Coat Color in Mice

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

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

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. Figure 10.18b Crosses between pure lines produce novel colors. Parental generation X Yellow Brown F1 generation Red Self-fertilization F2 generation Red Yellow Brown Green 9/16 3/16 3/16 1/16

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

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)