1. Coat Color in Mice 2 different genes determine only 3 different phenotypes, rather than 4 phenotypes typical of a dihybrid cross

5. Homozygous, recessive genotype at C-locus is epistatic to genotype at B-locus

6. Another epistasis example - flower color in peas Flower color is determined by two different genes The pigment in colored flowers is produced by a two-step process

7. GenotypeFlower colorEnzyme activities C_P_Flowers colored; anthocyanin produced Functional enzymes from both genes C_ppFlowers white; no anthocynain produced p enzyme not functional ccP_Flowers white; no anthocynain produced c enzyme not functional ccppFlowers white; no anthocynain produced P and c enzyme not functional

8. The result is therefore a ratio of 9 flowered plants: 7 white plants

9. Pleiotropic genes

10. Yellow and gray coat color in mice In 1904, researchers begin with a true- breeding strain of gray mice crossed with yellow mice The F1 generation was 50% gray and 50% yellow –Yellow must be dominant to gray –The yellow mice must have been heterozygotes

11. Yellow and gray coat color Next a cross of two yellow mice was made –One predicts a 3:1 ratio of yellow to gray mice –The result was a 2:1 ratio of yellow to gray mice

12. The ratio of 2:1 suggests a lethal gene In the heterozygous condition, the Y allele causes a yellowing of the coat In the homozygous condition, the Y alleles produce enough gene product to cause the mouse to die The Y allele is said to be pleiotropic; it affects more than one phenotypic characteristic

13. Punnett Square predictions Male FemaleYy YYYYy y yy

14. Phenylketonuria - another example of pleiotropy Metabolic defect caused by homozygous recessive alleles for enzyme phenylalanine hydroxylase

15. Phenylketonuria - another example of pleiotropy Primary effect of mutant gene is to cause toxic substances to build up in the brain, leading to mental impairment The mutant gene also affects: – the synthesis of melanin pigment, resulting in PKU patients having light brown or blond hair –Posture –Organ function

16. Figure 10.18a Fruit color is highly variable in bell peppers.

17. YellowBrown X F 2 generation F 1 generation Parental generation Red 9/16 Yellow 3/16 Brown 3/16 Green 1/16 Red Self-fertilization Figure 10.18b Crosses between pure lines produce novel colors.

18. GenotypeColorExplanation of color R-Y- rrY- R-yy Red Yellow Brown Red pigment + no chlorophyll Yellow pigment + no chlorophyll Red pigment + chlorophyll rryyGreenYellow pigment + chlorophyll Gene 1Gene 2 R = Red r = Yellow (-) = R or r Y = Absence of green (no chlorophyll) y = Presence of green (+ chlorophyll) (-) = Y or y Figure 10.18c Model to explain 9 : 3 : 3 : 1 pattern observed above: Two genes interact to produce pepper color.

19. Skin color in corn snakes

20. Gene interactions in corn snakes Two loci –One allele causes black pigment to be deposited (dominant allele is B + and recessive is b) –One allele causes orange pigment to be deposited (dominant allele is O + and recessive is o)