks the effect of the second allele at a second locus (One gene masks gene, regardless of the alleles there. the phenotypic effects of another). The Complete dominance at both gene pairs, alleles of ‘A’ locus express themselves as but one gene, when dominant, epistatic to a recessive allele b.(Become A-bb = the other. (Become A-B- = A-bb). aabb) The phenotypic ratio is 9: 4: 3. The phenotypic ratio is 12: 3: 1. Example: Example: Skin color onions Red 9/16 – yellow 3/16 – white 4/16 In Mouse coat color Fruit color in summer squash 12/16 white – 3/16 yellow – 1/16 Green Agouti 9/16 – Black 3/16 – Albino 4/16 Dominant white hides the effect of yellow or green. Genetic anaylsis: Genetic anaylsis: R yellow > r white C no effect > c white C-R- red W white > w green Y yellow > y green P: WWyy x wwYY Principles of genetics 3 P: CCRR x ccrr red white F1: C-R- (red) F2:C-R-: C-rr :ccR- :ccrr 9 red : 3 yellow: 4 white white yellow F1: W-Y- (white ) F2:W-Y-: W-yy: wwY-: wwyy 12 white : 3 yellow : 1 green 3. Duplicate recessive epistasis (9:7) 4. Dominant and recessive epistasis (13:3) If both gene loci have homozygous Sometimes the dominant alleles of one recessive alleles and both of them produce gene locus (A) in homozygous and identical phenotype the F2 ratio 9:3:3:1 heterozygous (AA, Aa) condition and would be 9:7. The genotype aaBB, aaBb, homozygous recessive alleles bb of AAbb, Aabb and aabb produce same another gene locus (B) produce the same phenotype. Both dominant alleles when phenotype. The F2 ratio will become 13: 3. are present together only then they can The genotype AABB, AaBB, AAbb, Aabb complement each other. This is known as and aabb produce one type of phenotype complementary gene. and genotype aaBb, aaBB will produce another type of phenotype. Examples: Example: In sweet pea flower color 9/16 purple – 7/16 white Note that the C and P Feather Color of Fowl 13/16 white : 3/16 color genes Gene pair ‘A’: color inhibition is dominant independently assort, and remember that to color appearance. the presence of a recessive genotype at Gene pair ‘B’: color is dominant to white one locus (i.e., cc; or pp) masks the effects of the alleles at the other locus. Principles of genetics 4 Genetic anaylsis: Genetic anaylsis: P: AABB (purple) x aabb (white) F1: A-B- (purple) F2: A-B-:A-bb: aaB-: aabb 9 purple : 7 white C color > c white I “no color” > i white P: CCII white x ccii white F1: CcIi (white ) F2:C-I- : C-ii :ccI- : ccii 13 white : 3 color ****Duplicate recessive epistasis Many years after Bateson first described this 9:7 phenotypic ratio in pea plants, researchers were finally able to determine the two genes responsible for it. These genes control flower color by controlling pea plant biochemistry, in particular that related to pigment compounds called anthocyanins. In peas, there is a two-step chemical reaction that forms anthocyanins; gene C is responsible for the first step, and gene P is responsible for the second. If either step is nonfunctional, then no purple pigment is produced, and the affected pea plant bears only white flowers. The dominant C and P alleles code for functional steps in anthocyanin production, whereas the recessive c and p alleles code for nonfunctional steps. Thus, if two recessive alleles occur for either gene, white flowers will result. Principles of genetics 5 Epistasis Gene interaction “Novel phenotype” 4. Duplicate dominant epistasis (15:1) 5. Duplicate interaction (9:6:1) If a dominant allele of both gene loci Certain phenotypic traits depend on the produces the same phenotype without dominant alleles of two gene loci. When cumulative effect, i.e., independently the dominant is present it will show its ratio will be 15:1. phenotype. The ratio will be 9: 