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

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

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

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

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

Pleiotropic genes

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

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

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

Punnett Square predictions Male FemaleYy YYYYy y yy

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

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

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

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.

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.

Skin color in corn snakes

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)