University:
California State University, Northridge
Course:
BIOL 360 | Genetics
Academic year:

2007
Views:

362

Pages:

28
Author:

brokolaf7g

Chapter 4 !Extensions to Mendelian Genetics ! Gene Interactions 1 Gene Interactions – Extensions to Mendelian Genetics • Just as different alleles of 1 gene can interact in complex ways, • 2 different genes can also act together to modify a phenotype: •2 genes 1 phenotype (Additive Gene Action) Complementation (complementary gene action) Epistasis (recessive and dominant) Redundancy 2 Multifactorial Inheritance • Vast majority of traits are determined by multiple factors: – genetic as well as environmental. • Gene interactions between two or more genes – Example: Lentil Seed color. • F1 all same, F2: 4 different phenotypes • F2 phenotypic ratio is 9:3:3:1 – (same as F2 dihybrids in Mendel’s original crosses). • Difference: – in original crosses: 2 independent traits/phenotypes=2 independent genes; • Seed color and seed shape – here: multiple phenotypes of 1 trait=2 independent genes • Seed color only. 3 (Additive Gene Action) You can tell this genotype is caused by more than one gene : • because there are 4 phenotypes not 3 in F2 •1 gene F2 would have 3 phenotypes 1:2:1 ratio 4 F2 phenotypes • Dominance Relationships: – – – – Tan is dominant to green Gray is dominant to green Brown is dominant to gray, green and tan. Tan and Gray are incompletely dominant, giving rise to brown. • Genotypic classes: – – – – Brown: A_B_ Tan: A_bb Gray: aaB_ Green: aabb 5 Complementary Gene Action • Each genotypic class may not always dictate a unique phenotype • A pair of genes can often work together to create a specific phenotype. We call this complementary interaction. • With this type of interaction we see 2 different phenotypes instead of the 4 seen in 2 genes 1 phenotype • Two or more genotypic classes may display an identical phenotype. – Example: Two lines of pure breeding white flowered pea plants falling into different genotypic classes: AAbb & aaBB The must have a dominant allele in both genes to result in the purple flower phenotype 6 7 8 Epistasis • One gene’s allele masks the phenotype of the other gene’s alleles. • Four genotypic classes produce fewer than four phenotypes. • Different types of epistasis: • Recessive epistasis: when the recessive allele of one gene masks the effects of either allele of the second gene. • Dominant epistasis: when the dominant allele of one gene masks the effects of either allele of the second gene. 9 Recessive Epistasis • Example 1: Coat color of Labrador retriever • Example 2: ABO blood groups: Bombay phenotype. • Phenotypic ratios are 9:3:4 in F2. 10 Coat-Color Inheritance in Labrador Retrievers X P black golden F1 black 11 Recessive Epistasis: a recessive mutation in one gene masks the phenotypic effects of another F1 Appears like incomplete dominance because some of the progeny look like neither parent, but the ratio is wrong. X F2 9 : 3 : 4 12 Dihybrid Cross: x BbEe BE F2 Be bE BbEe be BE AACC BBEE AACc BBEe Be BBEe AACc AAcc BBee bE cCC BbEE Aa BbEe AacCc cacCC a cacCc bbEe abbEE be AACc BbEe Aaccc Bbee cacCc acaccc abbEe bbee cCC cCc Aa BbEe BbEE Aa cCc Aa BbEe ccc Aa Bbee 9 black: 3 brown: 4 golden (9 B-E-: 3 bbE-: 3 B-ee: 1 bbee) 13 Molecular Explanation Pigment production (B) and subsequent incorporation (E) into the hair shaft are controlled by two separate genes. To be black, both genes must function. Mutations in B (b) lead to brown pigment. Mutations in E (e) lead to no pigment in coat. gene B gene E 3 4 9 14 Recessive Epistasis • Two genes involved in coat color determination. • Gene B determines whether black (B) or brown (bb) pigment is produced. • Gene E determines if pigment is deposited in hair – golden retrievers (ee) make either black (B-) or brown (bb) pigment (look at noses)… but not in fur • The recessive allele is epistatic to (stands over) other genes when homozygous -- hence the name “recessive epistasis” • Phenotypes do not segregate according to Mendelian ratios (the phenotypic ratios are modified Mendelian ratios). • epistasis - (Greek, to stand upon or stop) the differential phenotypic expression of a genotype at one locus caused by the genotype at another, non allelic, locus. A mutation that exerts its expression by canceling the expression of the 15 alleles of another gene. Dominant Epistasis • caused by the dominant allele of one gene, masking the action of