6: 1. Examples: Example: Shepherd’s purse 15/16 triangular : 1/16 ovate Fruit shape in summer squash Complete dominance at both gene pairs, Disc shaped fruits 9/16 but either gene when dominant, epistatic to the other. Seed capsule of shepherd’s purse. Sphere shaped fruits 6/16 Long shaped fruit colored 1/16 Gene pair ‘A’: Triangular shape dominant over ovoid Gene pair ‘B’: Triangular shape dominant over ovoid (double recessive) Principles of genetics 6 Genetic anaylsis: Genetic anaylsis: P: AABB (Tria.) x aabb (ovate) F1: A-B- (tria.) F2:A-B-: A-bb : aaB-:aabb 15 tria. : 1 ovate P:AAbb (round) x aaBB (round) F1: A-B- (disc ) F2: A-B-:A-bb: aaB-: aabb 9 disc : 6 round : 1 long Principles of genetics 7 Epistatic Ratios: Ratio Description 9:4:3 Name(s) of Relationship Recessive epistasis A-B- (red): A-bb (white): aaB- (yellow) : aabb (white) 9/16 (agouti) : 4/16 (albino) : 3/16 (black) 12:3:1 Dominant epistasis A-B- = A-bb : aaB- : aabb 12 White 9:7 : 3 yellow : 1 green Duplicate recessive epistasis A-B- : A-bb = aaB- = aabb 9 Purple : 7 white 13:3 Dominant and recessive epistasis A-B- = aaB- = aabb : A-bb 13 white : 3 color 15:1 Duplicate dominant epistasis A-B- = A-bb = aaB- : aabb 15 triangular : 1 ovate 9:6:1 Duplicate interaction A-B- : A-bb = aaB- : aabb 9 Disc : 6 round : 1 long References: (Dooner et al., 1991). http://seattlecentral.edu/faculty/jwhorley/Extensions.pdf http://www.slideshare.net/zohaibkhan404/epistatic-gene-interaction-by-biotechnology-ciit-abbottabad http://www.roosterrooperformances.com/roo-testiments.html http://www.slideshare.net/shainamavreenvillaroza/general-genetics-gene-segregation-and-integration-part-2 http://www.biologydiscussion.com/inheritance-in-plants/inheritance-types-top-14-types-of-inheritance-in-plants/18393 http://iweb.langara.bc.ca/biology/mario/Biol2330notes/biol2330chap13.html Principles of genetics 8 Exercise 11 a. In chickens, most individuals have un-feathered shanks when they are homozygous for recessive genes at two loci; the presence of a single dominant gene at either locus causes feathers. What is the feathered-un-feathered ratio in their offspring if chickens heterozygous at both loci are crossed? a. 9:7 b. 12:4 c. 13:3 d. 15:1 2. The agouti fur colour in mice actually results from alternating dark and light bands on each hair. Given this information and what you know about the gene interaction from the previous problem, propose a mechanism that could explain how each of the two genes is actually affecting the overall fur colour (agouti, black, or albino). Principles of genetics 9 3. In summer squash, there are two pairs of alleles that determine fruit colour. The two genes sort independently. Two white-fruited plants are crossed. Both parents are known to be heterozygous for both genes. The cross produces the following offspring: 20 green-fruited plants, 58 yellow-fruited plants, and 218 white-fruited plants. (a) Based on the observed ratio, which common type of epistasis is operating here? That is, which kind of modified dihybrid ratio most closely fits these data? (b) List the four genotype classes in the offspring and give the corresponding phenotype of each. (c) If a doubly heterozygous white plant is crossed with a green plant, what phenotype ratio would you expect in the progeny? Principles of genetics 10 4. Which of the following phenotypic ratios does not indicate epistasis in a dihybrid cross? a. b. c. d. e. 9:3:3:1 9:3:4 12:3:1 9:7 Unknown 5. Mating between purple starfish of identical genotype produced offspring as follows: 18 pink, 54 purple, and 24 albino. (a) What epistatic ratio is approximated by these offspring? (b)What are the genotypes of the parents and the offspring (use your own symbols)? Principles of genetics 11