either allele of the other gene. • Ratio is 12:3:1 instaed f 9:3:3:1 • Example: Summer Squash 16 17 Redundancy: Duplicate Genes P Petal color in snapdragons if Mendel had used snap dragons for his experiments, he wouldn’t be famous! X AABB F1 F2 aabb A-B- 15/16 red; 1/16 white 18 X A-B- A-B- AB Ab AB AACC AABB AACc AABb AacCC AaBb AaBB Ab AABb AACc AAcc AAbb cCc Aa AaBb aB cCC AaBb AaBB Aa AacCc aaBB acacCC aaBb ab AACc AaBb 15/16 A-B- Aaccc Aabb aB ab Aabb caccc acacCc a aaBb aabb red; 1/16 aabb white Whenever a dominant gene is present, the trait is expressed.19 One allele is sufficient to produce the pigment. Hints for figuring out gene interactions: Look at the F2 phenotypic ratios!! • If one gene is involved in the trait, then the monohybrid phenotypic ratio is: 3:1 or 1:2:1 or 2:1 • If two genes are involved in the trait, then the dihybrid phenotypic ratio is: 9:3:3:1 or some permutation (9:4:3 or 9:7 or 12:3:1) "The 1/16 class is always the double homozygous recessive. " Look for internal 3:1 ratios, which will indicate 20 dominance/recessive relationships for alleles within a gene. Hints for figuring out gene interactions: • 2 Genes 1 Phenotype (Additive Gene Action): You can tell this genotype is caused by more than one gene because there are 4 phenotypes not 3 in F2 (9:3:3:1) – 1 gene F2 would have 3 phenotypes 1:2:1 ratio • Complementary Gene Action: one good copy of each gene is needed for expression of the final phenotype – 9:7 ratio • Epistasis: one gene can mask the effect of another gene – 9:3:4 ratio for recessive epistasis – 12:3:1 ratio for dominant epistasis • Duplicate genes: only double mutant has mutant phenotype – 15:1 ratio 21 variations on Mendelian inheritance Gene interaction Inheritance pattern A-/B- A-/bb aa/B- aabb ratio 9 3 3 1 9:3:3:1 At least one dominant Complementary allele from each of two genes needed for phenotype 9 3 3 1 9:7 Recessive Epistasis Homozyous recessive genotype at one locus masks expression at second locus 9 3 3 1 9:3:4 Dominant Epistasis Dominant allele at one locus masks expression at second locus 9 3 3 1 12:3:1 One dominant allele from either of twogenes needed for phenotype 9 3 3 1 15:1 Additive Duplicate Genes Each genotype results in a unique phenotype 22 Sample Problem true breeding brown dogs X true breeding white dogs F1 = all white F2 = 118 white 12 32 black 3 10 brown 1 # Find the genotypes of the dogs in each class: What is the ratio? How many genes? 2 What is the ratio of white to colored dogs? 12:4 = 3:1 This means that white is dominant to colored so let’s call one gene: W= white w=colored 23 F2 = 118 white 32 black 10 brown What is the ratio of black to brown dogs? 3 : 1 So black must be dominant to brown. So we will call the second gene: B=black and b=brown What class of dogs are the double recessive homozygotes and what is their genotype? Brown - wwbb What is the genotype of the black dogs? Must be wwBWhat are the genotypes of the white dogs? W_ B_ and W_bb # This is an example of dominant 24 epistasis (white). Same Genotype may produce different Phenotypes • Penetrance: Genotype does not necessarily define phenotype. The proportion of individuals with a given genotype express the phenotype determines penetrance. • 100% penetrance = all individuals show phenotype. • 50% penetrance = half the individuals show phenotype. – Example: retinoblastoma: only 75% individuals affected. • Expressivity: the degree or intensity with which a particular genotype is expressed in a phenotype in a given individual. – Retinoblastoma: some have both eyes affected, some only one. 25 Modifier Effects • Modifier Genes: they have a subtle, secondary effect which alters the phenotypes produced by the primary genes. – E.G. Tail length in mice. The mutant allele t causes a shortening of the tail. Not all short tails are of the same length: another gene affects the actual length. (Variable expressivity). • Modifying environment: The environment may influence the effect of a genotype on the phenotype. – E.G.: Siamese cats: temperature dependent color of coat. Color shows up only in extremities, where the temp is lower (enzyme for pigment formation is active only at lower temp.) 26 Modifying environment: The environmental influence of a genotype on the phenotype= phenocopy 27 Modifying environment: The environmental influence of a genotype on the phenotype= phenocopy 28

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